The Objective Caml system
release 1.07
Documentation and user's manual
Xavier Leroy
December 10, 1997
Copyright oc 1997 Institut National de Recherche en Informatique et
Automatique
Contents
I An introduction to Objective Caml 6
1 The core language 7
2 Objects in Caml 19
3 The module system 34
II The Objective Caml language 42
4 The Objective Caml language 43
4.1 Lexical conventions . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3 Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.4 Type expressions. . . . . . . . . . . . . . . . . . . . . . . . . 49
4.5 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.6 Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.7 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.8 Type and exception definitions. . . . . . . . . . . . . . . . . . 61
4.9 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.10 Module types (module specifications). . . . . . . . . . . . . . . 65
4.11 Module expressions (module implementations) . . . . . . . . . . . 69
4.12 Compilation units . . . . . . . . . . . . . . . . . . . . . . . . 71
5 Language extensions 72
5.1 Streams and stream parsers. . . . . . . . . . . . . . . . . . . . 72
5.2 Range patterns. . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3 Assertion checking. . . . . . . . . . . . . . . . . . . . . . . . 73
5.4 Deferred computations . . . . . . . . . . . . . . . . . . . . . . 74
III The Objective Caml tools 75
6 Batch compilation (ocamlc) 76
6.1 Overview of the compiler. . . . . . . . . . . . . . . . . . . . . 76
6.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.3 Modules and the file system . . . . . . . . . . . . . . . . . . . 79
6.4 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . 80
7 The toplevel system (ocaml) 83
7.1 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.2 Toplevel directives . . . . . . . . . . . . . . . . . . . . . . . 85
7.3 The toplevel and the module system. . . . . . . . . . . . . . . . 86
7.4 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7.5 Building custom toplevel systems: ocamlmktop . . . . . . . . . . 87
7.6 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
8 The runtime system (ocamlrun) 89
1
2
8.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.3 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . 90
9 Native-code compilation (ocamlopt) 92
9.1 Overview of the compiler. . . . . . . . . . . . . . . . . . . . . 92
9.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
9.3 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . 95
9.4 Compatibility with the bytecode compiler. . . . . . . . . . . . . 95
10 Lexer and parser generators (ocamllex, ocamlyacc) 97
10.1 Overview of ocamllex. . . . . . . . . . . . . . . . . . . . . . . 97
10.2 Syntax of lexer definitions . . . . . . . . . . . . . . . . . . . 97
10.3 Overview of ocamlyacc . . . . . . . . . . . . . . . . . . . . . . 99
10.4 Syntax of grammar definitions . . . . . . . . . . . . . . . . . . 100
10.5 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
10.6 A complete example. . . . . . . . . . . . . . . . . . . . . . . . 102
11 Dependency generator (ocamldep) 104
11.1 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
11.2 A typical Makefile. . . . . . . . . . . . . . . . . . . . . . . . 104
12 The debugger (ocamldebug) 106
12.1 Compiling for debugging . . . . . . . . . . . . . . . . . . . . . 106
12.2 Invocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
12.3 Commands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
12.4 Executing a program . . . . . . . . . . . . . . . . . . . . . . . 108
12.5 Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
12.6 The call stack. . . . . . . . . . . . . . . . . . . . . . . . . . 111
12.7 Examining variable values . . . . . . . . . . . . . . . . . . . . 112
12.8 Controlling the debugger. . . . . . . . . . . . . . . . . . . . . 113
12.9 Miscellaneous commands. . . . . . . . . . . . . . . . . . . . . . 115
12.10Running the debugger under Emacs. . . . . . . . . . . . . . . . . 116
13 Profiling (ocamlprof) 118
13.1 Compiling for profiling . . . . . . . . . . . . . . . . . . . . . 118
13.2 Profiling an execution. . . . . . . . . . . . . . . . . . . . . . 118
13.3 Printing profiling information. . . . . . . . . . . . . . . . . . 119
13.4 Time profiling. . . . . . . . . . . . . . . . . . . . . . . . . . 119
14 Interfacing C with Objective Caml 120
14.1 Overview and compilation information. . . . . . . . . . . . . . . 120
14.2 The value type. . . . . . . . . . . . . . . . . . . . . . . . . . 123
14.3 Representation of Caml data types . . . . . . . . . . . . . . . . 124
14.4 Operations on values. . . . . . . . . . . . . . . . . . . . . . . 125
14.5 Living in harmony with the garbage collector. . . . . . . . . . . 128
14.6 Callbacks from C to Caml. . . . . . . . . . . . . . . . . . . . . 130
14.7 A complete example. . . . . . . . . . . . . . . . . . . . . . . . 133
14.8 Advanced example with callbacks . . . . . . . . . . . . . . . . . 135
IV The Objective Caml library 137
15 The core library 138
15.1 Module Pervasives: the initially opened module . . . . . . . . . 138
16 The standard library 153
16.1 Module Arg: parsing of command line arguments. . . . . . . . . . 154
16.2 Module Array: array operations . . . . . . . . . . . . . . . . . 155
3
16.3 Module Callback: registering Caml values with the C runtime. . . 157
16.4 Module Char: character operations. . . . . . . . . . . . . . . . 158
16.5 Module Digest: MD5 message digest. . . . . . . . . . . . . . . . 158
16.6 Module Filename: operations on file names. . . . . . . . . . . . 159
16.7 Module Format: pretty printing . . . . . . . . . . . . . . . . . 160
16.8 Module Gc: memory management control and statistics. . . . . . . 166
16.9 Module Genlex: a generic lexical analyzer. . . . . . . . . . . . 169
16.10Module Hashtbl: hash tables and hash functions . . . . . . . . . 170
16.11Module Lazy: deferred computations.. . . . . . . . . . . . . . . 172
16.12 Module Lexing: the run-time library for lexers generated by
camllex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
16.13Module List: list operations . . . . . . . . . . . . . . . . . . 173
16.14Module Map: association tables over ordered types. . . . . . . . 176
16.15Module Marshal: marshaling of data structures. . . . . . . . . . 177
16.16Module Oo: object-oriented extension . . . . . . . . . . . . . . 179
16.17Module Parsing: the run-time library for parsers generated by
camlyacc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
16.18Module Printexc: a catch-all exception handler . . . . . . . . . 180
16.19Module Printf: formatting printing functions . . . . . . . . . . 180
16.20Module Queue: first-in first-out queues. . . . . . . . . . . . . 181
16.21Module Random: pseudo-random number generator. . . . . . . . . . 182
16.22Module Set: sets over ordered types. . . . . . . . . . . . . . . 182
16.23Module Sort: sorting and merging lists . . . . . . . . . . . . . 184
16.24Module Stack: last-in first-out stacks . . . . . . . . . . . . . 185
16.25Module Stream: streams and parsers . . . . . . . . . . . . . . . 185
16.26Module String: string operations . . . . . . . . . . . . . . . . 187
16.27Module Sys: system interface . . . . . . . . . . . . . . . . . . 189
16.28Module Weak: arrays of weak pointers . . . . . . . . . . . . . . 191
17 The unix library: Unix system calls 192
17.1 Module Unix: interface to the Unix system. . . . . . . . . . . . 193
18 The num library: arbitrary-precision rational arithmetic 213
18.1 Module Num: operation on arbitrary-precision numbers . . . . . . 214
18.2 Module Arith_status: flags that control rational arithmetic. . . 216
19 The str library: regular expressions and string processing 218
19.1 Module Str: regular expressions and high-level string processing 218
20 The threads library 222
20.1 Module Thread: lightweight threads . . . . . . . . . . . . . . . 223
20.2 Module Mutex: locks for mutual exclusion . . . . . . . . . . . . 224
20.3 Module Condition: condition variables to synchronize between
threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
20.4 Module Event: first-class synchronous communication. . . . . . . 226
20.5 Module ThreadUnix: thread-compatible system calls. . . . . . . . 227
21 The graphics library 229
21.1 Module Graphics: machine-independent graphics primitives . . . . 230
22 The dbm library: access to NDBM databases 236
22.1 Module Dbm: interface to the NDBM databases. . . . . . . . . . . 236
V Appendix 238
Index to the library 239
Index of keywords 246
Foreword
This manual documents the release 1.07 of the Objective Caml system. It is
organized as follows.
- Part I, ``An introduction to Objective Caml'', gives an overview of the
language.
- Part II, ``The Objective Caml language'', is the reference description of
the language.
- Part III, ``The Objective Caml tools, documents the compilers, toplevel
system, and programming utilities.
- Part IV, ``The Objective Caml library'', describes the modules provided
in the standard library.
- Part V, ``Appendix'', contains an index of all identifiers defined in the
standard library, and an index of keywords.
Conventions
Objective Caml runs on several operating systems. The parts of this manual
that are specific to one operating system are presented as shown below:
Unix:
This is material specific to Unix.
Windows:
This is material specific to MS Windows (NT and 95).
License
c
The Objective Caml system is copyright o 1996, 1997 Institut National de
Recherche en Informatique et en Automatique (INRIA). INRIA holds all ownership
rights to the Objective Caml system. See the file LICENSE in the distribution
for licensing information.
The Objective Caml system can be freely redistributed. More precisely,
INRIA grants any user of the Objective Caml system the right to reproduce it,
provided that the copies are distributed under the conditions given in the
LICENSE file. The present documentation is distributed under the same
conditions.
Availability
The complete Objective Caml distribution resides on the machine ftp.inria.fr.
The distribution files can be transferred by anonymous FTP:
4
5
Host: ftp.inria.fr (Internet address 192.93.2.54)
Login name: anonymous
Password: your e-mail address
Directory: lang/caml-light
Files: see the index in file README
More information on the Caml family of languages is also available on the
World Wide Web, http://pauillac.inria.fr/caml/.
Part I
An introduction to Objective Caml
6
Chapter 1
The core language
This part of the manual is a tutorial introduction to the Objective Caml
language. A good familiarity with programming in a conventional languages
(say, Pascal or C) is assumed, but no prior exposure to functional languages
is required. The present chapter introduces the core language. Chapter 2
deals with the object-oriented features, and chapter 3 with the module system.
Basics
For this overview of Caml, we use the interactive system, which is started by
running ocaml from the Unix shell, or by launching the OCamlwin.exe
application under Windows. This tutorial is presented as the transcript of a
session with the interactive system: lines starting with # represent user
input; the system responses are printed below, without a leading #.
Under the interactive system, the user types Caml phrases, terminated by ;;,
in response to the # prompt, and the system compiles them on the fly, executes
them, and prints the outcome of evaluation. Phrases are either simple
expressions, or let definitions of identifiers (either values or functions).
#1+2*3;;
- : int = 7
#let pi = 4.0 *. atan 1.0;;
val pi : float = 3.14159265359
#let square x = x *. x;;
val square : float -> float = <fun>
#square(sin pi) +. square(cos pi);;
- : float = 1
The Caml system computes both the value and the type for each phrase. Even
function parameters need no explicit type declaration: the system infers
their types from their usage in the function. Notice also that integers and
floating-point numbers are distinct types, with distinct operators: + and *
operate on integers, but +. and *. operate on floats.
#1.0 * 2;;
Characters 0-3:
This expression has type float but is here used with type int
Recursive functions are defined with the let rec binding:
#let rec fib n =
7
Chapter 1. The core language 8
# if n < 2 then 1 else fib(n-1) + fib(n-2);;
val fib : int -> int = <fun>
#fib 10;;
- : int = 89
Data types
In addition to integers and floating-point numbers, Caml offers the usual
basic data types: booleans, characters, and character strings.
#(1 < 2) = false;;
- : bool = false
#'a';;
- : char = 'a'
#"Hello world";;
- : string = "Hello world"
Predefined data structures include tuples, arrays, and lists. General
mechanisms for defining your own data structures are also provided. They will
be covered in more details later; for now, we concentrate on lists. Lists are
either given in extension as a bracketed list of semicolon-separated elements,
or built from the empty list [] (pronounce ``nil'') by adding elements in
front using the :: (``cons'') operator.
#let l = ["is"; "a"; "tale"; "told"; "etc."];;
val l : string list = ["is"; "a"; "tale"; "told"; "etc."]
#"Life" :: l;;
- : string list = ["Life"; "is"; "a"; "tale"; "told"; "etc."]
As with all other Caml data structures, lists do not need to be explicitly
allocated and deallocated from memory: all memory management is entirely
automatic in Caml. Similarly, there is no explicit handling of pointers: the
Caml compiler silently introduces pointers where necessary.
As with most Caml data structures, inspecting and destructuring lists is
performed by pattern-matching. List patterns have the exact same shape as
list expressions, with identifier representing unspecified parts of the list.
As an example, here is insertion sort on a list:
#let rec sort lst =
# match lst with
# [] -> []
# | head :: tail -> insert head (sort tail)
#and insert elt lst =
# match lst with
# [] -> [elt]
# | head :: tail -
> if elt <= head then elt :: lst else head :: insert elt tail
#;;
val sort : 'a list -> 'a list = <fun>
val insert : 'a -> 'a list -> 'a list = <fun>
#sort l;;
- : string list = ["a"; "etc."; "is"; "tale"; "told"]
Chapter 1. The core language 9
The type inferred for sort, 'a list -> 'a list, means that sort can actually
apply to lists of any type, and returns a list of the same type. The type 'a
is a type variable, and stands for any given type. The reason why sort can
apply to lists of any type is that the comparisons (=, <=, etc.) are
polymorphic in Caml: they operate between any two values of the same type.
This makes sort itself polymorphic over all list types.
#sort [6;2;5;3];;
- : int list = [2; 3; 5; 6]
#sort [3.14; 2.718];;
- : float list = [2.718; 3.14]
The sort function above does not modify its input list: it builds and
returns a new list containing the same elements as the input list, in
ascending order. There is actually no way in Caml to modify in-place a list
once it is built: we say that lists are immutable data structures. Most Caml
data structures are immutable, but a few (most notably arrays) are mutable,
meaning that they can be modified in-place at any time.
Functions as values
Caml is a functional language: functions in the full mathematical sense are
supported and can be passed around freely just as any other piece of data.
For instance, here is a deriv function that takes any float function as
argument and returns an approximation of its derivative function:
#let deriv f dx = function x -> (f(x +. dx) -. f(x)) /. dx;;
val deriv : (float -> float) -> float -> float -> float = <fun>
#let cos' = deriv sin 1e-6;;
val cos' : float -> float = <fun>
#cos' pi;;
- : float = -1.00000000014
Even function composition is definable:
#let compose f g = function x -> f(g(x));;
val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = <fun>
#let cos2 = compose square cos;;
val cos2 : float -> float = <fun>
Functions that take other functions as arguments are called ``functionals'',
or ``higher-order functions''. Functionals are especially useful to provide
iterators or similar generic operations over a data structure. For instance,
the standard Caml library provides a List.map functional that applies a given
function to each element of a list, and returns the list of the results:
#List.map (function n -> n * 2 + 1) [0;1;2;3;4];;
- : int list = [1; 3; 5; 7; 9]
This functional, along with a number of other list and array functionals, is
predefined because it is often useful, but there is nothing magic with it: it
can easily be defined as follows.
#let rec map f l =
Chapter 1. The core language 10
# match l with
# [] -> []
# | hd :: tl -> f hd :: map f tl;;
val map : ('a -> 'b) -> 'a list -> 'b list = <fun>
Records and variants
User-defined data structures include records and variants. Both are defined
with the type declaration. Here, we declare a record type to represent
rational numbers.
#type ratio = {num: int; denum: int};;
type ratio = { num: int; denum: int }
#let add_ratio r1 r2 =
# {num = r1.num * r2.denum + r2.num * r1.denum;
# denum = r1.denum * r2.denum};;
val add_ratio : ratio -> ratio -> ratio = <fun>
#add_ratio {num=1; denum=3} {num=2; denum=5};;
- : ratio = {num=11; denum=15}
The declaration of a variant type lists all possible shapes for values of
that type. Each case is identified by a name, called a constructor, which
serves both for constructing values of the variant type and inspecting them by
pattern-matching. Constructor names are capitalized to distinguish them from
variable names (which must start with a lowercase letter). For instance, here
is a variant type for doing mixed arithmetic (integers and floats):
#type number = Int of int | Float of float | Error;;
type number = | Int of int | Float of float | Error
This declaration expresses that a value of type number is either an integer, a
floating-point number, or the constant Error representing the result of an
invalid operation (e.g. a division by zero).
Enumerated types are a special case of variant types, where all alternatives
are constants:
#type sign = Positive | Negative;;
type sign = | Positive | Negative
#let sign_int n = if n >= 0 then Positive else Negative;;
val sign_int : int -> sign = <fun>
To define arithmetic operations for the number type, we use pattern-matching
on the two numbers involved:
#let add_num n1 n2 =
# match (n1, n2) with
# (Int i1, Int i2) ->
# (* Check for overflow of integer addition *)
# if sign_int i1 = sign_int i2 && sign_int(i1 + i2) <> sign_int i1
# then Float(float i1 +. float i2)
# else Int(i1 + i2)
# | (Int i1, Float f2) -> Float(float i1 +. f2)
# | (Float f1, Int i2) -> Float(f1 +. float i2)
# | (Float f1, Float f2) -> Float(f1 +. f2)
Chapter 1. The core language 11
# | (Error, _) -> Error
# | (_, Error) -> Error;;
val add_num : number -> number -> number = <fun>
#add_num (Int 123) (Float 3.14159);;
- : number = Float 126.14159
The most common usage of variant types is to describe recursive data
structures. Consider for example the type of binary trees:
#type 'a btree = Empty | Node of 'a * 'a btree * 'a btree;;
type 'a btree = | Empty | Node of 'a * 'a btree * 'a btree
This definition reads as follow: a binary tree containing values of type 'a
(an arbitrary type) is either empty, or is a node containing one value of type
'a and two subtrees containing also values of type 'a, that is, two 'a btree.
Operations on binary trees are naturally expressed as recursive functions
following the same structure as the type definition itself. For instance,
here are functions performing lookup and insertion in ordered binary trees
(elements increase from left to right):
#let rec member x btree =
# match btree with
# Empty -> false
# | Node(y, left, right) ->
# if x = y then true else
# if x < y then member x left else member x right;;
val member : 'a -> 'a btree -> bool = <fun>
#let rec insert x btree =
# match btree with
# Empty -> Node(x, Empty, Empty)
# | Node(y, left, right) ->
# if x <= y then Node(y, insert x left, right)
# else Node(y, left, insert x right);;
val insert : 'a -> 'a btree -> 'a btree = <fun>
Imperative features
Though all examples so far were written in purely applicative style, Caml is
also equipped with full imperative features. This includes the usual while
and for loops, as well as mutable data structures such as arrays. Arrays are
either given in extension between [| and |] brackets, or allocated and
initialized with the Array.create function, then filled up later by
assignments. For instance, the function below sums two vectors (represented
as float arrays) componentwise.
#let add_vect v1 v2 =
# let len = min (Array.length v1) (Array.length v2) in
# let res = Array.create len 0.0 in
# for i = 0 to len - 1 do
# res.(i) <- v1.(i) +. v2.(i)
# done;
# res;;
val add_vect : float array -> float array -> float array = <fun>
#add_vect [| 1.0; 2.0 |] [| 3.0; 4.0 |];;
Chapter 1. The core language 12
- : float array = [|4; 6|]
Record fields can also be modified by assignment, provided they are declared
mutable in the definition of the record type:
#type mutable_point = { mutable x: float; mutable y: float };;
type mutable_point = { mutable x: float; mutable y: float }
#let translate p dx dy =
# p.x <- p.x +. dx; p.y <- p.y +. dy;;
val translate : mutable_point -> float -> float -> unit = <fun>
#let mypoint = { x = 0.0; y = 0.0 };;
val mypoint : mutable_point = {x=0; y=0}
#translate mypoint 1.0 2.0;;
- : unit = ()
#mypoint;;
- : mutable_point = {x=1; y=2}
Caml has no built-in notion of variable -- identifiers whose current value
can be changed by assignment. (The let binding is not an assignment, it
introduces a new identifier with a new scope.) However, the standard library
provides references, which are mutable indirection cells (or one-element
arrays), with operators ! to fetch the current contents of the reference and
:= to assign the contents. Variables can then be emulated by let-binding a
reference. For instance, here is an in-place insertion sort over arrays:
#let insertion_sort a =
# for i = 1 to Array.length a - 1 do
# let val_i = a.(i) in
# let j = ref i in
# while !j > 0 && val_i < a.(!j - 1) do
# a.(!j) <- a.(!j - 1);
# j := !j - 1
# done;
# a.(!j) <- val_i
# done;;
val insertion_sort : 'a array -> unit = <fun>
References are also useful to write functions that maintain a current state
between two calls to the function. For instance, the following pseudo-random
number generator keeps the last returned number in a reference:
#let current_rand = ref 0;;
val current_rand : int ref = {contents=0}
#let random () =
# current_rand := !current_rand * 25713 + 1345;
# !current_rand;;
val random : unit -> int = <fun>
Again, there is nothing magic with references: they are implemented as a
one-field mutable record, as follows.
#type 'a ref = { mutable contents: 'a };;
type 'a ref = { mutable contents: 'a }
Chapter 1. The core language 13
#let (!) r = r.contents;;
val ! : 'a ref -> 'a = <fun>
#let (:=) r newval = r.contents <- newval;;
val := : 'a ref -> 'a -> unit = <fun>
Exceptions
Caml provides exceptions for signalling and handling exceptional conditions.
Exceptions can also be used as a general-purpose non-local control structure.
Exceptions are declared with the exception construct, and signalled with the
raise operator. For instance, the function below for taking the head of a
list uses an exception to signal the case where an empty list is given.
#exception Empty_list;;
exception Empty_list
#let head l =
# match l with
# [] -> raise Empty_list
# | hd :: tl -> hd;;
val head : 'a list -> 'a = <fun>
#head [1;2];;
- : int = 1
#head [];;
Uncaught exception: Empty_list
Exceptions are used throughout the standard library to signal cases where
the library functions cannot complete normally. For instance, the List.assoc
function, which returns the data associated with a given key in a list of
(key, data) pairs, raises the predefined exception Not_found when the key does
not appear in the list:
#List.assoc 1 [(0, "zero"); (1, "one")];;
- : string = "one"
#List.assoc 2 [(0, "zero"); (1, "one")];;
Uncaught exception: Not_found
Exceptions can be trapped with the try...with construct:
#let name_of_binary_digit digit =
# try
# List.assoc digit [0, "zero"; 1, "one"]
# with Not_found ->
# "not a binary digit";;
val name_of_binary_digit : int -> string = <fun>
#name_of_binary_digit 0;;
- : string = "zero"
#name_of_binary_digit (-1);;
- : string = "not a binary digit"
The with part is actually a regular pattern-matching on the exception value.
Chapter 1. The core language 14
Thus, several exceptions can be caught by one try...with construct. Also,
finalization can be performed by trapping all exceptions, performing the
finalization, then raising again the exception:
#let temporarily_set_reference ref newval funct =
# let oldval = !ref in
# try
# ref := newval;
# let res = funct () in
# ref := oldval;
# res
# with x ->
# ref := oldval;
# raise x;;
val temporarily_set_reference : 'a ref -> 'a -> (unit -> 'b) -> 'b = <fun>
Symbolic processing of expressions
We finish this introduction with a more complete example representative of the
use of Caml for symbolic processing: formal manipulations of arithmetic
expressions containing variables. The following variant type describes the
expressions we shall manipulate:
#type expression =
# Const of float
# | Var of string
# | Sum of expression * expression (* e1 + e2 *)
# | Diff of expression * expression (* e1 - e2 *)
# | Prod of expression * expression (* e1 * e2 *)
# | Quot of expression * expression (* e1 / e2 *)
#;;
type expression =
| Const of float
| Var of string
| Sum of expression * expression
| Diff of expression * expression
| Prod of expression * expression
| Quot of expression * expression
We first define a function to evaluate an expression given an environment
that maps variable names to their values. For simplicity, the environment is
represented as an association list.
#exception Unbound_variable of string;;
exception Unbound_variable of string
#let rec eval env exp =
# match exp with
# Const c -> c
# | Var v ->
# (try List.assoc v env with Not_found -> raise(Unbound_variable v))
# | Sum(f, g) -> eval env f +. eval env g
# | Diff(f, g) -> eval env f -. eval env g
# | Prod(f, g) -> eval env f *. eval env g
# | Quot(f, g) -> eval env f /. eval env g;;
val eval : (string * float) list -> expression -> float = <fun>
Chapter 1. The core language 15
#eval [("x", 1.0); ("y", 3.14)] (Prod(Sum(Var "x", Const 2.0), Var "y"));;
- : float = 9.42
Now for a real symbolic processing, we define the derivative of an
expression with respect to a variable dv:
#let rec deriv exp dv =
# match exp with
# Const c -> Const 0.0
# | Var v -> if v = dv then Const 1.0 else Const 0.0
# | Sum(f, g) -> Sum(deriv f dv, deriv g dv)
# | Diff(f, g) -> Diff(deriv f dv, deriv g dv)
# | Prod(f, g) -> Sum(Prod(f, deriv g dv), Prod(deriv f dv, g))
# | Quot(f, g) -> Quot(Diff(Prod(deriv f dv, g), Prod(f, deriv g dv)),
# Prod(g, g))
#;;
val deriv : expression -> string -> expression = <fun>
#deriv (Quot(Const 1.0, Var "x")) "x";;
- : expression =
Quot
(Diff (Prod (Const 0, Var "x"), Prod (Const 1, Const 1)),
Prod (Var "x", Var "x"))
Pretty-printing and parsing
As shown in the examples above, the internal representation (also called
abstract syntax) of expressions quickly becomes hard to read and write as the
expressions get larger. We need a printer and a parser to go back and forth
between the abstract syntax and the concrete syntax, which in the case of
expressions is the familiar algebraic notation (e.g. 2*x+1).
For the printing function, we take into account the usual precedence rules
(i.e. * binds tighter than +) to avoid printing unnecessary parentheses. To
this end, we maintain the current operator precedence and print parentheses
around an operator only if its precedence is less than the current precedence.
#let print_expr exp =
# (* Local function definitions *)
# let open_paren prec op_prec =
# if prec > op_prec then print_string "(" in
# let close_paren prec op_prec =
# if prec > op_prec then print_string ")" in
# let rec print prec exp = (* prec is the current precedence *)
# match exp with
# Const c -> print_float c
# | Var v -> print_string v
# | Sum(f, g) ->
# open_paren prec 0;
# print 0 f; print_string " + "; print 0 g;
# close_paren prec 0
# | Diff(f, g) ->
# open_paren prec 0;
# print 0 f; print_string " - "; print 1 g;
# close_paren prec 0
# | Prod(f, g) ->
# open_paren prec 2;
# print 2 f; print_string " * "; print 2 g;
Chapter 1. The core language 16
# close_paren prec 2
# | Quot(f, g) ->
# open_paren prec 2;
# print 2 f; print_string " / "; print 3 g;
# close_paren prec 2
# in print 0 exp;;
val print_expr : expression -> unit = <fun>
#let e = Sum(Prod(Const 2.0, Var "x"), Const 1.0);;
val e : expression = Sum (Prod (Const 2, Var "x"), Const 1)
#print_expr e; print_newline();;
2 * x + 1
- : unit = ()
#print_expr (deriv e "x"); print_newline();;
2 * 1 + 0 * x + 0
- : unit = ()
Parsing (transforming concrete syntax into abstract syntax) is usually more
delicate. Caml offers several tools to help write parsers: on the one hand,
Caml versions of the lexer generator Lex and the parser generator Yacc (see
chapter 10), which handle LALR(1) languages using push-down automata; on the
other hand, a predefined type of streams (of characters or tokens) and
pattern-matching over streams, which facilitate the writing of
recursive-descent parsers for LL(1) languages. An example using ocamllex and
ocamlyacc is given in chapter 10. Here, we will use stream parsers.
#open Genlex;;
#let lexer = make_lexer ["("; ")"; "+"; "-"; "*"; "/"];;
val lexer : char Stream.t -> Genlex.token Stream.t = <fun>
For the lexical analysis phase (transformation of the input text into a stream
of tokens), we use a ``generic'' lexer provided in the standard library module
Genlex. The make_lexer function takes a list of keywords and returns a lexing
function that ``tokenizes'' an input stream of characters. Tokens are either
identifiers, keywords, or literals (integer, floats, characters, strings).
Whitespace and comments are skipped.
#let token_stream = lexer(Stream.of_string "1.0 +x");;
val token_stream : Genlex.token Stream.t = <abstr>
#Stream.next token_stream;;
- : Genlex.token = Float 1
#Stream.next token_stream;;
- : Genlex.token = Kwd "+"
#Stream.next token_stream;;
- : Genlex.token = Ident "x"
The parser itself operates by pattern-matching on the stream of tokens. As
usual with recursive descent parsers, we use several intermediate parsing
functions to reflect the precedence and associativity of operators.
Pattern-matching over streams is more powerful than on regular data
structures, as it allows recursive calls to parsing functions inside the
patterns, for matching sub-components of the input stream. See chapter 5 for
Chapter 1. The core language 17
more details.
#let rec parse_expr = parser
# [< e1 = parse_mult; e = parse_more_adds e1 >] -> e
#and parse_more_adds e1 = parser
# [< 'Kwd "+"; e2 = parse_mult; e = parse_more_adds (Sum(e1, e2)) >] -> e
# | [< 'Kwd "-"; e2 = parse_mult; e = parse_more_adds (Diff(e1, e2)) >] -> e
# | [< >] -> e1
#and parse_mult = parser
# [< e1 = parse_simple; e = parse_more_mults e1 >] -> e
#and parse_more_mults e1 = parser
# [< 'Kwd "*"; e2 = parse_simple; e = parse_more_mults (Prod(e1, e2)) >] -
> e
# | [< 'Kwd "/"; e2 = parse_simple; e = parse_more_mults (Quot(e1, e2)) >] -
> e
# | [< >] -> e1
#and parse_simple = parser
# [< 'Ident s >] -> Var s
# | [< 'Int i >] -> Const(float i)
# | [< 'Float f >] -> Const f
# | [< 'Kwd "("; e = parse_expr; 'Kwd ")" >] -> e;;
val parse_expr : Genlex.token Stream.t -> expression = <fun>
val parse_more_adds : expression -> Genlex.token Stream.t -> expression =
<fun>
val parse_mult : Genlex.token Stream.t -> expression = <fun>
val parse_more_mults : expression -> Genlex.token Stream.t -> expression =
<fun>
val parse_simple : Genlex.token Stream.t -> expression = <fun>
Composing the lexer and parser, we finally obtain a function to read an
expression from a character string:
#let read_expr s = parse_expr(lexer(Stream.of_string s));;
val read_expr : string -> expression = <fun>
#read_expr "2*(x+y)";;
- : expression = Prod (Const 2, Sum (Var "x", Var "y"))
Standalone Caml programs
All examples given so far were executed under the interactive system. Caml
code can also be compiled separately and executed non-interactively using the
batch compilers ocamlc or ocamlopt. The source code must be put in a file
with extension .ml. It consists of a sequence of phrases, which will be
evaluated at runtime in their order of appearance in the source file. Unlike
in interactive mode, types and values are not printed automatically; the
program must call printing functions explicitly to produce some output. Here
is a sample standalone program to print Fibonacci numbers:
(* File fib.ml *)
let rec fib n =
if n < 2 then 1 else fib(n-1) + fib(n-2);;
let main () =
let arg = int_of_string Sys.argv.(1) in
print_int(fib arg);
print_newline();
exit 0;;
Chapter 1. The core language 18
main ();;
Sys.argv is an array of strings containing the command-line parameters.
Sys.argv.(1) is thus the first command-line parameter. The program above is
compiled and executed with the following shell commands:
$ ocamlc -o fib fib.ml
$ ./fib 10
89
$ ./fib 20
10946
Chapter 2
Objects in Caml
(Chapter written by Jerome Vouillon and Didier Remy)
This chapter gives an overview of the object-oriented features of Objective
Caml.
Classes and objects
The class point has one instance variable x and two methods get_x and move.
The initial value of the instance variable is given here by the class
parameter x_init. The variable x is declared mutable, so the method move can
change its value.
#class point x_init =
# val mutable x = x_init
# method get_x = x
# method move d = x <- x + d
#end;;
class point (int) =
val mutable x : int
method get_x : int
method move : int -> unit
end
We now create a new point p, giving the initialization argument 7.
#let p = new point 7;;
val p : point = <obj>
Note that the type of p is point. This is an abbreviation automatically
defined by the class definition above. It stands for the object type <get_x :
int; move : int -> unit>, listing the methods of class point along with their
types.
Let us apply some methods to p:
#p#get_x;;
- : int = 7
#p#move 3;;
- : unit = ()
#p#get_x;;
- : int = 10
19
Chapter 2. Objects in Caml 20
The library function Oo.copy makes a shallow copy of an object. Its type is
< .. > as 'a -> 'a (which is parsed as (< .. > as 'a) -> 'a). The keyword as
in that type binds the type variable 'a to the object type < .. >. Therefore,
Oo.copy takes an object with any methods (represented by the ellipsis), and
returns an object of the same type. The type of Oo.copy is different from
type < .. > -> < .. > as each ellipsis represents a different set of methods.
Ellipsis actually behaves as a type variable.
#let q = Oo.copy p;;
val q : point = <obj>
#q#move 7; (p#get_x, q#get_x);;
- : int * int = 10, 17
Objects can be compared using the generic comparison functions (=, <, ...).
Two objects are equal if and only if they are physically equal. In
particular, an object and its copy are not equal.
#let q = Oo.copy p;;
val q : point = <obj>
#p = q, p = p;;
- : bool * bool = false, true
Inheritance
We now define a new class colored_point. This class inherits from class
point. So, it has all the instance variable and all the methods of point,
plus a new instance variable c and a new method color.
#class colored_point x (c : string) =
# inherit point x
# val c = c
# method color = c
#end;;
class colored_point (int) (string) =
val c : string
val mutable x : int
method color : string
method get_x : int
method move : int -> unit
end
#let p' = new colored_point 5 "red";;
val p' : colored_point = <obj>
#p'#get_x, p'#color;;
- : int * string = 5, "red"
A point and a colored point have incompatible types: a point has no method
color. However, the function get_x below is a generic function applying
method get_x to any object p that has this method (and possibly some others,
which are represented by an ellipsis in the type). Thus, it applies to both
points and colored points.
#let get_succ_x p = p#get_x + 1;;
val get_succ_x : < get_x : int; .. > -> int = <fun>
Chapter 2. Objects in Caml 21
#get_succ_x p + get_succ_x p';;
- : int = 17
Methods need not be declared previously, as shown by the example:
#let set_x p = p#set_x;;
val set_x : < set_x : 'a; .. > -> 'a = <fun>
#let incr p = set_x p (get_succ_x p);;
val incr : < get_x : int; set_x : int -> 'a; .. > -> 'a = <fun>
Parameterized classes
Reference cells can also be implemented as objects. The naive definition
fails to typecheck:
#class ref x_init =
# val mutable x = x_init
# method get = x
# method set y = x <- y
#end;;
Characters 5-85:
The type variable 'a is not bound in implicit type definition
ref = < get : 'a; set : 'a -> unit >
It should be captured by a class type parameter
The reason is that at least one of the methods has a polymorphic type (here,
the type of the value stored in the reference cell), thus the class should be
parametric. A monomorphic instance of the class could be defined by:
#class ref (x_init:int) =
# val mutable x = x_init
# method get = x
# method set y = x <- y
#end;;
class ref (int) =
val mutable x : int
method get : int
method set : int -> unit
end
A class for polymorphic references must explicitly list the type parameters in
its declaration. The type parameters must also be bound somewhere in the
class body by a type constraint.
#class 'a ref x_init =
# val mutable x = (x_init : 'a)
# method get = x
# method set y = x <- y
#end;;
class 'a ref ('a) =
val mutable x : 'a
method get : 'a
method set : 'a -> unit
end
#let r = new ref 1 in r#set 2; (r#get);;
Chapter 2. Objects in Caml 22
- : int = 2
The type parameter in the declaration may actually be constrained in the body
of the class definition. In the class type, the actual value of the type
parameter is displayed in the constraint clause.
#class 'a ref (x_init:'a) =
# val mutable x = x_init + 1
# method get = x
# method set y = x <- y
#end;;
class 'a ref ('a) =
constraint 'a = int
val mutable x : int
method get : int
method set : int -> unit
end
Let us consider a more realistic example. We put an additional type
constraint in method move, since no free variables must remain uncaptured by a
type parameter.
#class 'a circle (c : 'a) =
# val mutable center = c
# method center = center
# method set_center c = center <- c
# method move = (center#move : int -> unit)
#end;;
class 'a circle ('a) =
constraint 'a = < move : int -> unit; .. >
val mutable center : 'a
method center : 'a
method move : int -> unit
method set_center : 'a -> unit
end
An alternate definition of circle, using a constraint clause in the class
definition, is shown below. The type #point used below in the constraint
clause is an abbreviation produced by the definition of class point. This
abbreviation unifies with the type of any object belonging to a subclass of
class point. It actually expands to < get_x : int; move : int -> unit; .. >.
This leads to the following alternate definition of circle, which has slightly
stronger constraints on its argument, as we now expect center to have a method
get_x.
#class 'a circle (c : 'a) =
# constraint 'a = #point
# val mutable center = c
# method center = center
# method set_center c = center <- c
# method move = center#move
#end;;
class 'a circle ('a) =
constraint 'a = #point
val mutable center : 'a
method center : 'a
method move : int -> unit
method set_center : 'a -> unit
Chapter 2. Objects in Caml 23
end
The class colored_circle is a specialized version of class circle which
requires the type of the center to unify with #colored_point, and adds a
method color.
#class 'a colored_circle c =
# constraint 'a = #colored_point
# inherit ('a) circle c
# method color = center#color
#end;;
class 'a colored_circle ('a) =
constraint 'a = #colored_point
val mutable center : 'a
method center : 'a
method color : string
method move : int -> unit
method set_center : 'a -> unit
end
Reference to self
A method can also send messages to the object that invoked the method. For
that, self must be explicitly bound, here to the variable s.
#class printable_point y as s =
# inherit point y
# method print = print_int s#get_x
#end;;
class printable_point (int) =
val mutable x : int
method get_x : int
method move : int -> unit
method print : unit
end
#let p = new printable_point 7;;
val p : printable_point = <obj>
#p#print;;
7- : unit = ()
The variable s is bound at the invocation of a method. In particular, if the
class printable_point is inherited, the variable s will correctly be bound to
an object of the subclass.
Multiple inheritance
Multiple inheritance is allowed. Only the last definition of a method (or of
an instance variable) is kept. Previous definitions of a method can be reused
by binding the related ancestor. Below, super is bound to the ancestor
printable_point. The name super is not actually a variable and can only be
used to select a method as in super#print.
#class printable_colored_point y c as self =
# inherit colored_point y c
Chapter 2. Objects in Caml 24
# inherit printable_point y as super
# method print =
# print_string "(";
# super#print;
# print_string ", ";
# print_string (self#color);
# print_string ")"
#end;;
class printable_colored_point (int) (string) =
val c : string
val mutable x : int
method color : string
method get_x : int
method move : int -> unit
method print : unit
end
#let p' = new printable_colored_point 7 "red";;
val p' : printable_colored_point = <obj>
#p'#print;;
(7, red)- : unit = ()
Non-mutable objects
It is possible to write a version of class point without assignments on the
instance variables. The construct {< ... >} returns a copy of ``self'' (that
is, the current object), possibly changing the value of some instance
variables.
#class functional_point y =
# val x = y
# method get_x = x
# method move d = {< x = x + d >}
#end;;
class functional_point (int) : 'a =
val x : int
method get_x : int
method move : int -> 'a
end
#let p = new functional_point 7;;
val p : functional_point = <obj>
#p#get_x;;
- : int = 7
#(p#move 3)#get_x;;
- : int = 10
#p#get_x;;
- : int = 7
Note that the type abbreviation functional_point is recursive, which can be
seen in the class type of functional_point: the type of self to 'a and 'a
appears inside the type of the move method.
Chapter 2. Objects in Caml 25
Virtual methods
The class comparable below is a template for classes with a binary method leq
of type 'a -> bool where the type variable 'a is bound to the type of self.
Since this class has a method declared but not defined, it must be flagged
virtual and cannot be instantiated (that is, no object of this class can be
created). It still defines abbreviations. In particular, #comparable expands
to < leq : 'a -> bool; .. > as 'a. We see here that the binder as also allows
to write recursive types.
#class virtual comparable () : 'a =
# virtual leq : 'a -> bool
#end;;
class virtual comparable (unit) : 'a = virtual leq : 'a -> bool end
We then define a subclass of comparable that wraps integers as comparable
objects. There is a type constraint on the class parameter x as the primitive
<= is a polymorphic comparison function in Objective Caml. The inherit clause
ensures that the type of objects of this class is an instance of #comparable.
#class int_comparable (x : int) =
# inherit comparable ()
# val x = x
# method x = x
# method leq p = x <= p#x
#end;;
class int_comparable (int) : 'a =
val x : int
method leq : 'a -> bool
method x : int
end
Objects of class int_comparable2 below can also modify the integer they
hold. The status of instance variable x is changed. It is now mutable. Note
that the type int_comparable2 is not a subtype of type int_comparable, as the
self type appears in contravariant position in the type of method leq.
#class int_comparable2 x =
# inherit int_comparable x
# val mutable x
# method set_x y = x <- y
#end;;
class int_comparable2 (int) : 'a =
val mutable x : int
method leq : 'a -> bool
method set_x : int -> unit
method x : int
end
The function min will return the minimum of any two objects whose type
unifies with #comparable. The type of min is not the same as #comparable ->
#comparable -> #comparable, as the abbreviation #comparable hides a type
variable (an ellipsis). Each occurrence of this abbreviation generates a new
variable.
#let min (x : #comparable) y =
# if x#leq y then x else y;;
val min : (#comparable as 'a) -> 'a -> 'a = <fun>
Chapter 2. Objects in Caml 26
This function can be applied to objects of type int_comparable or
int_comparable2.
#(min (new int_comparable 7) (new int_comparable 11))#x;;
- : int = 7
#(min (new int_comparable2 5) (new int_comparable2 3))#x;;
- : int = 3
Protected methods
Protected methods are methods that do not appear in object interfaces. They
can only be invoked from other methods of the same object.
#class restricted_point x_init as self =
# val mutable x = x_init
# method get_x = x
# method protected move d = x <- x + d
# method bump = self#move 1
#end;;
class restricted_point (int) =
val mutable x : int
method bump : unit
method get_x : int
method protected move : int -> unit
end
#let p = new restricted_point 0;;
val p : restricted_point = <obj>
#p#move 10;;
Characters 0-1:
This expression has no method move
#p#bump;;
- : unit = ()
Protected methods are inherited. They can be hidden by signature matching, as
described in the next section.
Class interfaces
Class interfaces are inferred from class definitions. They may also be
defined directly in interfaces of modules. For instance, the following is the
interface of a module defining class restricted_point.
#module type POINT = sig
# class restricted_point (int) =
# val mutable x : int
# method get_x : int
# method protected move : int -> unit
# method bump : unit
# end
#end;;
module type POINT =
sig
Chapter 2. Objects in Caml 27
class restricted_point (int) =
val mutable x : int
method bump : unit
method get_x : int
method protected move : int -> unit
end
end
#module Point : POINT = struct
# class restricted_point x = inherit restricted_point x end
#end;;
module Point : POINT
It is sometime necessary to restrict the interface of classes. Instance
variables and protected methods can be hidden by signature matching, as shown
below. However, public methods cannot be hidden.
#module type ABSPOINT = sig
# class restricted_point (int) =
# method get_x : int
# method bump : unit
# end
#end;;
module type ABSPOINT =
sig
class restricted_point (int) = method bump : unit method get_x : int end
end
#module Abspoint : ABSPOINT = Point;;
module Abspoint : ABSPOINT
Using coercions
Subtyping is never implicit. There are, however, two ways to perform
subtyping. The most general construction is fully explicit: both the domain
and the codomain of the type coercion must be given.
We have seen that points and colored points have incompatible types. For
instance, they cannot be mixed in the same list. However, a colored point can
be coerced to a point, hiding its color method:
#let colored_point_to_point cp = (cp : colored_point :> point);;
val colored_point_to_point : colored_point -> point = <fun>
#let p = new point 3 and q = new colored_point 4 "blue";;
val p : point = <obj>
val q : colored_point = <obj>
#let l = [p; (colored_point_to_point q)];;
val l : point list = [<obj>; <obj>]
An object of type t can be seen as an object of type t' only if t is a subtype
of t'. For instance, a point cannot be seen as a colored point.
#(p : point :> colored_point);;
Characters 0-28:
Type point = < get_x : int; move : int -> unit > is not a subtype of type
colored_point = < get_x : int; move : int -> unit; color : string >
Chapter 2. Objects in Caml 28
Indeed, backward coercions are unsafe, and should be combined with a type
case, possibly raising a runtime error. However, there is not such operation
available in the language.
Be aware that subtyping and inheritance are not related. Inheritance is a
syntactic relation between classes while subtyping is a semantic relation
between types. For instance, the class of colored points could have been
defined directly, without inheriting from the class of points; the type of
colored points would remain unchanged and thus still be a subtype of points.
Conversely, the class int_comparable inherits from class comparable, but type
int_comparable is not a subtype of comparable.
#function x -> (x : int_comparable :> comparable);;
Characters 14-48:
Type int_comparable = < leq : int_comparable -> bool; x : int >
is not a subtype of type comparable = < leq : comparable -> bool >
Type int_comparable -> bool is not a subtype of type comparable -> bool
Type comparable = < leq : comparable -> bool > is not a subtype of type
int_comparable = < leq : int_comparable -> bool; x : int >
Indeed, an object p of class int_comparable has a method leq that expects an
argument of type int_comparable since it accesses its x method. Considering p
of type comparable would allow to call method leq on p with an argument that
does not have a method x, which would be an error.
The domain of a coercion can usually be omitted. For instance, one can
define:
#let to_point cp = (cp :> point);;
val to_point : < get_x : int; move : int -> unit; .. > -> point = <fun>
In this case, the function colored_point_to_point is an instance of the
function to_point. This is not always true, however. The fully explicit
coercion is more precise and is sometimes unavoidable. Here is an example:
#class virtual c () = virtual m : c end;;
class virtual c (unit) = virtual m : c end
#class c' () as self =
# inherit c ()
# method m = (self :> c)
# method m' = 1
#end;;
Characters 51-55:
This expression cannot be coerced to type c = < m : c >; it has type
< m : c; m' : 'a; .. >
but is here used with type < m : 'b; m' : 'a; .. > as 'b
Type c = < m : c > is not compatible with type 'b
The type of the coercion to type c can be seen here:
#function x -> (x :> c);;
- : (< m : 'a; .. > as 'a) -> c = <fun>
As class c' inherits from class c, its method m must have type c. On the
other hand, in expression (self :> c) the type of self and the domain of the
coercion above must be unified. That is, the type of the method m in self
(i.e. c) is also the type of self. So, the type of self is c. This is a
contradiction, as the type of self has a method m', whereas type c does not.
The desired coercion of type <m : c;..> -> c can be obtained by using a
Chapter 2. Objects in Caml 29
fully explicit coercion:
#function x -> (x : #c :> c);;
- : #c -> c = <fun>
Thus one can define class c' as follows:
#class c' () as self =
# inherit c ()
# method m = (self : #c :> c)
# method m' = 1
#end;;
class c' (unit) = method m : c method m' : int end
An alternative is to define class c as follows (of course this definition is
not equivalent to the previous one):
#class virtual c () : 'a = virtual m : 'a end;;
class virtual c (unit) : 'a = virtual m : 'a end
Then, a coercion operator is not even required.
#class c' () as self =
# inherit c ()
# method m = self
# method m' = 1
#end;;
class c' (unit) : 'a = method m : 'a method m' : int end
Here, the simple coercion operator (e :> c) can be used to coerce an object
expression e from type c' to type c. Semi implicit coercions are actually
defined so as to work correctly with classes returning self.
#(new c' () :> c);;
- : c = <obj>
Another common problem may occur when one tries to define a coercion to a
class c inside the definition of class c. The problem is due to the type
abbreviation not being completely defined yet, and so its subtypes are not
clearly known. Then, a coercion (_ : #c :> c) is taken to be the identity
function, as in
#function x -> (x :> 'a);;
- : 'a -> 'a = <fun>
As a consequence, if the coercion is applied to self, as in the following
example, the type of self is unified with the closed type c. This constrains
the class to be closed:
#class c () as self = method m = (self : #c :> c) end;;
Characters 32-48:
Type #c = < m : 'a; .. > is not a subtype of type c = < m : 'b; .. >
Although declaring the class as closed will typecheck, this is not usually
what is desired.
#class closed c () as self = method m = (self : #c :> c) end;;
Characters 39-55:
Chapter 2. Objects in Caml 30
Type < m : 'b; .. > as 'a is not a subtype of type c = 'a
This problem can sometimes be avoided by first defining the abbreviation,
using a virtual class:
#class virtual c0 () = virtual m : c0 end;;
class virtual c0 (unit) = virtual m : c0 end
#class c () as self = method m = (self : #c0 :> c0) end;;
class c (unit) = method m : c0 end
The class c may be also declared to inherit from the virtual class c0, so as
to simultaneously enforce all methods of c to have the same type as the
methods of c0.
#class c () as self = inherit c0 () method m = (self : #c0 :> c0) end;;
class c (unit) = method m : c0 end
One could think of defining the type abbreviation directly:
#type c1 = <m : c1>;;
type c1 = < m : c1 >
However, the abbreviation #c1 cannot be defined this way (the abbreviation #c0
is defined from the class c0, not from the type c0), and should be expanded:
#class c () as self = method m = (self : <m : c1; ..> as 'a :> c1) end;;
class c (unit) = method m : c1 end
Recursive classes
Recursive classes can be used to define objects whose types are mutually
recursive.
#class window () =
# val mutable top_widget = (None : widget option)
# method top_widget = top_widget
#and widget (w : window) =
# val window = w
# method window = window
#end;;
class window (unit) =
val mutable top_widget : widget option
method top_widget : widget option
end
class widget (window) = val window : window method window : window end
Although their types are mutually recursive, the classes widget and window are
themselves independent.
Simple modules as classes
There is sometime an alternative between using modules or classes. Indeed,
there are situations when the two approaches are quite similar. For instance,
a stack can be straightforwardly implemented as a class:
Chapter 2. Objects in Caml 31
#exception Empty;;
exception Empty
#class 'a stack () =
# val mutable l = ([] : 'a list)
# method push x = l <- x::l
# method pop = match l with [] -> raise Empty | a::l' -> l <- l'; a
# method clear = l <- []
# method length = List.length l
#end;;
class 'a stack (unit) =
val mutable l : 'a list
method clear : unit
method length : int
method pop : 'a
method push : 'a -> unit
end
However, writing a method for iterating over a stack is more problematic. A
method fold would have type ('b -> 'a -> 'b) -> 'b -> 'b. Here 'a is the
parameter of the stack. The parameter 'b is not related to the class 'a stack
but to the argument that will be passed to the method fold. The intuition is
that method fold should be polymorphic, i.e. of type All ('a) ('b -> 'a ->
'b) -> 'b -> 'b, which is not currently possible. One possibility would be to
make 'b an extra parameter of class stack
#class ('a, 'b) stack2 () =
# inherit ('a) stack ()
# method fold f (x : 'b) = List.fold_left f x l
#end;;
class ('a, 'b) stack2 (unit) =
val mutable l : 'a list
method clear : unit
method fold : ('b -> 'a -> 'b) -> 'b -> 'b
method length : int
method pop : 'a
method push : 'a -> unit
end
However, method fold of a given object can only be applied to functions that
all have the same type:
#let s = new stack2 ();;
val s : ('_a, '_b) stack2 = <obj>
#s#fold (+) 0;;
- : int = 0
#s;;
- : (int, int) stack2 = <obj>
The best solution would be to make method fold polymorphic. However Ocaml
does not currently allow methods to be polymorphic.
Thus, the current solution is to leave the function fold outside of the
class.
#class 'a stack3 () =
# inherit ('a) stack ()
Chapter 2. Objects in Caml 32
# method iter f = List.iter (f : 'a -> unit) l
#end;;
class 'a stack3 (unit) =
val mutable l : 'a list
method clear : unit
method iter : ('a -> unit) -> unit
method length : int
method pop : 'a
method push : 'a -> unit
end
#let stack_fold (s : 'a #stack3) f x =
# let accu = ref x in
# s#iter (fun e -> accu := f !accu e);
# !accu;;
val stack_fold : 'a #stack3 -> ('b -> 'a -> 'b) -> 'b -> 'b = <fun>
Implementing sets leads to another difficulty. Indeed, the method union
needs to be able to access the internal representation of another object of
the same class. For that, a set class must have an additional method
returning this representation. However, this representation should not be
public. This result is obtained by making the type of the representation
abstract via a module signature constraint. From outside, the additional
method appears like a tag ensuring that an object belongs to class set.
#module type SET =
# sig
# type 'a tag
# class 'a c (unit) : 'b =
# method is_empty : bool
# method mem : 'a -> bool
# method add : 'a -> 'b
# method union : 'b -> 'b
# method iter : ('a -> unit) -> unit
# method tag : 'a tag
# end
# end;;
module type SET =
sig
type 'a tag
class 'a c (unit) : 'b =
method add : 'a -> 'b
method is_empty : bool
method iter : ('a -> unit) -> unit
method mem : 'a -> bool
method tag : 'a tag
method union : 'b -> 'b
end
end
#module Set : SET =
# struct
# let rec merge l1 l2 =
# match l1 with
# [] -> l2
# | h1 :: t1 ->
# match l2 with
# [] -> l1
Chapter 2. Objects in Caml 33
# | h2 :: t2 ->
# if h1 < h2 then h1 :: merge t1 l2
# else if h1 > h2 then h2 :: merge l1 t2
# else merge t1 l2
# type 'a tag = 'a list
# class 'a c () : 'b =
# val repr = ([] : 'a list)
# method is_empty = (repr = [])
# method mem x = List.exists ((=) x) repr
# method add x = {< repr = merge [x] repr >}
# method union (s : 'b) = {< repr = merge repr s#tag >}
# method iter (f : 'a -> unit) = List.iter f repr
# method tag = repr
# end
# end;;
module Set : SET
Chapter 3
The module system
This chapter introduces the module system of Objective Caml.
Structures
A primary motivation for modules is to package together related definitions
(such as the definitions of a data type and associated operations over that
type) and enforce a consistent naming scheme for these definitions. This
avoids running out of names or accidentally confusing names. Such a package
is called a structure and is introduced by the struct...end construct, which
contains an arbitrary sequence of definitions. The structure is usually given
a name with the module binding. Here is for instance a structure packaging
together a type of priority queues and their operations:
#module PrioQueue =
# struct
# type priority = int
# type 'a queue = Empty | Node of priority * 'a * 'a queue * 'a queue
# let empty = Empty
# let rec insert queue prio elt =
# match queue with
# Empty -> Node(prio, elt, Empty, Empty)
# | Node(p, e, left, right) ->
# if prio <= p
# then Node(prio, elt, insert right p e, left)
# else Node(p, e, insert right prio elt, left)
# exception Queue_is_empty
# let rec remove_top = function
# Empty -> raise Queue_is_empty
# | Node(prio, elt, left, Empty) -> left
# | Node(prio, elt, Empty, right) -> right
# | Node(prio, elt, (Node(lprio, lelt, _, _) as left),
# (Node(rprio, relt, _, _) as right)) ->
# if lprio <= rprio
# then Node(lprio, lelt, remove_top left, right)
# else Node(rprio, relt, left, remove_top right)
# let extract = function
# Empty -> raise Queue_is_empty
# | Node(prio, elt, _, _) as queue -> (prio, elt, remove_top queue)
# end;;
module PrioQueue :
sig
type priority = int
type 'a queue = | Empty | Node of priority * 'a * 'a queue * 'a queue
34
Chapter 3. The module system 35
val empty : 'a queue
val insert : 'a queue -> priority -> 'a -> 'a queue
exception Queue_is_empty
val remove_top : 'a queue -> 'a queue
val extract : 'a queue -> priority * 'a * 'a queue
end
Outside the structure, its components can be referred to using the ``dot
notation'', that is, identifiers qualified by a structure name. For instance,
PrioQueue.insert in a value context is the function insert defined inside the
structure PrioQueue. Similarly, PrioQueue.queue in a type context is the type
queue defined in PrioQueue.
#PrioQueue.insert PrioQueue.empty 1 "hello";;
- : string PrioQueue.queue =
PrioQueue.Node (1, "hello", PrioQueue.Empty, PrioQueue.Empty)
Signatures
Signatures are interfaces for structures. A signature specifies which
components of a structure are accessible from the outside, and with which
type. It can be used to hide some components of a structure (e.g. local
function definitions) or export some components with a restricted type. For
instance, the signature below specifies the three priority queue operations
empty, insert and extract, but not the auxiliary function remove_top.
Similarly, it makes the queue type abstract (by not providing its actual
representation as a concrete type).
#module type PRIOQUEUE =
# sig
# type priority = int (* still concrete *)
# type 'a queue (* now abstract *)
# val empty : 'a queue
# val insert : 'a queue -> int -> 'a -> 'a queue
# val extract : 'a queue -> int * 'a * 'a queue
# exception Queue_is_empty
# end;;
module type PRIOQUEUE =
sig
type priority = int
type 'a queue
val empty : 'a queue
val insert : 'a queue -> int -> 'a -> 'a queue
val extract : 'a queue -> int * 'a * 'a queue
exception Queue_is_empty
end
Restricting the PrioQueue structure by this signature results in another view
of the PrioQueue structure where the remove_top function is not accessible and
the actual representation of priority queues is hidden:
#module AbstractPrioQueue = (PrioQueue : PRIOQUEUE);;
module AbstractPrioQueue : PRIOQUEUE
#AbstractPrioQueue.remove_top;;
Characters 0-28:
Unbound value AbstractPrioQueue.remove_top
Chapter 3. The module system 36
#AbstractPrioQueue.insert AbstractPrioQueue.empty 1 "hello";;
- : string AbstractPrioQueue.queue = <abstr>
The restriction can also be performed during the definition of the structure,
as in
module PrioQueue = (struct ... end : PRIOQUEUE);;
An alternate syntax is provided for the above:
module PrioQueue : PRIOQUEUE = struct ... end;;
Functors
Functors are ``functions'' from structures to structures. They are used to
express parameterized structures: a structure A parameterized by a structure
B is simply a functor F with a formal parameter B (along with the expected
signature for B) which returns the actual structure A itself. The functor F
can then be applied to one or several implementations B1 ...Bn of B, yielding
the corresponding structures A1 ...An.
For instance, here is a structure implementing sets as sorted lists,
parameterized by a structure providing the type of the set elements and an
ordering function over this type (used to keep the sets sorted):
#type comparison = Less | Equal | Greater;;
type comparison = | Less | Equal | Greater
#module type ORDERED_TYPE =
# sig
# type t
# val cmp: t -> t -> comparison
# end;;
module type ORDERED_TYPE = sig type t val cmp : t -> t -> comparison end
#module Set =
# functor (Elt: ORDERED_TYPE) ->
# struct
# type element = Elt.t
# type set = element list
# let empty = []
# let rec add x s =
# match s with
# [] -> [x]
# | hd::tl ->
# match Elt.cmp x hd with
# Equal -> s (* x is already in s *)
# | Less -> x :: s (* x is smaller than all elements of s *)
# | Greater -> hd :: add x tl
# let rec member x s =
# match s with
# [] -> false
# | hd::tl ->
# match Elt.cmp x hd with
# Equal -> true (* x belongs to s *)
# | Less -> false (* x is smaller than all elements of s *)
# | Greater -> member x tl
# end;;
module Set :
Chapter 3. The module system 37
functor(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set = element list
val empty : 'a list
val add : Elt.t -> Elt.t list -> Elt.t list
val member : Elt.t -> Elt.t list -> bool
end
By applying the Set functor to a structure implementing an ordered type, we
obtain set operations for this type:
#module OrderedString =
# struct
# type t = string
# let cmp x y = if x = y then Equal else if x < y then Less else Greater
# end;;
module OrderedString :
sig type t = string val cmp : 'a -> 'a -> comparison end
#module StringSet = Set(OrderedString);;
module StringSet :
sig
type element = OrderedString.t
type set = element list
val empty : 'a list
val add : OrderedString.t -> OrderedString.t list -> OrderedString.t list
val member : OrderedString.t -> OrderedString.t list -> bool
end
#StringSet.member "bar" (StringSet.add "foo" StringSet.empty);;
- : bool = false
Functors and type abstraction
As in the PrioQueue example, it would be good style to hide the actual
implementation of the type set, so that users of the structure will not rely
on sets being lists, and we can switch later to another, more efficient
representation of sets without breaking their code. This can be achieved by
restricting Set by a suitable functor signature:
#module type SETFUNCTOR =
# functor (Elt: ORDERED_TYPE) ->
# sig
# type element = Elt.t (* concrete *)
# type set (* abstract *)
# val empty : set
# val add : element -> set -> set
# val member : element -> set -> bool
# end;;
module type SETFUNCTOR =
functor(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set
val empty : set
val add : element -> set -> set
Chapter 3. The module system 38
val member : element -> set -> bool
end
#module AbstractSet = (Set : SETFUNCTOR);;
module AbstractSet : SETFUNCTOR
#module AbstractStringSet = AbstractSet(OrderedString);;
module AbstractStringSet :
sig
type element = OrderedString.t
type set = AbstractSet(OrderedString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
#AbstractStringSet.add "gee" AbstractStringSet.empty;;
- : AbstractStringSet.set = <abstr>
In an attempt to write the type constraint above more elegantly, one may
wish to name the signature of the structure returned by the functor, then use
that signature in the constraint:
#module type SET =
# sig
# type element
# type set
# val empty : set
# val add : element -> set -> set
# val member : element -> set -> bool
# end;;
module type SET =
sig
type element
type set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
#module WrongSet = (Set : functor(Elt: ORDERED_TYPE) -> SET);;
module WrongSet : functor(Elt : ORDERED_TYPE) -> SET
#module WrongStringSet = WrongSet(OrderedString);;
module WrongStringSet :
sig
type element = WrongSet(OrderedString).element
type set = WrongSet(OrderedString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
#WrongStringSet.add "gee" WrongStringSet.empty;;
Characters 19-24:
This expression has type string but is here used with type
WrongStringSet.element = WrongSet(OrderedString).element
Chapter 3. The module system 39
The problem here is that SET specifies the type element abstractly, so that
the type equality between element in the result of the functor and t in its
argument is forgotten. Consequently, WrongStringSet.element is not the same
type as string, and the operations of WrongStringSet cannot be applied to
strings. As demonstrated above, it is important that the type element in the
signature SET be declared equal to Elt.t; unfortunately, this is impossible
above since SET is defined in a context where Elt does not exist. To overcome
this difficulty, Objective Caml provides a with type construct over signatures
that allows to enrich a signature with extra type equalities:
#module AbstractSet =
# (Set : functor(Elt: ORDERED_TYPE) -> (SET with type element = Elt.t));;
module AbstractSet :
functor(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
As in the case of simple structures, an alternate syntax is provided for
defining functors and restricting their result:
module AbstractSet(Elt: ORDERED_TYPE) : (SET with type element = Elt.t) =
struct ... end;;
Abstracting a type component in a functor result is a powerful technique
that provides a high degree of type safety, as we now illustrate. Consider an
ordering over character strings that is different from the standard ordering
implemented in the OrderedString structure. For instance, we compare strings
without distinguishing upper and lower case.
#module NoCaseString =
# struct
# type t = string
# let cmp s1 s2 =
# OrderedString.cmp (String.lowercase s1) (String.lowercase s2)
# end;;
module NoCaseString :
sig type t = string val cmp : string -> string -> comparison end
#module NoCaseStringSet = AbstractSet(NoCaseString);;
module NoCaseStringSet :
sig
type element = NoCaseString.t
type set = AbstractSet(NoCaseString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
#NoCaseStringSet.add "FOO" AbstractStringSet.empty;;
Characters 26-49:
This expression has type
AbstractStringSet.set = AbstractSet(OrderedString).set
but is here used with type
Chapter 3. The module system 40
NoCaseStringSet.set = AbstractSet(NoCaseString).set
Notice that the two types AbstractStringSet.set and NoCaseStringSet.set are
not compatible, and values of these two types do not match. This is the
correct behavior: even though both set types contain elements of the same
type (strings), both are built upon different orderings of that type, and
different invariants need to be maintained by the operations (being strictly
increasing for the standard ordering and for the case-insensitive ordering).
Applying operations from AbstractStringSet to values of type
NoCaseStringSet.set could give incorrect results, or build lists that violate
the invariants of NoCaseStringSet.
Modules and separate compilation
All examples of modules so far have been given in the context of the
interactive system. However, modules are most useful for large,
batch-compiled programs. For these programs, it is a practical necessity to
split the source into several files, called compilation units, that can be
compiled separately, thus minimizing recompilation after changes.
In Objective Caml, compilation units are special cases of structures and
signatures, and the relationship between the units can be explained easily in
terms of the module system. A compilation unit a comprises two files:
- the implementation file a.ml, which contains a sequence of definitions,
analogous to the inside of a struct...end construct;
- the interface file a.mli, which contains a sequence of specifications,
analogous to the inside of a sig...end construct.
Both files define a structure named A (same name as the base name a of the two
files, with the first letter capitalized), as if the following definition was
entered at top-level:
module A: sig (* contents of file a.mli *) end
= struct (* contents of file a.ml *) end;;
The files defining the compilation units can be compiled separately using the
ocaml -c command (the -c option means ``compile only, do not try to link'');
this produces compiled interface files (with extension .cmi) and compiled
object code files (with extension .cmo). When all units have been compiled,
their .cmo files are linked together using the ocaml command. For instance,
the following commands compile and link a program composed of two compilation
units aux and main:
$ ocamlc -c aux.mli # produces aux.cmi
$ ocamlc -c aux.ml # produces aux.cmo
$ ocamlc -c main.mli # produces main.cmi
$ ocamlc -c main.ml # produces main.cmo
$ ocamlc -o theprogram aux.cmo main.cmo
The program behaves exactly as if the following phrases were entered at
top-level:
module Aux: sig (* contents of aux.mli *) end
= struct (* contents of aux.ml *) end;;
module Main: sig (* contents of main.mli *) end
= struct (* contents of main.ml *) end;;
Chapter 3. The module system 41
In particular, Main can refer to Aux: the definitions and declarations
contained in main.ml and main.mli can refer to definition in aux.ml, using the
Aux.ident notation, provided these definitions are exported in aux.mli.
The order in which the .cmo files are given to ocaml during the linking
phase determines the order in which the module definitions occur. Hence, in
the example above, Aux appears first and Main can refer to it, but Aux cannot
refer to Main.
Notice that only top-level structures can be mapped to separately-compiled
files, but not functors nor module types. However, all module-class objects
can appear as components of a structure, so the solution is to put the functor
or module type inside a structure, which can then be mapped to a file.
Part II
The Objective Caml language
42
Chapter 4
The Objective Caml language
Foreword
This document is intended as a reference manual for the Objective Caml
language. It lists the language constructs, and gives their precise syntax
and informal semantics. It is by no means a tutorial introduction to the
language: there is not a single example. A good working knowledge of Caml is
assumed.
No attempt has been made at mathematical rigor: words are employed with
their intuitive meaning, without further definition. As a consequence, the
typing rules have been left out, by lack of the mathematical framework
required to express them, while they are definitely part of a full formal
definition of the language.
Notations
The syntax of the language is given in BNF-like notation. Terminal symbols
are set in typewriter font (like this). Non-terminal symbols are set in
italic font (like that). Square brackets [...] denote optional components.
Curly brackets {...} denotes zero, one or several repetitions of the enclosed
components. Curly bracket with a trailing plus sign {...}+ denote one or
several repetitions of the enclosed components. Parentheses (...) denote
grouping.
4.1 Lexical conventions
Blanks
The following characters are considered as blanks: space, newline, horizontal
tabulation, carriage return, line feed and form feed. Blanks are ignored, but
they separate adjacent identifiers, literals and keywords that would otherwise
be confused as one single identifier, literal or keyword.
Comments
Comments are introduced by the two characters (*, with no intervening blanks,
and terminated by the characters *), with no intervening blanks. Comments are
treated as blank characters. Comments do not occur inside string or character
literals. Nested comments are handled correctly.
43
Chapter 4. The Objective Caml language 44
Identifiers
ident ::= (letter | _) {letter | 0...9 | _ | '}
letter ::= A...Z | a...z
Identifiers are sequences of letters, digits, _ (the underscore character),
and ' (the single quote), starting with a letter or an underscore. Letters
contain at least the 52 lowercase and uppercase letters from the ASCII set.
The current implementation also recognizes as letters all accented characters
from the ISO 8859-1 (``ISO Latin 1'') set, and also allows an underscore _ as
the first character of an identifier. All characters in an identifier are
meaningful. The current implementation places no limits on the number of
characters of an identifier.
Integer literals
integer-literal ::= [-] {0...9}+
| [-] (0x | 0X) {0...9 | A...F | a...f}+
| [-] (0o | 0O) {0...7}+
| [-] (0b | 0B) {0...1}+
An integer literal is a sequence of one or more digits, optionally preceded
by a minus sign. By default, integer literals are in decimal (radix 10). The
following prefixes select a different radix:
--------------------------------
|Prefix|Radix |
--------------------------------
|0x, 0X|hexadecimal (radix 16) |
|0o, 0O|octal (radix 8) |
|0b, 0B|binary (radix 2) |
--------------------------------
(The initial 0 is the digit zero; the O for octal is the letter O.) The
interpretation of integer literals that fall outside the range of
representable integer values is undefined.
Floating-point literals
float-literal ::= [-] {0...9}+ [. {0...9}] [(e | E) [+ | -] {0...9}+]
Floating-point decimals consist in an integer part, a decimal part and an
exponent part. The integer part is a sequence of one or more digits,
optionally preceded by a minus sign. The decimal part is a decimal point
followed by zero, one or more digits. The exponent part is the character e or
E followed by an optional + or - sign, followed by one or more digits. The
decimal part or the exponent part can be omitted, but not both to avoid
ambiguity with integer literals. The interpretation of floating-point
literals that fall outside the range of representable floating-point values is
undefined.
Character literals
char-literal ::= ' regular-char '
| ' \ (\ | ' | n | t | b | r) '
| ' \ (0...9) (0...9) (0...9) '
Character literals are delimited by ' (single quote) characters. The two
single quotes enclose either one character different from ' and \, or one of
the escape sequences below:
Chapter 4. The Objective Caml language 45
--------------------------------------------------------
|Sequence|Character denoted |
--------------------------------------------------------
|\\ |backslash (\) |
|\' |single quote (') |
|\n |newline (LF) |
|\r |return (CR) |
|\t |horizontal tabulation (TAB) |
|\b |backspace (BS) |
|\ddd |the character with ASCII code ddd in decimal |
--------------------------------------------------------
String literals
string-literal ::= " {string-character} "
string-character ::= regular-char
| \ (\ | " | n | t | b | r)
| \ (0...9) (0...9) (0...9)
String literals are delimited by " (double quote) characters. The two
double quotes enclose a sequence of either characters different from " and \,
or escape sequences from the table below:
--------------------------------------------------------
|Sequence|Character denoted |
--------------------------------------------------------
|\\ |backslash (\) |
|\" |double quote (") |
|\n |newline (LF) |
|\r |return (CR) |
|\t |horizontal tabulation (TAB) |
|\b |backspace (BS) |
|\ddd |the character with ASCII code ddd in decimal |
--------------------------------------------------------
The current implementation places no restrictions on the length of string
literals.
Prefix and infix symbols
infix-symbol ::= (= | < | > | @ | ^ | | | & | + | - | * | / | $ | %) {operator-char}
prefix-symbol ::= (! | ? | ~) {operator-char}
operator-char ::= ! | $ | % | & | * | + | - | . | / | : | < | = | > | ? | @ | ^ | | | ~
Sequences of ``operator characters'', such as <=> or !!, are read as a
single token from the infix-symbol or prefix-symbol class. These symbols are
parsed as prefix and infix operators inside expressions, but otherwise behave
much as identifiers.
Keywords
The identifiers below are reserved as keywords, and cannot be employed
otherwise:
and as assert asr begin class
closed constraint do done downto else
end exception external false for fun
function functor if in include inherit
land lazy let lor lsl lsr
lxor match method mod module mutable
new of open or parser private
rec sig struct then to true
Chapter 4. The Objective Caml language 46
try type val virtual when while
with
The following character sequences are also keywords:
# & ' ( ) * , -> ?
. .. .( .[ : :: := ; ;;
<- = [ [| [< {< ] |] >]
>} _ { | }
Ambiguities
Lexical ambiguities are resolved according to the ``longest match'' rule:
when a character sequence can be decomposed into two tokens in several
different ways, the decomposition retained is the one with the longest first
token.
4.2 Values
This section describes the kinds of values that are manipulated by Caml Light
programs.
4.2.1 Base values
Integer numbers
30 30
Integer values are integer numbers from -2 to 2 -1, that is -1073741824
to 1073741823. The implementation may support a wider range of integer
values: on 64-bit platforms, the current implementation supports integers
62 62
ranging from -2 to 2 -1.
Floating-point numbers
Floating-point values are numbers in floating-point representation. The
current implementation uses double-precision floating-point numbers conforming
to the IEEE 754 standard, with 53 bits of mantissa and an exponent ranging
from -1022 to 1023.
Characters
Character values are represented as 8-bit integers between 0 and 255.
Character codes between 0 and 127 are interpreted following the ASCII
standard. The current implementation interprets character codes between 128
and 255 following the ISO 8859-1 standard.
Character strings
String values are finite sequences of characters. The current implementation
24
supports strings containing up to 2 -6 characters (16777210 characters).
Chapter 4. The Objective Caml language 47
4.2.2 Tuples
Tuples of values are written (v1,...,vn), standing for the n-tuple of values
22
v1 to vn. The current implementation supports tuple of up to 2 -1 elements
(4194303 elements).
4.2.3 Records
Record values are labeled tuples of values. The record value written
{label1=v1 ;...;labeln =vn} associates the value vi to the record label
labeli, for i=1...n. The current implementation supports records with up to
22
2 -1 fields (4194303 fields).
4.2.4 Arrays
Arrays are finite, variable-sized sequences of values of the same type. The
22
current implementation supports arrays containing to 2 -1 elements (4194303
elements).
4.2.5 Variant values
Variant values are either a constant constructor, or a pair of a non-constant
constructor and a value. The former case is written cconstr; the latter case
is written ncconstr(v), where v is said to be the argument of the non-constant
constructor ncconstr.
The following constants are treated like built-in constant constructors:
------------------------------
|Constant|Constructor |
------------------------------
|false |the boolean false |
|true |the boolean true |
|() |the ``unit'' value |
|[] |the empty list |
------------------------------
The current implementation limits the number of distinct constructors in a
given variant type to at most 249.
4.2.6 Functions
Functional values are mappings from values to values.
4.2.7 Objects
Objects are composed of a hidden internal state which is a record of instance
variables, and a set of methods for accessing and modifying these variables.
The structure of an object is described by the toplevel class that created it.
4.3 Names
Identifiers are used to give names to several classes of language objects and
refer to these objects by name later:
- value names (syntactic class value-name),
Chapter 4. The Objective Caml language 48
- value constructors (constant -- class cconstr-name -- or non-constant --
class ncconstr-name),
- type constructors (typeconstr-name),
- record labels (label-name),
- class names (class-name),
- method names (method-name),
- instance variable names (inst-var-name),
- module names (module-name),
- module type names (modtype-name).
These nine name spaces are distinguished both by the context and by the
capitalization of the identifier: whether the first letter of the identifier
is in lowercase (written lowercase-ident below) or in uppercase (written
capitalized-ident). Underscore is considered a lowercase letter for this
purpose.
Naming objects
value-name ::= lowercase-ident
| ( operator-name )
operator-name ::= prefix-symbol | infix-symbol | * | = | or | & | :=
cconstr-name ::= capitalized-ident
| false
| true
| [ ]
| ( )
ncconstr-name ::= capitalized-ident
| ::
typeconstr-name ::= lowercase-ident
label-name ::= lowercase-ident
module-name ::= capitalized-ident
modtype-name ::= ident
class-name ::= lowercase-ident
inst-var-name ::= lowercase-ident
method-name ::= lowercase-ident
As shown above, prefix and infix symbols as well as some keywords can be used
as value names, provided they are written between parentheses. Keywords such
as '::' and 'false' are also constructor names. The capitalization rules are
summarized in the table below.
-----------------------------------------
|Name space |Case of first letter |
-----------------------------------------
|Values |lowercase |
|Constructors |uppercase |
|Type constructors |lowercase |
|Record labels |lowercase |
|Classes |lowercase |
|Methods |lowercase |
|Modules |uppercase |
|Module types |any |
-----------------------------------------
Chapter 4. The Objective Caml language 49
Referring to named objects
value-path ::= value-name
| module-path . lowercase-ident
cconstr ::= cconstr-name
| module-path . capitalized-ident
ncconstr ::= ncconstr-name
| module-path . capitalized-ident
typeconstr ::= typeconstr-name
| extended-module-path . lowercase-ident
label ::= label-name
| module-path . lowercase-ident
module-path ::= module-name
| module-path . capitalized-ident
extended-module-path ::= module-name
| extended-module-path . capitalized-ident
| extended-module-path ( extended-module-path )
modtype-path ::= modtype-name
| extended-module-path . ident
class-path ::= class-name
| module-path . lowercase-ident
A named object can be referred to either by its name (following the usual
static scoping rules for names) or by an access path prefix . name, where
prefix designates a module and name is the name of an object defined in that
module. The first component of the path, prefix, is either a simple module
name or an access path name1 . name2..., in case the defining module is
itself nested inside other modules. For referring to type constructors or
module types, the prefix can also contain simple functor applications (as in
the syntactic class extended-module-path above), in case the defining module
is the result of a functor application.
Instance variable names and method names need not be qualified: the former
are local to a class while the latter are global labels.
4.4 Type expressions
typexpr ::= ' ident
| ( typexpr )
| typexpr -> typexpr
| typexpr {* typexpr}+
| typeconstr
| typexpr typeconstr
| ( typexpr {, typexpr} ) typeconstr
| typexpr as ' ident
| < [..] >
| < method-type {; method-type} [; ..] >
| # class-path
| typexpr # class-path
| ( typexpr {, typexpr}) # class-path
method-type ::= method-name : typexpr
The table below shows the relative precedences and associativity of
operators and non-closed type constructions. The constructions with higher
precedences come first.
Chapter 4. The Objective Caml language 50
---------------------------------------------
|Operator |Associativity |
---------------------------------------------
|Type constructor application |-- |
|* |-- |
|-> |right |
|as |-- |
---------------------------------------------
Type expressions denote types in definitions of data types as well as in
type constraints over patterns and expressions.
Type variables
The type expression ' ident stands for the type variable named ident. In data
type definitions, type variables are names for the data type parameters. In
type constraints, they represent unspecified types that can be instantiated by
any type to satisfy the type constraint.
Parenthesized types
The type expression ( typexpr ) denotes the same type as typexpr.
Function types
The type expression typexpr1 -> typexpr2 denotes the type of functions
mapping arguments of type typexpr1 to results of type typexpr2.
Tuple types
The type expression typexpr1 *...* typexprn denotes the type of tuples whose
elements belong to types typexpr1,...typexprn respectively.
Constructed types
Type constructors with no parameter, as in typeconstr, are type expressions.
The type expression typexpr typeconstr, where typeconstr is a type
constructor with one parameter, denotes the application of the unary type
constructor typeconstr to the type typexpr.
The type expression (typexpr1,...,typexprn) typeconstr, where typeconstr is
a type constructor with n parameters, denotes the application of the n-ary
type constructor typeconstr to the types typexpr1 through typexprn.
Recursive types
The type expression typexpr as ' ident denotes the same type as typexpr, and
also binds the type variable ident to type typexpr both in typexpr and in the
remaining part of the type. If the type variable ident actually occurs in
typexpr, a recursive type is created. Recursive types are only allowed when
any recursion crosses an object type.
Object types
An object type < method-type {; method-type} > is a record of method types.
The type < method-type {; method-type} ; .. > is the type of an object with
methods and their associated types are described by
method-type1,...,method-typen, and possibly some other methods represented by
the ellipsis. This ellipsis actually is a special kind of type variable,
named row variable.
Chapter 4. The Objective Caml language 51
#-types
The type # class-path is a special kind of abbreviation. This abbreviation
unifies with the type of any object belonging to a subclass of class
class-path. It is handled in a special way as it usually hides a type
variable (an ellipsis, representing the methods that may be added in a
subclass). In particular, it vanishes when the ellipsis gets instantiated.
Each type expression # class-path defines a new type variable, so type
# class-path -> # class-path is usually not the same as type
# class-path as ' ident -> ' ident.
4.5 Constants
constant ::= integer-literal
| float-literal
| char-literal
| string-literal
| cconstr
The syntactic class of constants comprises literals from the four base types
(integers, floating-point numbers, characters, character strings), and
constant constructors.
4.6 Patterns
pattern ::= value-name
| _
| constant
| pattern as value-name
| ( pattern )
| ( pattern : typexpr )
| pattern | pattern
| ncconstr pattern
| pattern {, pattern}
| { label = pattern {; label = pattern} }
| [ pattern {; pattern} ]
| pattern :: pattern
The table below shows the relative precedences and associativity of
operators and non-closed pattern constructions. The constructions with higher
precedences come first.
----------------------------------------
|Operator |Associativity |
----------------------------------------
|Constructor application|-- |
|:: |right |
|, |-- |
|| |left |
|as |-- |
----------------------------------------
Patterns are templates that allow selecting data structures of a given
shape, and binding identifiers to components of the data structure. This
selection operation is called pattern matching; its outcome is either ``this
value does not match this pattern'', or ``this value matches this pattern,
resulting in the following bindings of names to values''.
Chapter 4. The Objective Caml language 52
Variable patterns
A pattern that consists in a value name matches any value, binding the name to
the value. The pattern _ also matches any value, but does not bind any name.
Patterns are linear: a variable cannot appear several times in a given
pattern. In particular, there is no way to test for equality between two
parts of a data structure using only a pattern (but when guards can be used
for this purpose).
Constant patterns
A pattern consisting in a constant matches the values that are equal to this
constant.
Alias patterns
The pattern pattern1 as value-name matches the same values as pattern1. If
the matching against pattern1 is successful, the name name is bound to the
matched value, in addition to the bindings performed by the matching against
pattern1.
Parenthesized patterns
The pattern ( pattern1 ) matches the same values as pattern1. A type
constraint can appear in a parenthesized pattern, as in
( pattern1 : typexpr ). This constraint forces the type of pattern1 to be
compatible with type.
``Or'' patterns
The pattern pattern1 | pattern2 represents the logical ``or'' of the two
patterns pattern1 and pattern2. A value matches pattern1 | pattern2 either
if it matches pattern1 or if it matches pattern2. The two sub-patterns
pattern1 and pattern2 must contain no identifiers. Hence no bindings are
returned by matching against an ``or'' pattern.
Variant patterns
The pattern ncconstr pattern1 matches all variants whose constructor is equal
to ncconstr, and whose argument matches pattern1.
The pattern pattern1 :: pattern2 matches non-empty lists whose heads match
pattern1, and whose tails match pattern2. This pattern behaves like
( :: ) ( pattern1 , pattern2 ).
The pattern [ pattern1 ;...; patternn ] matches lists of length n whose
elements match pattern1 ...patternn, respectively. This pattern behaves like
pattern1 ::...:: patternn :: [].
Tuple patterns
The pattern pattern1 ,..., patternn matches n-tuples whose components match
the patterns pattern1 through patternn. That is, the pattern matches the
tuple values (v1,...,vn) such that patterni matches vi for i =1, ...,n.
Record patterns
The pattern { label1 = pattern1 ;...; labeln = patternn } matches records
that define at least the labels label1 through labeln, and such that the
Chapter 4. The Objective Caml language 53
value associated to labeli match the pattern patterni, for i= 1,...,n. The
record value can define more labels than label1 ...labeln; the values
associated to these extra labels are not taken into account for matching.
4.7 Expressions
expr ::= value-path
| constant
| ( expr )
| begin expr end
| ( expr : typexpr )
| expr , expr {, expr}
| ncconstr expr
| expr :: expr
| [ expr {; expr} ]
| [| expr {; expr} |]
| { label = expr {; label = expr} }
| expr expr
| prefix-symbol expr
| expr infix-op expr
| expr . label
| expr . label <- expr
| expr .( expr )
| expr .( expr ) <- expr
| expr .[ expr ]
| expr .[ expr ] <- expr
| if expr then expr [else expr]
| while expr do expr done
| for ident = expr (to | downto) expr do expr done
| expr ; expr
| match expr with pattern-matching
| function pattern-matching
| fun multiple-matching
| try expr with pattern-matching
| let [rec] let-binding {and let-binding} in expr
| new class-path
| expr # method-name
| ( expr :> typexpr )
| ( expr : typexpr :> typexpr )
| {< inst-var-name = expr {; inst-var-name = expr} >}
pattern-matching ::= pattern [when expr] -> expr {| pattern [when expr] -> expr}
multiple-matching ::= {pattern}+ [when expr] -> expr
let-binding ::= pattern = expr
| value-name {pattern}+ [: typexpr] = expr
infix-op ::= infix-symbol
| * | = | or | &
The table below shows the relative precedences and associativity of
operators and non-closed constructions. The constructions with higher
precedence come first. For infix and prefix symbols, we write ``*...'' to
mean ``any symbol starting with *''.
Chapter 4. The Objective Caml language 54
----------------------------------------------------------------------
|Construction or operator |Associativity |
----------------------------------------------------------------------
|prefix-symbol |-- |
|. .( .[ |-- |
|function application |left |
|constructor application |-- |
|- -. (prefix) |-- |
|**... |right |
|*... /... %... mod |left |
|+... -... |left |
|:: |right |
|@ ^ |right |
|comparisons (= == < etc.), all other infix symbols|left |
|not |-- |
|& && |left |
|or || |left |
|, |-- |
|<- := |right |
|if |-- |
|; |right |
|let match fun function try |-- |
----------------------------------------------------------------------
4.7.1 Basic expressions
Constants
Expressions consisting in a constant evaluate to this constant.
Value paths
Expressions consisting in an access path evaluate to the value bound to this
path in the current evaluation environment. The path can be either a value
name or an access path to a value component of a module.
Parenthesized expressions
The expressions ( expr ) and begin expr end have the same value as expr. Both
constructs are semantically equivalent, but it is good style to use
begin...end inside control structures:
if ... then begin ... ; ... end else begin ... ; ... end
and (...) for the other grouping situations.
Parenthesized expressions can contain a type constraint, as in
( expr : type ). This constraint forces the type of expr to be compatible
with type.
Parenthesized expressions can also contain coercions
( expr [: type] :> type ) (see subsection 4.7.5 below).
Function application
Function application is denoted by juxtaposition of expressions. The
expression expr1 expr2...exprn evaluates the expressions expr1 to exprn. The
expression expr1 must evaluate to a functional value, which is then applied
to the values of expr2,...,exprn. The order in which the expressions
expr1,...,exprn are evaluated is not specified.
Chapter 4. The Objective Caml language 55
Function definition
Two syntactic forms are provided to define functions. The first form is
introduced by the keyword function:
function pattern1 -> expr1
| ...
| patternn -> exprn
This expression evaluates to a functional value with one argument. When this
function is applied to a value v, this value is matched against each pattern
pattern1 to patternn. If one of these matchings succeeds, that is, if the
value v matches the pattern patterni for some i, then the expression expri
associated to the selected pattern is evaluated, and its value becomes the
value of the function application. The evaluation of expri takes place in an
environment enriched by the bindings performed during the matching.
If several patterns match the argument v, the one that occurs first in the
function definition is selected. If none of the patterns matches the
argument, the exception Match_failure is raised.
The other form of function definition is introduced by the keyword fun:
fun pattern1...patternn -> expr
This expression is equivalent to:
function pattern1 ->...function patternn -> expr
That is, the fun expression above evaluates to a curried function with n
arguments: after applying this function n times to the values v1 ... vm, the
values will be matched in parallel against the patterns pattern1...patternn.
If the matching succeeds, the function returns the value of expr in an
environment enriched by the bindings performed during the matchings. If the
matching fails, the exception Match_failure is raised.
Guards in pattern-matchings
Cases of a pattern matching (in the function, fun, match and try constructs)
can include guard expressions, which are arbitrary boolean expressions that
must evaluate to true for the match case to be selected. Guards occur just
before the -> token and are introduced by the when keyword:
function pattern1 [whencond1] -> expr1
| ...
| patternn [whencondn] -> exprn
Matching proceeds as described before, except that if the value matches some
pattern patterni which has a guard condi, then the expression condi is
evaluated (in an environment enriched by the bindings performed during
matching). If condi evaluates to true, then expri is evaluated and its value
returned as the result of the matching, as usual. But if condi evaluates to
false, the matching is resumed against the patterns following patterni.
Local definitions
The let and let rec constructs bind value names locally. The construct
let pattern1 = expr1 and...and patternn = exprn in expr
Chapter 4. The Objective Caml language 56
evaluates expr1...exprn in some unspecified order, then matches their values
against the patterns pattern1...patternn. If the matchings succeed, expr is
evaluated in the environment enriched by the bindings performed during
matching, and the value of expr is returned as the value of the whole let
expression. If one of the matchings fails, the exception Match_failure is
raised.
An alternate syntax is provided to bind variables to functional values:
instead of writing
let ident = fun pattern1...patternm -> expr
in a let expression, one may instead write
let ident pattern1 ...patternm = expr
Recursive definitions of names are introduced by let rec:
let rec pattern1 = expr1 and...and patternn = exprn in expr
The only difference with the let construct described above is that the
bindings of names to values performed by the pattern-matching are considered
already performed when the expressions expr1 to exprn are evaluated. That
is, the expressions expr1 to exprn can reference identifiers that are bound
by one of the patterns pattern1,...,patternn, and expect them to have the
same value as in expr, the body of the let rec construct.
The recursive definition is guaranteed to behave as described above if the
expressions expr1 to exprn are function definitions (fun... or function...),
and the patterns pattern1...patternn are just value names, as in:
let rec name1 = fun...and...and namen = fun...in expr
This defines name1...namen as mutually recursive functions local to expr.
The behavior of other forms of let rec definitions is
implementation-dependent. The current implementation also supports a certain
class of recursive definitions of non-functional values, such as
let rec name1 = 1 :: name2 and name2 = 2 :: name1 in expr
which binds name1 to the cyclic list 1::2::1::2::..., and name2 to the cyclic
list 2::1::2::1::...Informally, the class of accepted definitions consists of
those definitions where the defined names occur only inside function bodies or
as argument to a data constructor.
4.7.2 Control structures
Sequence
The expression expr1 ; expr2 evaluates expr1 first, then expr2, and returns
the value of expr2.
Conditional
The expression if expr1 then expr2 else expr3 evaluates to the value of expr2
if expr1 evaluates to the boolean true, and to the value of expr3 if expr1
evaluates to the boolean false.
The else expr3 part can be omitted, in which case it defaults to else ().
Chapter 4. The Objective Caml language 57
Case expression
The expression
match expr
with pattern1 -> expr1
| ...
| patternn -> exprn
matches the value of expr against the patterns pattern1 to patternn. If the
matching against patterni succeeds, the associated expression expri is
evaluated, and its value becomes the value of the whole match expression. The
evaluation of expri takes place in an environment enriched by the bindings
performed during matching. If several patterns match the value of expr, the
one that occurs first in the match expression is selected. If none of the
patterns match the value of expr, the exception Match_failure is raised.
Boolean operators
The expression expr1 & expr2 evaluates to true if both expr1 and expr2
evaluate to true; otherwise, it evaluates to false. The first component,
expr1, is evaluated first. The second component, expr2, is not evaluated if
the first component evaluates to false. Hence, the expression expr1 & expr2
behaves exactly as
if expr1 then expr2 else false.
The expression expr1 or expr2 evaluates to true if one of expr1 and expr2
evaluates to true; otherwise, it evaluates to false. The first component,
expr1, is evaluated first. The second component, expr2, is not evaluated if
the first component evaluates to true. Hence, the expression expr1 or expr2
behaves exactly as
if expr1 then true else expr2.
Loops
The expression while expr1 do expr2 done repeatedly evaluates expr2 while
expr1 evaluates to true. The loop condition expr1 is evaluated and tested at
the beginning of each iteration. The whole while...done expression evaluates
to the unit value ().
The expression for name = expr1 to expr2 do expr3 done first evaluates the
expressions expr1 and expr2 (the boundaries) into integer values n and p.
Then, the loop body expr3 is repeatedly evaluated in an environment where
name is successively bound to the values n, n+1, ..., p -1, p. The loop
body is never evaluated if n>p.
The expression for name = expr1 downto expr2 do expr3 done evaluates
similarly, except that name is successively bound to the values n, n-1, ...,
p+1, p. The loop body is never evaluated if n <p.
In both cases, the whole for expression evaluates to the unit value ().
Chapter 4. The Objective Caml language 58
Exception handling
The expression
try expr
with pattern1 -> expr1
| ...
| patternn -> exprn
evaluates the expression expr and returns its value if the evaluation of expr
does not raise any exception. If the evaluation of expr raises an exception,
the exception value is matched against the patterns pattern1 to patternn. If
the matching against patterni succeeds, the associated expression expri is
evaluated, and its value becomes the value of the whole try expression. The
evaluation of expri takes place in an environment enriched by the bindings
performed during matching. If several patterns match the value of expr, the
one that occurs first in the try expression is selected. If none of the
patterns matches the value of expr, the exception value is raised again,
thereby transparently ``passing through'' the try construct.
4.7.3 Operations on data structures
Products
The expression expr1 ,..., exprn evaluates to the n-tuple of the values of
expressions expr1 to exprn. The evaluation order for the subexpressions is
not specified.
Variants
The expression ncconstr expr evaluates to the variant value whose constructor
is ncconstr, and whose argument is the value of expr.
For lists, some syntactic sugar is provided. The expression expr1 :: expr2
stands for the constructor ( :: ) applied to the argument ( expr1 , expr2 ),
and therefore evaluates to the list whose head is the value of expr1 and
whose tail is the value of expr2. The expression [ expr1 ;...; exprn ] is
equivalent to expr1 ::...:: exprn :: [], and therefore evaluates to the list
whose elements are the values of expr1 to exprn.
Records
The expression { label1 = expr1 ;...; labeln = exprn } evaluates to the
record value { label1 = v1 ;...; labeln = vn }, where vi is the value of
expri for i=1, ...,n. The labels label1 to labeln must all belong to the
same record types; all labels belonging to this record type must appear
exactly once in the record expression, though they can appear in any order.
The order in which expr1 to exprn are evaluated is not specified.
The expression expr1 . label evaluates expr1 to a record value, and returns
the value associated to label in this record value.
The expression expr1 . label <- expr2 evaluates expr1 to a record value,
which is then modified in-place by replacing the value associated to label in
this record by the value of expr2. This operation is permitted only if label
has been declared mutable in the definition of the record type. The whole
expression expr1 . label <- expr2 evaluates to the unit value ().
Chapter 4. The Objective Caml language 59
Arrays
The expression [| expr1 ;...; exprn |] evaluates to a n-element array, whose
elements are initialized with the values of expr1 to exprn respectively. The
order in which these expressions are evaluated is unspecified.
The expression expr1 .( expr2 ) returns the value of element number expr2
in the array denoted by expr1. The first element has number 0; the last
element has number n-1, where n is the size of the array. The exception
Invalid_argument is raised if the access is out of bounds.
The expression expr1 .( expr2 ) <- expr3 modifies in-place the array
denoted by expr1, replacing element number expr2 by the value of expr3. The
exception Invalid_argument is raised if the access is out of bounds. The
value of the whole expression is ().
Strings
The expression expr1 .[ expr2 ] returns the value of character number expr2
in the string denoted by expr1. The first character has number 0; the last
character has number n-1, where n is the length of the string. The exception
Invalid_argument is raised if the access is out of bounds.
The expression expr1 .[ expr2 ] <- expr3 modifies in-place the string
denoted by expr1, replacing character number expr2 by the value of expr3.
The exception Invalid_argument is raised if the access is out of bounds. The
value of the whole expression is ().
4.7.4 Operators
Symbols from the class infix-symbols, as well as the keywords *, =, or and &,
can appear in infix position (between two expressions). Symbols from the
class prefix-symbols can appear in prefix position (in front of an
expression).
Infix and prefix symbols do not have a fixed meaning: they are simply
interpreted as applications of functions bound to the names corresponding to
the symbols. The expression prefix-symbol expr is interpreted as the
application ( prefix-symbol ) expr. Similarly, the expression
expr1 infix-symbol expr2 is interpreted as the application
( infix-symbol ) expr1 expr2.
The table below lists the symbols defined in the initial environment and
their initial meaning. (See the description of the standard library module
Pervasive in chapter 15 for more details). Their meaning may be changed at
any time using let ( infix-op ) name1 name2 =...
Chapter 4. The Objective Caml language 60
------------------------------------------------------------------------
|Operator |Initial meaning |
------------------------------------------------------------------------
|+ |Integer addition. |
|- (infix) |Integer subtraction. |
|- (prefix) |Integer negation. |
|* |Integer multiplication. |
|/ |Integer division. Raise Division_by_zero if second |
| |argument is zero. The result is unspecified if either |
| |argument is negative. |
|mod |Integer modulus. Raise Division_by_zero if second |
| |argument is zero. The result is unspecified if either |
| |argument is negative. |
|land |Bitwise logical ``and'' on integers. |
|lor |Bitwise logical ``or on integers. |
|lxor |Bitwise logical ``exclusive or'' on integers. |
|lsl |Bitwise logical shift left on integers. |
|lsr |Bitwise logical shift right on integers. |
|asr |Bitwise arithmetic shift right on integers. |
|+. |Floating-point addition. |
|-. (infix) |Floating-point subtraction. |
|-. (prefix) |Floating-point negation. |
|*. |Floating-point multiplication. |
|/. |Floating-point division. |
|** |Floating-point exponentiation. |
|@ |List concatenation. |
|^ |String concatenation. |
|! |Dereferencing (return the current contents of a |
| |reference). |
|:= |Reference assignment (update the reference given as |
| |first argument with the value of the second argument). |
|= |Structural equality test. |
|<> |Structural inequality test. |
|== |Physical equality test. |
|!= |Physical inequality test. |
|< |Test ``less than''. |
|<= |Test ``less than or equal''. |
|> |Test ``greater than''. |
|>= |Test ``greater than or equal'' |
------------------------------------------------------------------------
4.7.5 Objects
Object creation
The expression new class-path denotes a function that takes some
initialization arguments and returns a new object of class class-path.
Message sending
The expression expr # method-name invokes the method method-name of the object
denoted by expr.
Coercion
The type of an object can be coerced (weakened) to a supertype. The
expression ( expr :> typexpr ) coerces the expression expr to type typexpr.
The expression ( expr : typexpr1 :> typexpr2 ) coerces the expression expr
from type typexpr1 to type typexpr2. The former operator will sometimes fail
to coerce an expression expr from a type t1 to a type t2 even if type t1 is a
subtype of type t2. In this case, the latter operator should be used.
Chapter 4. The Objective Caml language 61
In a class definition, coercion to the type this class defines is the
identity, as this type abbreviation is not yet completely defined.
Object duplication
An object can be duplicated using the library function Oo.copy (see
section 16.16). Inside a method, the expression
{< inst-var-name = expr {; inst-var-name = expr} >} returns a copy of self
with the given instance variables replaced by the values of the associated
expressions; other instance variables have the same value in the returned
object as in self.
4.8 Type and exception definitions
4.8.1 Type definitions
Type definitions bind type constructors to data types: either variant types,
record types, type abbreviations, or abstract data types. They also bind the
value constructors and record labels associated with the definition.
type-definition ::= type typedef {and typedef}
typedef ::= [type-params] typeconstr-name [type-equation] [type-representation] {constraint}
type-equation ::= = typexpr
type-representation ::= = constr-decl {| constr-decl}
| = { label-decl {; label-decl} }
type-params ::= ' ident
| ( ' ident {, ' ident} )
constr-decl ::= cconstr-name
| ncconstr-name of typexpr
label-decl ::= label-name : typexpr
| mutable label-name : typexpr
constraint ::= constraint ' ident = typexpr
Type definitions are introduced by the type keyword, and consist in one or
several simple definitions, possibly mutually recursive, separated by the and
keyword. Each simple definition defines one type constructor.
A simple definition consists in a lowercase identifier, possibly preceded by
one or several type parameters, and followed by an optional type equation,
then an optional type representation, and then a constraint clause. The
identifier is the name of the type constructor being defined.
The optional type parameters are either one type variable ' ident, for type
constructors with one parameter, or a list of type variables
(' ident1,...,' identn), for type constructors with several parameters.
These type parameters can appear in the type expressions of the right-hand
side of the definition.
The optional type equation = typexpr makes the defined type equivalent to
the type expression typexpr on the right of the = sign: one can be
substituted for the other during typing. If no type equation is given, a new
type is generated: the defined type is incompatible with any other type.
The optional type representation describes the data structure representing
the defined type, by giving the list of associated constructors (if it is a
variant type) or associated labels (if it is a record type). If no type
representation is given, nothing is assumed on the structure of the type
besides what is stated in the optional type equation.
Chapter 4. The Objective Caml language 62
The type representation = constr-decl {| constr-decl} describes a variant
type. The constructor declarations constr-decl1,...,constr-decln describe
the constructors associated to this variant type. The constructor declaration
ncconstr-name of typexpr declares the name ncconstr-name as a non-constant
constructor, whose argument has type typexpr. The constructor declaration
cconstr-name declares the name cconstr-name as a constant constructor.
Constructor names must be capitalized.
The type representation = { label-decl {; label-decl} } describes a record
type. The label declarations label-decl1,...,label-decln describe the labels
associated to this record type. The label declaration label-name : typexpr
declares label-name as a label whose argument has type typexpr. The label
declaration mutable label-name : typexpr behaves similarly; in addition, it
allows physical modification over the argument to this label.
The two components of a type definition, the optional equation and the
optional representation, can be combined independently, giving rise to four
typical situations:
Abstract type: no equation, no representation.
When appearing in a module signature, this definition specifies nothing
on the type constructor, besides its number of parameters: its
representation is hidden and it is assumed incompatible with any other
type.
Type abbreviation: an equation, no representation.
This defines the type constructor as an abbreviation for the type
expression on the right of the = sign.
New variant type or record type: no equation, a representation.
This generates a new type constructor and defines associated constructors
or labels, through which values of that type can be directly built or
inspected.
Re-exported variant type or record type: an equation, a representation.
In this case, the type constructor is defined as an abbreviation for the
type expression given in the equation, but in addition the constructors
or labels given in the representation remain attached to the defined type
constructor. The type expression in the equation part must agree with
the representation: it must be of the same kind (record or variant) and
have exactly the same constructors or labels, in the same order, with the
same arguments.
The construct constraint ' ident = typexpr allows to specify type
parameters. Any actual type argument corresponding to the type parameter
ident have to be an instance of typexpr (more precisely, ident and typexpr are
unified). Type variables of typexpr can appear in the type equation and the
type declaration.
4.8.2 Exception definitions
exception-definition ::= exception constr-decl
Exception definitions add new constructors to the built-in variant type exn
of exception values. The constructors are declared as for a definition of a
variant type.
Chapter 4. The Objective Caml language 63
4.9 Classes
4.9.1 Class definitions
class-definition ::= class-header = {constraint} {class-fields}
class-header ::= class-tags parameterized-class-name class-params class-binders
class-tags ::= [virtual] [closed]
parameterized-class-name ::= class-name
| ' ident class-name
| ( ' ident {, ' ident} ) class-name
class-params ::= {pattern}+
class-binders ::= [as value-name] [: ' ident]
class-fields ::= inherit ancestor
| val value
| virtual [protected] method-type
| method [protected] method
ancestor ::= [( typexpr {, typexpr} )] class-path {expr}+ [as value-name]
value ::= [private] [mutable] inst-var-name [= expr]
method ::= method-name {pattern} = expr
method-type ::= method-name : typexpr
Class definitions
A class definition class class-definition {and class-definition} is recursive.
Each class-definition defines a class-name that can be used in the whole
expression except for inheritance. It can also be used for inheritance, but
only in the definitions that follow its own.
Type parameters
A class class-name automatically defines two abbreviations : class-name and
# class-name. The first one is the type of objects of this class, while the
second is more general as it unifies with the type of any object belonging to
a subclass (see section 4.4).
The class type parameters correspond to the ones of these two abbreviations.
They must be bound to actual types in the class definition using type
constraints. So that the abbreviations are well-formed, type variables of the
inferred type of the class must either be type parameters or be bound in the
constraint clause.
Class parameters
The parameters class-params are the ones of the object creation function
new class-path, as well as the ones of the inheritance construct.
Self and self type binders
The binders class-binders, i.e. as value-name and : ' ident, allow to bind
self (the current object) and its type, respectively. The variable value-name
can then be used as any variable in method body.
Constraints on type parameters
The construct constraint ' ident = typexpr allows to specify type parameters.
The value of the type parameter ident will be an instance of typexpr (more
precisely, ident and typexpr are unified).
Chapter 4. The Objective Caml language 64
Inheritance
The inheritance construct inherit [( typexpr {, typexpr} )] class-path {expr}+
allows to reuse methods from other classes. It adds the instance variables
and methods from class class-path into the current class, possibly overriding
previously defined ones of the same name. Parent instance variables are
initialized with parent class parameters bound to the arguments {expr}+.
Parent type parameters are unified with type arguments typexpr1,...,typexprn.
An ancestor can be bound by prepending the construct as value-name to the
inheritance construct above. value-name is not a true variable and can only
be used to select a method, i.e. in an expression value-name # method-name.
This gives access to the method method-name as it was defined in the parent
class even if it is redefined in the current class.
Instance variable definition
Instance variables can be defined or their status can be changed using the
construct val value.
The definition val [private] [mutable] inst-var-name = expr adds an instance
variable inst-var-name whose initial value is the value of expression expr.
If the variable was previously defined, its previous initial value is
overridden. By default, this variable is visible in subclasses of current
class. The flag private makes this variable only visible in the current
class. The flag mutable allows physical modification of this variable by
methods.
The construct val [private] [mutable] inst-var-name enables to change the
variable status (makes it private and/or mutable), while keeping the same
initial value.
Method definition
Method definition is written method method. The definition of a method
overrides any previous definition of this method. The method will be public
(that is, not protected) if any of the definition states so.
A protected method, method protected method or virtualOprotected method, is
a method that can only be invoked on self (from other methods of the current
class as well as of subclasses of the current class). This invocation is
performed using the expression value-name # method-name, where value-name is
directly bound to self at the beginning of the class definition. Protected
methods do not appear in object types.
Method bodies do not have access to class parameters, but to instance
variables. Some special expressions are available in method bodies for
manipulating instance variables and duplicating self:
expr ::= ...
| inst-var-name
| inst-var-name <- expr
| {< [inst-var-name = expr {; inst-var-name = expr}] >}
The expression inst-var-name evaluates to the value of the corresponding
instance variable in the current object, while the expression
inst-var-name <- expr modifies in-place the current object by replacing the
value associated to inst-var-name by the value of expr. Of course, this
instance variable must have been declared mutable.
The expression {< [inst-var-name = expr {; inst-var-name = expr}] >}
evaluates to a copy of the current object in which the values of instance
variables inst-var-name1,...,inst-var-namen have been replaced by the values
Chapter 4. The Objective Caml language 65
of the corresponding expressions expr1,...,exprn.
Virtual class
Methods can be declared, without being defined, with the construct
virtual method-type. Methods that are declared in this way or applied to self
but not actually defined are said to be virtual. A class must be flagged
virtual if one of its methods is virtual. Objects cannot be created from a
virtual class.
Closed class
A class can be flagged as closed. A closed class is a class to which
subclasses cannot add methods. If the compiler complains that a class must be
closed, this usually means that the type of self has been unintentionally
unified with a closed object type.
4.9.2 Class types
Class types are specifications for class definitions. The syntax of class
types is closely modeled on class definitions.
The type of a concrete method can be omitted. That way, the status of a
method can be changed from virtual to concrete without having the method type
be given.
Some type information can be hidden in a class type: instance variables and
protected concrete methods can be omitted; previously non-mutable instance
variables can be flagged mutable.
class-type ::= class-type-header = {constraint} {class-type-fields}
class-type-header ::= class-tags parameterized-class-name class-type-params [: ' ident]
class-type-params ::= {( typexpr )}+
class-type-fields ::= inherit ancestor-type
| val value-type
| virtual [protected] method-type
| method [protected] method-name [: typexpr]
ancestor-type ::= [( typexpr {, typexpr} )] class-path
value-type ::= [private] [mutable] inst-var-name [: typexpr]
method-type ::= method-name : typexpr
4.10 Module types (module specifications)
Module types are the module-level equivalent of type expressions: they
specify the general shape and type properties of modules.
Chapter 4. The Objective Caml language 66
module-type ::= modtype-path
| sig {specification [;;]} end
| functor ( module-name : module-type ) -> module-type
| module-type with constraint {and constraint}
| ( module-type )
specification ::= val value-name : typexpr
| external value-name : typexpr = external-declaration
| type-definition
| exception-definition
| class class-type {and class-type} end
| module module-name : module-type
| module module-name {( module-name : module-type )} : module-type
| module type modtype-name
| module type modtype-name = module-type
| open module-path
constraint ::= type [type-parameters] typeconstr = typexp
| module module-path = extended-module-path
4.10.1 Simple module types
The expression modtype-path is equivalent to the module type bound to the name
modtype-path. The expression ( module-type ) denotes the same type as
module-type.
4.10.2 Signatures
Signatures are type specifications for structures. Signatures sig...end are
collections of type specifications for value names, type names, exceptions,
module names and module type names. A structure will match a signature if the
structure provides definitions (implementations) for all the names specified
in the signature (and possibly more), and these definitions meet the type
requirements given in the signature.
For compatibility with Caml Light, an optional ;; is allowed after each
specification in a signature. The ;; has no semantic meaning.
Value specifications
A specification of a value component in a signature is written
val value-name : typexpr, where value-name is the name of the value and
typexpr its expected type.
The form external value-name : typexpr = external-declaration is similar,
except that it requires in addition the name to be implemented as the external
function specified in external-declaration (see chapter 14).
Type specifications
A specification of one or several type components in a signature is written
type typedef {and typedef} and consists of a sequence of mutually recursive
definitions of type names.
Each type definition in the signature specifies an optional type equation
= typexp and an optional type representation = constr-decl... or
= { label-decl...}. The implementation of the type name in a matching
structure must be compatible with the type expression specified in the
equation (if given), and have the specified representation (if given).
Conversely, users of that signature will be able to rely on the type equation
or type representation, if given. More precisely, we have the following four
situations:
Chapter 4. The Objective Caml language 67
Abstract type: no equation, no representation.
Names that are defined as abstract types in a signature can be
implemented in a matching structure by any kind of type definition
(provided it has the same number of type parameters). The exact
implementation of the type will be hidden to the users of the structure.
In particular, if the type is implemented as a variant type or record
type, the associated constructors and labels will not be accessible to
the users; if the type is implemented as an abbreviation, the type
equality between the type name and the right-hand side of the
abbreviation will be hidden from the users of the structure. Users of
the structure consider that type as incompatible with any other type: a
fresh type has been generated.
Type abbreviation: an equation = typexp, no representation.
The type name must be implemented by a type compatible with typexp. All
users of the structure know that the type name is compatible with typexp.
New variant type or record type: no equation, a representation.
The type name must be implemented by a variant type or record type with
exactly the constructors or labels specified. All users of the structure
have access to the constructors or labels, and can use them to create or
inspect values of that type. However, users of the structure consider
that type as incompatible with any other type: a fresh type has been
generated.
Re-exported variant type or record type: an equation, a representation.
This case combines the previous two: the representation of the type is
made visible to all users, and no fresh type is generated.
Exception specification
The specification exception constr-decl in a signature requires the matching
structure to provide an exception with the name and arguments specified in the
definition, and makes the exception available to all users of the structure.
Class specifications
A class specification is composed of one or several mutually recursive class
types, as described in section 4.9.2.
Module specifications
A specification of a module component in a signature is written
module module-name : module-type, where module-name is the name of the module
component and module-type its expected type. Modules can be nested
arbitrarily; in particular, functors can appear as components of structures
and functor types as components of signatures.
For specifying a module component that is a functor, one may write
module module-name ( name : module-type )...( name : module-type ) : module-type
1 1 n n
instead of
module module-name : functor ( name1 : module-type1 ) ->...-> module-type
Chapter 4. The Objective Caml language 68
Module type specifications
A module type component of a signature can be specified either as a manifest
module type or as an abstract module type.
An abstract module type specification module type modtype-name allows the
name modtype-name to be implemented by any module type in a matching
signature, but hides the implementation of the module type to all users of the
signature.
A manifest module type specification module type modtype-name = module-type
requires the name modtype-name to be implemented by the module type
module-type in a matching signature, but makes the equality between
modtype-name and module-type apparent to all users of the signature.
Opening a module path
The expression open module-path in a signature does not specify any
components. It simply affects the parsing of the following items of the
signature, allowing components of the module denoted by module-path to be
referred to by their simple names name instead of path accesses
module-path . name. The scope of the open stops at the end of the signature
expression.
4.10.3 Functor types
The module type expression
functor ( module-name : module-type1 ) -> module-type2 is the type of
functors (functions from modules to modules) that take as argument a module of
type module-type1 and return as result a module of type module-type2. The
module type module-type2 can use the name module-name to refer to type
components of the actual argument of the functor. No restrictions are placed
on the type of the functor argument; in particular, a functor may take another
functor as argument (``higher-order'' functor).
4.10.4 The with operator
Assuming module-type denotes a signature, the expression
module-type with constraint {and constraint} denotes the same signature where
type equations have been added to some of the type specifications, as
described by the constraints following the with keyword. The constraint
type [type-parameters] typeconstr = typexp adds the type equation = typexp to
the specification of the type component named typeconstr of the constrained
signature. The constraint module module-path = extended-module-path adds type
equations to all type components of the sub-structure denoted by module-path,
making them equivalent to the corresponding type components of the structure
denoted by extended-module-path.
For instance, if the module type name S is bound to the signature
sig type t module M: (sig type u end) end
then S with type t=int denotes the signature
sig type t=int module M: (sig type u end) end
and S with module M = N denotes the signature
sig type t module M: (sig type u=N.u end) end
Chapter 4. The Objective Caml language 69
A functor taking two arguments of type S that share their t component is
written
functor (A: S) (B: S with type t = A.t) ...
4.11 Module expressions (module implementations)
Module expressions are the module-level equivalent of value expressions: they
evaluate to modules, thus providing implementations for the specifications
expressed in module types.
module-expr ::= module-path
| struct {definition [;;]} end
| functor ( module-name : module-type ) -> module-expr
| module-expr ( module-expr )
| ( module-expr )
| ( module-expr : module-type )
definition ::= let [rec] let-binding {and let-binding}
| external value-name : typexpr = external-declaration
| type-definition
| exception-definition
| class class-definition {and class-definition} end
| module module-name {( module-name : module-type )} [: module-type] = module-expr
| module type modtype-name = module-type
| open module-path
4.11.1 Simple module expressions
The expression module-path evaluates to the module bound to the name
module-path.
The expression ( module-expr ) evaluates to the same module as module-expr.
The expression ( module-expr : module-type ) checks that the type of
module-expr is a subtype of module-type, that is, that all components
specified in module-type are implemented in module-expr, and their
implementation meets the requirements given in module-type. In other terms,
it checks that the implementation module-expr meets the type specification
module-type. The whole expression evaluates to the same module as
module-expr, except that all components not specified in module-type are
hidden and can no longer be accessed.
4.11.2 Structures
Structures struct...end are collections of definitions for value names, type
names, exceptions, module names and module type names. The definitions are
evaluated in the order in which they appear in the structure. The scope of
the bindings performed by the definitions extend to the end of the structure.
As a consequence, a definition may refer to names bound by earlier definitions
in the same structure.
For compatibility with toplevel phrases (chapter 7) and with Caml Light, an
optional ;; is allowed after each definition in a structure. The ;; has no
semantic meaning. Also for compatibility, ;; expr is allowed as a component
of a structure, meaning let _ = expr, i.e. evaluate expr for its
side-effects.
Chapter 4. The Objective Caml language 70
Value definitions
A value definition let [rec] let-binding {and let-binding} bind value names in
the same way as a let...in... expression (see section 4.7.1). The value names
appearing in the left-hand sides of the bindings are bound to the
corresponding values in the right-hand sides.
A value definition external value-name : typexpr = external-declaration
implements value-name as the external function specified in
external-declaration (see chapter 14).
Type definitions
A definition of one or several type components is written
type typedef {and typedef} and consists of a sequence of mutually recursive
definitions of type names.
Exception definitions
Exceptions are defined with the syntax exception constr-decl.
Class definitions
The class keyword introduces one or several mutually recursive class
definitions, as described in section 4.9.1.
Module definitions
The basic form for defining a module component is
module module-name = module-expr, which evaluates module-expr and binds the
result to the name module-name.
One can write
module module-name : module-type = module-expr
instead of
module module-name = ( module-expr : module-type ).
Another derived form is
module module-name ( name : module-type )...( name : module-type ) = module-expr
1 1 n n
which is equivalent to
module module-name = functor ( name1 : module-type1 ) ->...-> module-expr
Module type definitions
A definition for a module type is written
module type modtype-name = module-type. It binds the name modtype-name to the
module type denoted by the expression module-type.
Opening a module path
The expression open module-path in a structure does not define any components
nor perform any bindings. It simply affects the parsing of the following
items of the structure, allowing components of the module denoted by
Chapter 4. The Objective Caml language 71
module-path to be referred to by their simple names name instead of path
accesses module-path . name. The scope of the open stops at the end of the
structure expression.
4.11.3 Functors
Functor definition
The expression functor ( module-name : module-type ) -> module-expr evaluates
to a functor that takes as argument modules of the type module-type1, binds
module-name to these modules, evaluates module-expr in the extended
environment, and returns the resulting modules as results. No restrictions
are placed on the type of the functor argument; in particular, a functor may
take another functor as argument (``higher-order'' functor).
Functor application
The expression module-expr1 ( module-expr2 ) evaluates module-expr1 to a
functor and module-expr2 to a module, and applies the former to the latter.
The type of module-expr2 must match the type expected for the arguments of
the functor module-expr1.
4.12 Compilation units
unit-interface ::= {specification [;;]}
unit-implementation ::= {definition [;;]}
Compilation units bridge the module system and the separate compilation
system. A compilation unit is composed of two parts: an interface and an
implementation. The interface contains a sequence of specifications, just as
the inside of a sig...end signature expression. The implementation contains a
sequence of definitions, just as the inside of a struct...end module
expression. A compilation unit also has a name unit-name, derived from the
names of the files containing the interface and the implementation (see
chapter 6 for more details). A compilation unit behaves roughly as the module
definition
module unit-name : sig unit-interface end = struct unit-implementation end
A compilation unit can refer to other compilation units by their names, as
if they were regular modules. For instance, if U is a compilation unit that
defines a type t, other compilation units can refer to that type under the
name U.t; they can also refer to U as a whole structure. Except for names of
other compilation units, a unit interface or unit implementation must not have
any other free variables. In other terms, the type-checking and compilation
of an interface or implementation proceeds in the initial environment
name1 : sig interface1 end...namen : sig interfacen end
where name1...nameN are the names of the other compilation units available in
the search path (see chapter 6 for more details) and interface1...interfacen
are their respective interfaces.
Chapter 5
Language extensions
This chapter describes the language features that are implemented in Objective
Caml, but not described in the Objective Caml reference manual. In contrast
with the fairly stable kernel language that is described in the reference
manual, the extensions presented here are still experimental, and may be
removed or changed in the future.
5.1 Streams and stream parsers
Objective Caml comprises a library type for streams (possibly infinite
sequences of elements, that are evaluated on demand), and associated stream
expressions, to build streams, and stream patterns, to destructure streams.
Streams and stream patterns provide a natural approach to the writing of
recursive-descent parsers.
Streams are presented by the following extensions to the syntactic classes
of expressions:
expr ::= ...
| [< >]
| [< stream-component {; stream-component} >]
| parser [pattern] stream-matching
| match expr with parser [pattern] stream-matching
stream-component ::= ' expr
| expr
stream-matching ::= stream-pattern [pattern] -> expr {| stream-pattern [pattern] -> expr}
stream-pattern ::= [< >]
| [< stream-pat-comp {; stream-pat-comp [? expr]} >]
stream-pat-comp ::= ' pattern [when expr]
| pattern = expr
| ident
Stream expressions are bracketed by [< and >]. They represent the
concatenation of their components. The component ' expr represents the
one-element stream whose element is the value of expr. The component expr
represents a sub-stream. For instance, if both s and t are streams of
integers, then [<'1; s; t; '2>] is a stream of integers containing the element
1, then the elements of s, then those of t, and finally 2. The empty stream
is denoted by [< >].
Unlike any other kind of expressions in the language, stream expressions are
submitted to lazy evaluation: the components are not evaluated when the
stream is built, but only when they are accessed during stream matching. The
components are evaluated once, the first time they are accessed; the following
72
Chapter 5. Language extensions 73
accesses reuse the value computed the first time.
Stream patterns, also bracketed by [< and >], describe initial segments of
streams. In particular, the stream pattern [< >] matches all streams. Stream
pattern components are matched against the corresponding elements of a stream.
The component ' pattern matches the corresponding stream element against the
pattern; if followed by when, the match is accepted only if the result of the
guard expression is true. The component pattern = expr applies the function
denoted by expr to the current stream, then matches the result of the function
against pattern. Finally, the component ident simply binds the identifier to
the stream being matched.
Stream matching proceeds destructively: once a component has been matched,
it is discarded from the stream (by in-place modification).
Stream matching proceeds in two steps: first, a pattern is selected by
matching the stream against the first components of the stream patterns; then,
the following components of the selected pattern are checked against the
stream. If the following components do not match, the exception
Stream.Parse_error is raised. There is no backtracking here: stream matching
commits to the pattern selected according to the first element. If none of
the first components of the stream patterns match, the exception
Stream.Parse_failure is raised. The Stream.Parse_failure exception causes the
next alternative to be tried, if it occurs during the matching of the first
element of a stream, before matching has committed to one pattern.
The streams hold the count of their elements discarded. The optional
pattern before the first stream pattern is bound to the stream count before
the matching. The one after each stream pattern (optional, too) is bound to
the stream count after the matching.
The exception Parse_error has a string parameter coming from the optional
? expr after the stream pattern components (its default is the empty string).
This expression is evaluated only in case of error.
See Functional programming using Caml Light for a more gentle introductions
to streams, and for some examples of their use in writing parsers. A more
formal presentation of streams, and a discussion of alternate semantics, can
be found in Parsers in ML by Michel Mauny and Daniel de Rauglaudre, in the
proceedings of the 1992 ACM conference on Lisp and Functional Programming.
5.2 Range patterns
In patterns, Objective Caml recognizes the form ' c ' .. ' d ' (two character
literals separated by ..) as shorthand for the pattern
' c ' | ' c1 ' | ' c2 ' |...| ' cn ' | ' d '
where c1, c2, ..., cn are the characters that occur between c and d in the
ASCII character set. For instance, the pattern '0'..'9' matches all
characters that are digits.
5.3 Assertion checking
Objective Caml supports the assert construct to check debugging assertions.
The expression assert expr evaluates the expression expr and returns () if
expr evaluates to true. Otherwise, the exception Assert_failure is raised
with the location of expr in the source code as argument. As a special case,
assert false is reduced to raise (Assert_failure ...), which is polymorphic.
Assertion checking can be turned off with the -noassert compiler option.
Chapter 5. Language extensions 74
5.4 Deferred computations
The expression lazy expr returns a value v of type Lazy.t that encapsulates
the computation of expr. The argument expr is not evaluated at this point in
the program. Instead, its evaluation will be performed the first time
Lazy.force is applied to the value v, returning the actual value of expr.
Subsequent applications of Lazy.force to v do not evaluate expr again.
The expression lazy expr is equivalent to
ref (Lazy.Delayed (fun () -> expr)). For more information, see the
description of module Lazy in the standard library (section 16.11).
Part III
The Objective Caml tools
75
Chapter 6
Batch compilation (ocamlc)
This chapter describes the Objective Caml batch compiler ocamlc, which
compiles Caml source files to bytecode object files and link these object
files to produce standalone bytecode executable files. These executable files
are then run by the bytecode interpreter ocamlrun.
6.1 Overview of the compiler
The ocamlc command has a command-line interface similar to the one of most C
compilers. It accepts several types of arguments:
- Arguments ending in .mli are taken to be source files for compilation
unit interfaces. Interfaces specify the names exported by compilation
units: they declare value names with their types, define public data
types, declare abstract data types, and so on. From the file x.mli, the
ocamlc compiler produces a compiled interface in the file x.cmi.
- Arguments ending in .ml are taken to be source files for compilation unit
implementations. Implementations provide definitions for the names
exported by the unit, and also contain expressions to be evaluated for
their side-effects. From the file x.ml, the ocamlc compiler produces
compiled object bytecode in the file x.cmo.
If the interface file x.mli exists, the implementation x.ml is checked
against the corresponding compiled interface x.cmi, which is assumed to
exist. If no interface x.mli is provided, the compilation of x.ml
produces a compiled interface file x.cmi in addition to the compiled
object code file x.cmo. The file x.cmi produced corresponds to an
interface that exports everything that is defined in the implementation
x.ml.
- Arguments ending in .cmo are taken to be compiled object bytecode. These
files are linked together, along with the object files obtained by
compiling .ml arguments (if any), and the Caml Light standard library, to
produce a standalone executable program. The order in which .cmo and .ml
arguments are presented on the command line is relevant: compilation
units are initialized in that order at run-time, and it is a link-time
error to use a component of a unit before having initialized it. Hence,
a given x.cmo file must come before all .cmo files that refer to the unit
x.
- Arguments ending in .cma are taken to be libraries of object bytecode. A
library of object bytecode packs in a single file a set of object
76
Chapter 6. Batch compilation (ocamlc) 77
bytecode files (.cmo files). Libraries are built with ocamlc -a (see the
description of the -a option below). The object files contained in the
library are linked as regular .cmo files (see above), in the order
specified when the .cma file was built. The only difference is that if
an object file contained in a library is not referenced anywhere in the
program, then it is not linked in.
- Arguments ending in .c are passed to the C compiler, which generates a .o
object file. This object file is linked with the program if the -custom
flag is set (see the description of -custom below).
- Arguments ending in .o or .a are assumed to be C object files and
libraries. They are passed to the C linker when linking in -custom mode
(see the description of -custom below).
The output of the linking phase is a file containing compiled bytecode that
can be executed by the Objective Caml bytecode interpreter: the command named
ocamlrun. If caml.out is the name of the file produced by the linking phase,
the command
ocamlrun caml.out arg1 arg2 ... argn
executes the compiled code contained in caml.out, passing it as arguments the
character strings arg1 to argn. (See chapter 8 for more details.)
On most Unix systems, the file produced by the linking phase can be run
directly, as in:
./caml.out arg1 arg2 ... argn
The produced file has the executable bit set, and it manages to launch the
bytecode interpreter by itself.
6.2 Options
The following command-line options are recognized by ocamlc.
-a Build a library (.cma file) with the object files (.cmo files) given on
the command line, instead of linking them into an executable file. The
name of the library can be set with the -o option. The default name is
library.cma.
-c Compile only. Suppress the linking phase of the compilation. Source
code files are turned into compiled files, but no executable file is
produced. This option is useful to compile modules separately.
-cclib -llibname
Pass the -llibname option to the C linker when linking in ``custom
runtime'' mode (see the -custom option). This causes the given C library
to be linked with the program.
-ccopt option
Pass the given option to the C compiler and linker, when linking in
``custom runtime'' mode (see the -custom option). For instance, -ccopt
-Ldir causes the C linker to search for C libraries in directory dir.
-custom
Link in ``custom runtime'' mode. In the default linking mode, the linker
Chapter 6. Batch compilation (ocamlc) 78
produces bytecode that is intended to be executed with the shared runtime
system, ocamlrun. In the custom runtime mode, the linker produces an
output file that contains both the runtime system and the bytecode for
the program. The resulting file is larger, but it can be executed
directly, even if the ocamlrun command is not installed. Moreover, the
``custom runtime'' mode enables linking Caml code with user-defined C
functions, as described in chapter 14.
-g Add debugging information while compiling and linking. This option is
required in order to be able to debug the program with ocamldebug (see
chapter 12).
-i Cause the compiler to print all defined names (with their inferred types
or their definitions) when compiling an implementation (.ml file). This
can be useful to check the types inferred by the compiler. Also, since
the output follows the syntax of interfaces, it can help in writing an
explicit interface (.mli file) for a file: just redirect the standard
output of the compiler to a .mli file, and edit that file to remove all
declarations of unexported names.
-I directory
Add the given directory to the list of directories searched for compiled
interface files (.cmi) and compiled object code files (.cmo). By
default, the current directory is searched first, then the standard
library directory. Directories added with -I are searched after the
current directory, in the order in which they were given on the command
line, but before the standard library directory.
-impl filename
Compile the file filename as an implementation file, even if its
extension is not .ml.
-intf filename
Compile the file filename as an interface file, even if its extension is
not .mli.
-linkall
Force all modules contained in libraries to be linked in. If this flag
is not given, unreferenced modules are not linked in. When building a
library (-a flag), setting the -linkall flag forces all subsequent links
of programs involving that library to link all the modules contained in
the library.
-noassert
Turn assertion checking off: assertions are not compiled. This flag has
no effect when linking already compiled files.
-o exec-file
Specify the name of the output file produced by the linker. The default
output name is a.out, in keeping with the Unix tradition. If the -a
option is given, specify the name of the library produced. If the
-output-obj option is given, specify the name of the output file
produced.
-output-obj
Cause the linker to produce a C object file instead of a bytecode
executable file. This is useful to wrap Caml code as a C library,
callable from any C program. See chapter 14, section 14.6.5. The name
Chapter 6. Batch compilation (ocamlc) 79
of the output object file is camlprog.o by default; it can be set with
the -o option.
-pp command
Cause the compiler to call the given command as a preprocessor for each
source file. The output of command is redirected to an intermediate
file, which is compiled. If there are no compilation errors, the
intermediate file is deleted afterwards. The name of this file is built
from the basename of the source file with the extension .ppi for an
interface (.mli) file and .ppo for an implementation (.ml) file.
-thread
Compile or link multithreaded programs, in combination with the threads
library described in chapter 20. What this option actually does is
select a special, thread-safe version of the standard library.
-unsafe
Turn bound checking off on array and string accesses (the v.(i) and s.[i]
constructs). Programs compiled with -unsafe are therefore slightly
faster, but unsafe: anything can happen if the program accesses an array
or string outside of its bounds.
-v Print the version number of the compiler.
6.3 Modules and the file system
This short section is intended to clarify the relationship between the names
of the modules corresponding to compilation units and the names of the files
that contain their compiled interface and compiled implementation.
The compiler always derives the module name by taking the capitalized base
name of the source file (.ml or .mli file). That is, it strips the leading
directory name, if any, as well as the .ml or .mli suffix; then, it set the
first letter to uppercase, in order to comply with the requirement that module
names must be capitalized. For instance, compiling the file mylib/misc.ml
provides an implementation for the module named Misc. Other compilation units
may refer to components defined in mylib/misc.ml under the names Misc.name;
they can also do open Misc, then use unqualified names name.
The .cmi and .cmo files produced by the compiler have the same base name as
the source file. Hence, the compiled files always have their base name equal
(modulo capitalization of the first letter) to the name of the module they
describe (for .cmi files) or implement (for .cmo files).
When the compiler encounters a reference to a free module identifier Mod, it
looks in the search path for a file mod.cmi (note lowercasing of first letter)
and loads the compiled interface contained in that file. As a consequence,
renaming .cmi files is not advised: the name of a .cmi file must always
correspond to the name of the compilation unit it implements. It is
admissible to move them to another directory, if their base name is preserved,
and the correct -I options are given to the compiler. The compiler will flag
an error if it loads a .cmi file that has been renamed.
Compiled bytecode files (.cmo files), on the other hand, can be freely
renamed once created. That's because the linker never attempts to find by
itself the .cmo file that implements a module with a given name: it relies
instead on the user providing the list of .cmo files by hand.
Chapter 6. Batch compilation (ocamlc) 80
6.4 Common errors
This section describes and explains the most frequently encountered error
messages.
Cannot find file filename
The named file could not be found in the current directory, nor in the
directories of the search path. The filename is either a compiled
interface file (.cmi file), or a compiled bytecode file (.cmo file). If
filename has the format mod.cmi, this means you are trying to compile a
file that references identifiers from module mod, but you have not yet
compiled an interface for module mod. Fix: compile mod.mli or mod.ml
first, to create the compiled interface mod.cmi.
If filename has the format mod.cmo, this means you are trying to link a
bytecode object file that does not exist yet. Fix: compile mod.ml
first.
If your program spans several directories, this error can also appear
because you haven't specified the directories to look into. Fix: add
the correct -I options to the command line.
Corrupted compiled interface filename
The compiler produces this error when it tries to read a compiled
interface file (.cmi file) that has the wrong structure. This means
something went wrong when this .cmi file was written: the disk was full,
the compiler was interrupted in the middle of the file creation, and so
on. This error can also appear if a .cmi file is modified after its
creation by the compiler. Fix: remove the corrupted .cmi file, and
rebuild it.
This expression has type t1, but is used with type t2
This is by far the most common type error in programs. Type t1 is the
type inferred for the expression (the part of the program that is
displayed in the error message), by looking at the expression itself.
Type t2 is the type expected by the context of the expression; it is
deduced by looking at how the value of this expression is used in the
rest of the program. If the two types t1 and t2 are not compatible, then
the error above is produced.
In some cases, it is hard to understand why the two types t1 and t2 are
incompatible. For instance, the compiler can report that ``expression of
type foo cannot be used with type foo'', and it really seems that the two
types foo are compatible. This is not always true. Two type
constructors can have the same name, but actually represent different
types. This can happen if a type constructor is redefined. Example:
type foo = A | B
let f = function A -> 0 | B -> 1
type foo = C | D
f C
This result in the error message ``expression C of type foo cannot be
used with type foo''.
Chapter 6. Batch compilation (ocamlc) 81
The type of this expression, t, contains type variables that cannot be generalized
Type variables ('a, 'b, ...) in a type t can be in either of two states:
generalized (which means that the type t is valid for all possible
instantiations of the variables) and not generalized (which means that
the type t is valid only for one instantiation of the variables). In a
let binding let name = expr, the type-checker normally generalizes as
many type variables as possible in the type of expr. However, this leads
to unsoundness (a well-typed program can crash) in conjunction with
polymorphic mutable data structures. To avoid this, generalization is
performed at let bindings only if the bound expression expr belongs to
the class of ``syntactic values'', which includes constants, identifiers,
functions, tuples of syntactic values, etc. In all other cases (for
instance, expr is a function application), a polymorphic mutable could
have been created and generalization is therefore turned off.
Non-generalized type variables in a type cause no difficulties inside a
given structure or compilation unit (the contents of a .ml file, or an
interactive session), but they cannot be allowed inside signatures nor in
compiled interfaces (.cmi file), because they could be used
inconsistently later. Therefore, the compiler flags an error when a
structure or compilation unit defines a value name whose type contains
non-generalized type variables. There are two ways to fix this error:
- Add a type constraint or a .mli file to give a monomorphic type
(without type variables) to name. For instance, instead of writing
let sort_int_list = Sort.list (<)
(* inferred type 'a list -> 'a list, with 'a not generalized *)
write
let sort_int_list = (Sort.list (<) : int list -> int list);;
- If you really need name to have a polymorphic type, turn its defining
expression into a function by adding an extra parameter. For
instance, instead of writing
let map_length = List.map Array.length
(* inferred type 'a array list -> int list, with 'a not general-
ized *)
write
let map_length lv = List.map Array.length lv
Reference to undefined global mod
This error appears when trying to link an incomplete or incorrectly
Chapter 6. Batch compilation (ocamlc) 82
ordered set of files. Either you have forgotten to provide an
implementation for the compilation unit named mod on the command line
(typically, the file named mod.cmo, or a library containing that file).
Fix: add the missing .ml or .cmo file to the command line. Or, you have
provided an implementation for the module named mod, but it comes too
late on the command line: the implementation of mod must come before all
bytecode object files that reference mod. Fix: change the order of .ml
and .cmo files on the command line.
Of course, you will always encounter this error if you have mutually
recursive functions across modules. That is, function Mod1.f calls
function Mod2.g, and function Mod2.g calls function Mod1.f. In this
case, no matter what permutations you perform on the command line, the
program will be rejected at link-time. Fixes:
- Put f and g in the same module.
- Parameterize one function by the other. That is, instead of having
mod1.ml: let f x = ... Mod2.g ...
mod2.ml: let g y = ... Mod1.f ...
define
mod1.ml: let f g x = ... g ...
mod2.ml: let rec g y = ... Mod1.f g ...
and link mod1.cmo before mod2.cmo.
- Use a reference to hold one of the two functions, as in :
mod1.ml: let forward_g =
ref((fun x -> failwith "forward_g") : <type>)
let f x = ... !forward_g ...
mod2.ml: let g y = ... Mod1.f ...
let _ = Mod1.forward_g := g
This will not work if g is a polymorphic function, however.
The external function f is not available
This error appears when trying to link code that calls external functions
written in C in ``default runtime'' mode. As explained in chapter 14,
such code must be linked in ``custom runtime'' mode. Fix: add the
-custom option, as well as the C libraries and C object files that
implement the required external functions.
Chapter 7
The toplevel system (ocaml)
This chapter describes the toplevel system for Objective Caml, that permits
interactive use of the Objective Caml system through a read-eval-print loop.
In this mode, the system repeatedly reads Caml phrases from the input, then
typechecks, compile and evaluate them, then prints the inferred type and
result value, if any. The system prints a # (sharp) prompt before reading
each phrase.
Input to the toplevel can span several lines. It is terminated by ;; (a
double-semicolon). The toplevel input consists in one or several toplevel
phrases, with the following syntax:
toplevel-input ::= {toplevel-phrase} ;;
toplevel-phrase ::= definition
| expr
| # ident directive-argument
definition ::= let [rec] let-binding {and let-binding}
| external value-name : typexpr = external-declaration
| type-definition
| exception-definition
| module module-name [: module-type] = module-expr
| module type modtype-name = module-type
| open module-path
directive-argument ::= nothing
| string-literal
| integer-literal
| value-path
A phrase can consist of a definition, similar to those found in
implementations of compilation units or in struct...end module expressions.
The definition can bind value names, type names, an exception, a module name,
or a module type name. The toplevel system performs the bindings, then prints
the types and values (if any) for the names thus defined.
A phrase may also consist in a open directive (see section 4.11), or a value
expression (section 4.7). Expressions are simply evaluated, without
performing any bindings, and the value of the expression is printed.
Finally, a phrase can also consist in a toplevel directive, starting with #
(the sharp sign). These directives control the behavior of the toplevel; they
are listed below in section 7.2.
Unix:
The toplevel system is started by the command ocaml, as follows:
83
Chapter 7. The toplevel system (ocaml) 84
ocaml options # interactive mode
ocaml options scriptfile # script mode
If no filename is given on the command line, the toplevel system enters
interactive mode: phrases are read on standard input, results are
printed on standard output, errors on standard error. End-of-file on
standard input terminates ocaml (see also the #quit directive in
section 7.2).
On start-up (before the first phrase is read), if the file .ocamlinit
exists in the current directory, its contents are read as a sequence of
Objective Caml phrases and executed as per the #use directive described
in section 7.2. The evaluation outcode for each phrase are not
displayed.
The toplevel system does not perform line editing, but it can easily be
used in conjunction with an external line editor such as fep; just run
fep -emacs ocaml or fep -vi ocaml. Another option is to use ocaml under
Gnu Emacs, which gives the full editing power of Emacs (see the
subdirectory emacs of the Objective Caml distribution).
At any point, the parsing, compilation or evaluation of the current
phrase can be interrupted by pressing ctrl-C (or, more precisely, by
sending the sigintr signal to the ocaml process). The toplevel then
immediately returns to the # prompt.
If a filename is given on the command-line to ocaml, the toplevel system
enters script mode: the contents of the file are read as a sequence of
Objective Caml phrases and executed, as per the #use directive
(section 7.2). The outcome of the evaluation is not printed. On
reaching the end of file, the ocaml command exits immediately. No
commands are read from standard input.
In script mode, the first line of the script is ignored if it starts with
#!. Thus, it is theoretically possible to make the script itself
executable and put as first line #!/usr/local/bin/ocaml, thus calling the
toplevel system automatically when the script is run. However, ocaml
itself is a #! script on most installations of Objective Caml, and Unix
kernels usually do not handle nested #! scripts.
Windows:
In addition to the text-only command ocaml.exe, which works exactly as
under Unix (see above), a graphical user interface for the toplevel is
available under the name ocamlwin.exe. It should be launched from the
Windows file manager or program manager.
The ``Terminal'' windows is split in two panes. Phrases are entered and
edited in the bottom pane. The top pane displays a copy of the input
phrases as they are processed by the Caml Light toplevel, interspersed
with the toplevel responses. The ``Return'' key sends the contents of
the bottom pane to the Caml Light toplevel. The ``Enter'' key inserts a
newline without sending the contents of the Input window. (This can be
configured with the ``Preferences'' menu item.)
The contents of the input window can be edited at all times, with the
standard Windows interface. An history of previously entered phrases is
maintained and displayed in a separate window.
Chapter 7. The toplevel system (ocaml) 85
To quit the Camlwin application, either select ``Quit'' from the ``File''
menu, or use the quit function described below.
At any point, the parsing, compilation or evaluation of the current
phrase can be interrupted by selecting the ``Interrupt Caml Light'' menu
item. This goes back to the # prompt.
7.1 Options
The following command-line options are recognized by the ocaml command.
-I directory
Add the given directory to the list of directories searched for source
and compiled files. By default, the current directory is searched first,
then the standard library directory. Directories added with -I are
searched after the current directory, in the order in which they were
given on the command line, but before the standard library directory.
Directories can also be added to the search path once the toplevel is
running with the #directory directive (section 7.2).
-unsafe
See the corresponding option for ocamlc, chapter 6. Turn bound checking
off on array and string accesses (the v.(i) and s.[i] constructs).
Programs compiled with -unsafe are therefore slightly faster, but unsafe:
anything can happen if the program accesses an array or string outside of
its bounds.
Unix:
The following environment variables are also consulted:
LC_CTYPE
If set to iso_8859_1, accented characters (from the ISO Latin-1
character set) in string and character literals are printed as is;
otherwise, they are printed as decimal escape sequences (\ddd).
TERM
When printing error messages, the toplevel system attempts to
underline visually the location of the error. It consults the TERM
variable to determines the type of output terminal and look up its
capabilities in the terminal database.
7.2 Toplevel directives
The following directives control the toplevel behavior, load files in memory,
and trace program execution.
#quit;;
Exit the toplevel loop and terminate the ocaml command.
#directory "dir-name";;
Add the given directory to the list of directories searched for source
and compiled files.
Chapter 7. The toplevel system (ocaml) 86
#cd "dir-name";;
Change the current working directory.
#load "file-name";;
Load in memory a bytecode object file (.cmo file) produced by the batch
compiler ocamlc.
#use "file-name";;
Read, compile and execute source phrases from the given file. This is
textual inclusion: phrases are processed just as if they were typed on
standard input. The reading of the file stops at the first error
encountered.
#install_printer printer-name;;
This directive registers the function named printer-name (a value path)
as a printer for objects whose types match the argument type of the
function. That is, the toplevel loop will call printer-name when it has
such an object to print. The printing function printer-name must use the
Format library module to produce its output, otherwise its output will
not be correctly located in the values printed by the toplevel loop.
#remove_printer printer-name;;
Remove the named function from the table of toplevel printers.
#trace function-name;;
After executing this directive, all calls to the function named
function-name will be ``traced''. That is, the argument and the result
are displayed for each call, as well as the exceptions escaping out of
the function, raised either by the function itself or by another function
it calls. If the function is curried, each argument is printed as it is
passed to the function.
#untrace function-name;;
Stop tracing the given function.
#untrace_all;;
Stop tracing all functions traced so far.
#print_depth n;;
Limit the printing of values to a maximal depth of n. The parts of
values whose depth exceeds n are printed as ... (ellipsis).
#print_length n;;
Limit the number of value nodes printed to at most n. Remaining parts of
values are printed as ... (ellipsis).
7.3 The toplevel and the module system
Toplevel phrases can refer to identifiers defined in compilation units with
the same mechanisms as for separately compiled units: either by using
qualified names (Modulename.localname), or by using the open construct and
unqualified names (see section 4.3).
However, before referencing another compilation unit, an implementation of
that unit must be present in memory. At start-up, the toplevel system
contains implementations for all the modules in the the standard library.
Implementations for user modules can be entered with the #load directive
described above. Referencing a unit for which no implementation has been
Chapter 7. The toplevel system (ocaml) 87
provided results in the error ``Reference to undefined global `...'''.
Note that entering open mod merely accesses the compiled interface (.cmi
file) for mod, but does not load the implementation of mod, and does not cause
any error if no implementation of mod has been loaded. The error ``reference
to undefined global mod'' will occur only when executing a value or module
definition that refers to mod.
7.4 Common errors
This section describes and explains the most frequently encountered error
messages.
Cannot find file filename
The named file could not be found in the current directory, nor in the
directories of the search path.
If filename has the format mod.cmi, this means you have referenced the
compilation unit mod, but its compiled interface could not be found.
Fix: compile mod.mli or mod.ml first, to create the compiled interface
mod.cmi.
If filename has the format mod.cmo, this means you are trying to load
with #load a bytecode object file that does not exist yet. Fix: compile
mod.ml first.
If your program spans several directories, this error can also appear
because you haven't specified the directories to look into. Fix: use
the #directory directive to add the correct directories to the search
path.
This expression has type t1, but is used with type t2
See section 6.4.
Reference to undefined global mod
You have neglected to load in memory an implementation for a module with
#load. See section 7.3 above.
7.5 Building custom toplevel systems: ocamlmktop
The ocamlmktop command builds Objective Caml toplevels that contain user code
preloaded at start-up.
The ocamlmktop command takes as argument a set of .cmo and .cma files, and
links them with the object files that implement the Objective Caml toplevel.
The typical use is:
ocamlmktop -o mytoplevel foo.cmo bar.cmo gee.cmo
This creates the bytecode file mytoplevel, containing the Objective Caml
toplevel system, plus the code from the three .cmo files. This toplevel is
directly executable and is started by:
./mytoplevel
This enters a regular toplevel loop, except that the code from foo.cmo,
bar.cmo and gee.cmo is already loaded in memory, just as if you had typed:
#load "foo.cmo";;
Chapter 7. The toplevel system (ocaml) 88
#load "bar.cmo";;
#load "gee.cmo";;
on entrance to the toplevel. The modules Foo, Bar and Gee are not opened,
though; you still have to do
open Foo;;
yourself, if this is what you wish.
7.6 Options
The following command-line options are recognized by ocamlmktop.
-cclib libname
Pass the -llibname option to the C linker when linking in ``custom
runtime'' mode. See the corresponding option for ocamlc, in chapter 6.
-ccopt option
Pass the given option to the C compiler and linker, when linking in
``custom runtime'' mode. See the corresponding option for ocamlc, in
chapter 6.
-custom
Link in ``custom runtime'' mode. See the corresponding option for
ocamlc, in chapter 6.
-I directory
Add the given directory to the list of directories searched for compiled
object code files (.cmo and .cma).
-o exec-file
Specify the name of the toplevel file produced by the linker. The
default is a.out.
Chapter 8
The runtime system (ocamlrun)
The ocamlrun command executes bytecode files produced by the linking phase of
the ocamlc command.
8.1 Overview
The ocamlrun command comprises three main parts: the bytecode interpreter,
that actually executes bytecode files; the memory allocator and garbage
collector; and a set of C functions that implement primitive operations such
as input/output.
The usage for ocamlrun is:
ocamlrun options bytecode-executable arg1 ... argn
The first non-option argument is taken to be the name of the file containing
the executable bytecode. (That file is searched in the executable path as
well as in the current directory.) The remaining arguments are passed to the
Caml Light program, in the string array Sys.argv. Element 0 of this array is
the name of the bytecode executable file; elements 1 to n are the remaining
arguments arg1 to argn.
As mentioned in chapter 6, in most cases, the bytecode executable files
produced by the ocamlc command are self-executable, and manage to launch the
ocamlrun command on themselves automatically. That is, assuming caml.out is a
bytecode executable file,
caml.out arg1 ... argn
works exactly as
ocamlrun caml.out arg1 ... argn
Notice that it is not possible to pass options to ocamlrun when invoking
caml.out directly.
8.2 Options
The following command-line option is recognized by ocamlrun.
-v When set, the memory manager prints verbose messages on standard error to
signal garbage collections and heap extensions.
The following environment variable are also consulted:
89
Chapter 8. The runtime system (ocamlrun) 90
CAMLRUNPARAM
Set the garbage collection parameters. This variable must be a sequence
of parameter specifications. A parameter specification is an option
letter followed by an = sign, a decimal number, and an optional
multiplier. There are seven options, the first six correspond to the
fields of the control record documented in section 16.8:
s (minor_heap_size) Size of the minor heap.
i (major_heap_increment) Minimum size increment for the major heap.
o (space_overhead) The major GC speed setting.
O (max_overhead) The heap compaction trigger setting.
v (verbose) Whether to print GC messages or not. 0 is false; 1 is
true; other values may give unexpected results.
l (stack_limit) The limit (in words) of the stack size.
h The initial size of the major heap (in words).
10 20 30
The multiplier is k, M, or G, for multiplication by 2 , 2 , and 2
respectively. For example, on a 32-bit machine, under bash the command
export CAMLRUNPARAM='s=256k,v=1'
tells a subsequent ocamlrun to set its initial minor heap size to
1 megabyte and to print its GC messages.
PATH
List of directories searched to find the bytecode executable file.
8.3 Common errors
This section describes and explains the most frequently encountered error
messages.
filename: no such file or directory
If filename is the name of a self-executable bytecode file, this means
that either that file does not exist, or that it failed to run the
ocamlrun bytecode interpreter on itself. The second possibility
indicates that Objective Caml has not been properly installed on your
system.
Cannot exec camlrun
(When launching a self-executable bytecode file.) The ocamlrun could not
be found in the executable path. Check that Objective Caml has been
properly installed on your system.
Cannot find the bytecode file
The file that ocamlrun is trying to execute (e.g. the file given as
Chapter 8. The runtime system (ocamlrun) 91
first non-option argument to ocamlrun) either does not exist, or is not a
valid executable bytecode file.
Truncated bytecode file
The file that ocamlrun is trying to execute is not a valid executable
bytecode file. Probably it has been truncated or mangled since created.
Erase and rebuild it.
Uncaught exception
The program being executed contains a ``stray'' exception. That is, it
raises an exception at some point, and this exception is never caught.
This causes immediate termination of the program. The name of the
exception is printed, but not its arguments.
Out of memory
The program being executed requires more memory than available. Either
the program builds excessively large data structures; or the program
contains too many nested function calls, and the stack overflows. In
some cases, your program is perfectly correct, it just requires more
memory than your machine provides. In other cases, the ``out of memory''
message reveals an error in your program: non-terminating recursive
function, allocation of an excessively large array or string, attempts to
build an infinite list or other data structure, ...
To help you diagnose this error, run your program with the -v option to
ocamlrun. If it displays lots of ``Growing stack...'' messages, this is
probably a looping recursive function. If it displays lots of ``Growing
heap...'' messages, with the heap size growing slowly, this is probably
an attempt to construct a data structure with too many (infinitely many?)
cells. If it displays few ``Growing heap...'' messages, but with a huge
increment in the heap size, this is probably an attempt to build an
excessively large array or string.
Chapter 9
Native-code compilation (ocamlopt)
This chapter describes the Objective Caml high-performance native-code
compiler ocamlopt, which compiles Caml source files to native code object
files and link these object files to produce standalone executables.
The native-code compiler is only available on certain platforms. It
produces code that runs faster than the bytecode produced by ocamlc, at the
cost of increased compilation time and executable code size. Compatibility
with the bytecode compiler is extremely high: the same source code should run
identically when compiled with ocamlc and ocamlopt.
It is not possible to mix native-code object files produced by ocamlc with
bytecode object files produced by ocamlopt: a program must be compiled
entirely with ocamlopt or entirely with ocamlc. Native-code object files
produced by ocamlopt cannot be loaded in the toplevel system ocaml.
9.1 Overview of the compiler
The ocamlopt command has a command-line interface very close to that of
ocamlc. It accepts the same types of arguments:
- Arguments ending in .mli are taken to be source files for compilation
unit interfaces. Interfaces specify the names exported by compilation
units: they declare value names with their types, define public data
types, declare abstract data types, and so on. From the file x.mli, the
ocamlopt compiler produces a compiled interface in the file x.cmi. The
interface produced is identical to that produced by the bytecode compiler
ocamlc.
- Arguments ending in .ml are taken to be source files for compilation unit
implementations. Implementations provide definitions for the names
exported by the unit, and also contain expressions to be evaluated for
their side-effects. From the file x.ml, the ocamlopt compiler produces
two files: x.o, containing native object code, and x.cmx, containing
extra information for linking and optimization of the clients of the
unit. The compiled implementation should always be referred to under the
name x.cmx (when given a .o file, ocamlopt assumes that it contains code
compiled from C, not from Caml).
The implementation is checked against the interface file x.mli (if it
exists) as described in the manual for ocamlc (chapter 6).
- Arguments ending in .cmx are taken to be compiled object code. These
files are linked together, along with the object files obtained by
compiling .ml arguments (if any), and the Caml Light standard library, to
92
Chapter 9. Native-code compilation (ocamlopt) 93
produce a native-code executable program. The order in which .cmx and
.ml arguments are presented on the command line is relevant: compilation
units are initialized in that order at run-time, and it is a link-time
error to use a component of a unit before having initialized it. Hence,
a given x.cmx file must come before all .cmx files that refer to the unit
x.
- Arguments ending in .cmxa are taken to be libraries of object code. Such
a library packs in two files (lib.cmxa and lib.a) a set of object files
(.cmx/.o files). Libraries are build with ocamlopt -a (see the
description of the -a option below). The object files contained in the
library are linked as regular .cmx files (see above), in the order
specified when the library was built. The only difference is that if an
object file contained in a library is not referenced anywhere in the
program, then it is not linked in.
- Arguments ending in .c are passed to the C compiler, which generates a .o
object file. This object file is linked with the program.
- Arguments ending in .o or .a are assumed to be C object files and
libraries. They are linked with the program.
The output of the linking phase is a regular Unix executable file. It does
not need ocamlrun to run.
9.2 Options
The following command-line options are recognized by ocamlopt.
-a Build a library (.cmxa/.a file) with the object files (.cmx/.o files)
given on the command line, instead of linking them into an executable
file. The name of the library can be set with the -o option. The
default name is library.cmxa.
-c Compile only. Suppress the linking phase of the compilation. Source
code files are turned into compiled files, but no executable file is
produced. This option is useful to compile modules separately.
-cclib -llibname
Pass the -llibname option to the linker. This causes the given C library
to be linked with the program.
-ccopt option
Pass the given option to the C compiler and linker. For instance, -ccopt
-Ldir causes the C linker to search for C libraries in directory dir.
-compact
Optimize the produced code for space rather than for time. This results
in slightly smaller but slightly slower programs. The default is to
optimize for speed.
-i Cause the compiler to print all defined names (with their inferred types
or their definitions) when compiling an implementation (.ml file). This
can be useful to check the types inferred by the compiler. Also, since
the output follows the syntax of interfaces, it can help in writing an
explicit interface (.mli file) for a file: just redirect the standard
output of the compiler to a .mli file, and edit that file to remove all
Chapter 9. Native-code compilation (ocamlopt) 94
declarations of unexported names.
-I directory
Add the given directory to the list of directories searched for compiled
interface files (.cmi) and compiled object code files (.cmx). By
default, the current directory is searched first, then the standard
library directory. Directories added with -I are searched after the
current directory, in the order in which they were given on the command
line, but before the standard library directory.
-inline n
Set aggressiveness of inlining to n, where n is a positive integer.
Specifying -inline 0 prevents all functions from being inlined, except
those whose body is smaller than the call site. Thus, inlining causes no
expansion in code size. The default aggressiveness, -inline 1, allows
slightly larger functions to be inlined, resulting in a slight expansion
in code size. Higher values for the -inline option cause larger and
larger functions to become candidate for inlining, but can result in a
serious increase in code size.
-linkall
Forces all modules contained in libraries to be linked in. If this flag
is not given, unreferenced modules are not linked in. When building a
library (-a flag), setting the -linkall flag forces all subsequent links
of programs involving that library to link all the modules contained in
the library.
-o exec-file
Specify the name of the output file produced by the linker. The default
output name is a.out, in keeping with the Unix tradition. If the -a
option is given, specify the name of the library produced. If the
-output-obj option is given, specify the name of the output file
produced.
-output-obj
Cause the linker to produce a C object file instead of an executable
file. This is useful to wrap Caml code as a C library, callable from any
C program. See chapter 14, section 14.6.5. The name of the output
object file is camlprog.o by default; it can be set with the -o option.
-pp command
Cause the compiler to call the given command as a preprocessor for each
source file. The output of command is redirected to an intermediate
file, which is compiled. If there are no compilation errors, the
intermediate file is deleted afterwards. The name of this file is built
from the basename of the source file with the extension .ppi for an
interface (.mli) file and .ppo for an implementation (.ml) file.
-S Keep the assembly code produced during the compilation. The assembly
code for the source file x.ml is saved in the file x.s.
-thread
Compile or link multithreaded programs, in combination with the threads
library described in chapter 20. What this option actually does is
select a special, thread-safe version of the standard library.
-unsafe
Turn bound checking off on array and string accesses (the v.(i) and s.[i]
Chapter 9. Native-code compilation (ocamlopt) 95
constructs). Programs compiled with -unsafe are therefore faster, but
unsafe: anything can happen if the program accesses an array or string
outside of its bounds.
-v Print the version number of the compiler.
9.3 Common errors
The error messages are almost identical to those of ocamlc. See section 6.4.
9.4 Compatibility with the bytecode compiler
This section lists the known incompatibilities between the bytecode compiler
and the native-code compiler. Except on those points, the two compilers
should generate code that behave identically.
- The following operations abort the program (either by printing an error
message or just via an hardware trap or fatal Unix signal) instead of
raising an exception:
- out-of bounds accesses to arrays and strings;
- integer division by zero, modulus by zero;
- stack overflow;
- on the Alpha processor only, floating-point operations involving
infinite or denormalized numbers (all other processors supported by
ocamlopt treat these numbers correctly, as per the IEEE 754
standard).
In particular, notice that stack overflow caused by excessively deep
recursion is reported by most Unix kernels as a ``segmentation
violation'' signal.
- The following library functions print a fatal error message and abort the
program instead of raising the Invalid_argument exception:
- structural comparisons (=), (<>), etc., when encountering a
functional value;
- Array.create and String.create when the requested size is negative or
exceeds the memory manager limits;
- output_value when encountering a functional value or pointers outside
the Caml heap (such as input-output buffers).
- Signals are detected only when the program performs an allocation in the
heap. That is, if a signal is delivered while in a piece of code that
does not allocate, its handler will not be called until the next heap
allocation.
Chapter 9. Native-code compilation (ocamlopt) 96
The best way to avoid running into those incompatibilities is to never trap
the Invalid_argument, Division_by_zero, and Stack_overflow exceptions, thus
also treating them as fatal errors with the bytecode compiler as well as with
the native-code compiler. Test the divisor or array/string index before
performing the operation instead of trapping the exception afterwards.
Chapter 10
Lexer and parser generators (ocamllex, ocamlyacc)
This chapter describes two program generators: ocamllex, that produces a
lexical analyzer from a set of regular expressions with associated semantic
actions, and ocamlyacc, that produces a parser from a grammar with associated
semantic actions.
These program generators are very close to the well-known lex and yacc
commands that can be found in most C programming environments. This chapter
assumes a working knowledge of lex and yacc: while it describes the input
syntax for ocamllex and ocamlyacc and the main differences with lex and yacc,
it does not explain the basics of writing a lexer or parser description in lex
and yacc. Readers unfamiliar with lex and yacc are referred to ``Compilers:
principles, techniques, and tools'' by Aho, Sethi and Ullman (Addison-Wesley,
1986), or ``Lex & Yacc'', by Levine, Mason and Brown (O'Reilly, 1992).
10.1 Overview of ocamllex
The ocamllex command produces a lexical analyzer from a set of regular
expressions with attached semantic actions, in the style of lex. Assuming the
input file is lexer.mll, executing
ocamllex lexer.mll
produces Caml code for a lexical analyzer in file lexer.ml. This file defines
one lexing function per entry point in the lexer definition. These functions
have the same names as the entry points. Lexing functions take as argument a
lexer buffer, and return the semantic attribute of the corresponding entry
point.
Lexer buffers are an abstract data type implemented in the standard library
module Lexing. The functions Lexing.from_channel, Lexing.from_string and
Lexing.from_function create lexer buffers that read from an input channel, a
character string, or any reading function, respectively. (See the description
of module Lexing in chapter 15.)
When used in conjunction with a parser generated by ocamlyacc, the semantic
actions compute a value belonging to the type token defined by the generated
parsing module. (See the description of ocamlyacc below.)
10.2 Syntax of lexer definitions
The format of lexer definitions is as follows:
{ header }
rule entrypoint =
parse regexp { action }
97
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 98
| ...
| regexp { action }
and entrypoint =
parse ...
and ...
{ trailer }
Comments are delimited by (* and *), as in Caml.
10.2.1 Header and trailer
The header and trailer sections are arbitrary Caml text enclosed in curly
braces. Either or both can be omitted. If present, the header text is copied
as is at the beginning of the output file and the trailer text at the end.
Typically, the header section contains the open directives required by the
actions, and possibly some auxiliary functions used in the actions.
10.2.2 Entry points
The names of the entry points must be valid value identifiers.
10.2.3 Regular expressions
The regular expressions are in the style of lex, with a more Caml-like syntax.
' char '
A character constant, with the same syntax as Objective Caml character
constants. Match the denoted character.
_ (Underscore.) Match any character.
eof Match the end of the lexer input.
Note: On some systems, with interactive input, and end-of-file may be
followed by more characters. However, ocamllex will not correctly handle
regular expressions that contain eof followed by something else.
" string "
A string constant, with the same syntax as Objective Caml string
constants. Match the corresponding sequence of characters.
[ character-set ]
Match any single character belonging to the given character set. Valid
character sets are: single character constants ' c '; ranges of
characters ' c1 ' - ' c2 ' (all characters between c1 and c2, inclusive);
and the union of two or more character sets, denoted by concatenation.
[ ^ character-set ]
Match any single character not belonging to the given character set.
regexp *
(Repetition.) Match the concatenation of zero or more strings that match
regexp.
regexp +
(Strict repetition.) Match the concatenation of one or more strings that
match regexp.
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 99
regexp ?
(Option.) Match either the empty string, or a string matching regexp.
regexp1 | regexp2
(Alternative.) Match any string that matches either regexp1 or regexp2
regexp1 regexp2
(Concatenation.) Match the concatenation of two strings, the first
matching regexp1, the second matching regexp2.
( regexp )
Match the same strings as regexp.
Concerning the precedences of operators, * and + have highest precedence,
followed by ?, then concatenation, then | (alternation).
10.2.4 Actions
The actions are arbitrary Caml expressions. They are evaluated in a context
where the identifier lexbuf is bound to the current lexer buffer. Some
typical uses for lexbuf, in conjunction with the operations on lexer buffers
provided by the Lexing standard library module, are listed below.
Lexing.lexeme lexbuf
Return the matched string.
Lexing.lexeme_char lexbuf n
th
Return the n character in the matched string. The first character
corresponds to n=0.
Lexing.lexeme_start lexbuf
Return the absolute position in the input text of the beginning of the
matched string. The first character read from the input text has
position 0.
Lexing.lexeme_end lexbuf
Return the absolute position in the input text of the end of the matched
string. The first character read from the input text has position 0.
entrypoint lexbuf
(Where entrypoint is the name of another entry point in the same lexer
definition.) Recursively call the lexer on the given entry point.
Useful for lexing nested comments, for example.
10.3 Overview of ocamlyacc
The ocamlyacc command produces a parser from a context-free grammar
specification with attached semantic actions, in the style of yacc. Assuming
the input file is grammar.mly, executing
ocamlyacc options grammar.mly
produces Caml code for a parser in the file grammar.ml, and its interface in
file grammar.mli.
The generated module defines one parsing function per entry point in the
grammar. These functions have the same names as the entry points. Parsing
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 100
functions take as arguments a lexical analyzer (a function from lexer buffers
to tokens) and a lexer buffer, and return the semantic attribute of the
corresponding entry point. Lexical analyzer functions are usually generated
from a lexer specification by the ocamllex program. Lexer buffers are an
abstract data type implemented in the standard library module Lexing. Tokens
are values from the concrete type token, defined in the interface file
grammar.mli produced by ocamlyacc.
10.4 Syntax of grammar definitions
Grammar definitions have the following format:
%{
header
%}
declarations
%%
rules
%%
trailer
Comments are enclosed between /* and */ (as in C) in the ``declarations''
and ``rules'' sections, and between (* and *) (as in Caml) in the ``header''
and ``trailer'' sections.
10.4.1 Header and trailer
The header and the trailer sections are Caml code that is copied as is into
file grammar.ml. Both sections are optional. The header goes at the
beginning of the output file; it usually contains open directives and
auxiliary functions required by the semantic actions of the rules. The
trailer goes at the end of the output file.
10.4.2 Declarations
Declarations are given one per line. They all start with a % sign.
%token symbol...symbol
Declare the given symbols as tokens (terminal symbols). These symbols
are added as constant constructors for the token concrete type.
%token < type > symbol...symbol
Declare the given symbols as tokens with an attached attribute of the
given type. These symbols are added as constructors with arguments of
the given type for the token concrete type. The type part is an
arbitrary Caml type expression, except that all type constructor names
must be fully qualified (e.g. Modname.typename) for all types except
standard built-in types, even if the proper open directives (e.g.
open Modname) were given in the header section. That's because the
header is copied only to the .ml output file, but not to the .mli output
file, while the type part of a %token declaration is copied to both.
%start symbol...symbol
Declare the given symbols as entry points for the grammar. For each
entry point, a parsing function with the same name is defined in the
output module. Non-terminals that are not declared as entry points have
no such parsing function. Start symbols must be given a type with the
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 101
%type directive below.
%type < type > symbol...symbol
Specify the type of the semantic attributes for the given symbols. This
is mandatory for start symbols only. Other nonterminal symbols need not
be given types by hand: these types will be inferred when running the
output files through the Objective Caml compiler (unless the -s option is
in effect). The type part is an arbitrary Caml type expression, except
that all type constructor names must be fully qualified, as explained
above for %token.
%left symbol...symbol
%right symbol...symbol
%nonassoc symbol...symbol
Associate precedences and associativities to the given symbols. All
symbols on the same line are given the same precedence. They have higher
precedence than symbols declared before in a %left, %right or %nonassoc
line. They have lower precedence than symbols declared after in a %left,
%right or %nonassoc line. The symbols are declared to associate to the
left (%left), to the right (%right), or to be non-associative
(%nonassoc). The symbols are usually tokens. They can also be dummy
nonterminals, for use with the %prec directive inside the rules.
10.4.3 Rules
The syntax for rules is as usual:
nonterminal :
symbol ... symbol { semantic-action }
| ...
| symbol ... symbol { semantic-action }
;
Rules can also contain the %prec symbol directive in the right-hand side part,
to override the default precedence and associativity of the rule with the
precedence and associativity of the given symbol.
Semantic actions are arbitrary Caml expressions, that are evaluated to
produce the semantic attribute attached to the defined nonterminal. The
semantic actions can access the semantic attributes of the symbols in the
right-hand side of the rule with the $ notation: $1 is the attribute for the
first (leftmost) symbol, $2 is the attribute for the second symbol, etc.
The rules may contain the special symbol error to indicate resynchronization
points, as in yacc.
Actions occurring in the middle of rules are not supported.
10.4.4 Error handling
Error recovery is supported as follows: when the parser reaches an error
state (no grammar rules can apply), it calls a function named parse_error with
the string syntax error as argument. The default parse_error function does
nothing and returns, thus initiating error recovery (see below). The user can
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 102
define a customized parse_error function in the header section of the grammar
file.
The parser also enters error recovery mode if one of the grammar actions
raise the Parsing.Parse_error exception.
In error recovery mode, the parser discards states from the stack until it
reaches a place where the error token can be shifted. It then discards tokens
from the input until it finds three successive tokens that can be accepted,
and starts processing with the first of these. If no state can be uncovered
where the error token can be shifted, then the parser aborts by raising the
Parsing.Parse_error exception.
Refer to documentation on yacc for more details and guidance in how to use
error recovery.
10.5 Options
The ocamlyacc command recognizes the following options:
-v Generate a description of the parsing tables and a report on conflicts
resulting from ambiguities in the grammar. The description is put in
file grammar.output.
-bprefix
Name the output files prefix.ml, prefix.mli, prefix.output, instead of
the default naming convention.
10.6 A complete example
The all-time favorite: a desk calculator. This program reads arithmetic
expressions on standard input, one per line, and prints their values. Here is
the grammar definition:
/* File parser.mly */
%token <int> INT
%token PLUS MINUS TIMES DIV
%token LPAREN RPAREN
%token EOL
%left PLUS MINUS /* lowest precedence */
%left TIMES DIV /* medium precedence */
%nonassoc UMINUS /* highest precedence */
%start main /* the entry point */
%type <int> main
%%
main:
expr EOL { $1 }
;
expr:
INT { $1 }
| LPAREN expr RPAREN { $2 }
| expr PLUS expr { $1 + $3 }
| expr MINUS expr { $1 - $3 }
| expr TIMES expr { $1 * $3 }
| expr DIV expr { $1 / $3 }
| MINUS expr %prec UMINUS { - $2 }
;
Here is the definition for the corresponding lexer:
Chapter 10. Lexer and parser generators (ocamllex, ocamlyacc) 103
(* File lexer.mll *)
{
open Parser (* The type token is defined in parser.mli *)
exception Eof
}
rule token = parse
[' ' '\t'] { token lexbuf } (* skip blanks *)
| ['\n' ] { EOL }
| ['0'-'9']+ { INT(int_of_string(Lexing.lexeme lexbuf)) }
| '+' { PLUS }
| '-' { MINUS }
| '*' { TIMES }
| '/' { DIV }
| '(' { LPAREN }
| ')' { RPAREN }
| eof { raise Eof }
Here is the main program, that combines the parser with the lexer:
(* File calc.ml *)
let _ =
try
let lexbuf = Lexing.from_channel stdin in
while true do
let result = Parser.main Lexer.token lexbuf in
print_int result; print_newline(); flush stdout
done
with Lexer.Eof ->
exit 0
To compile everything, execute:
ocamllex lexer.mll # generates lexer.ml
ocamlyacc parser.mly # generates parser.ml and parser.mli
ocamlc -c parser.mli
ocamlc -c lexer.ml
ocamlc -c parser.ml
ocamlc -c calc.ml
ocamlc -o calc lexer.cmo parser.cmo calc.cmo
Chapter 11
Dependency generator (ocamldep)
The ocamldep command scans a set of Objective Caml source files (.ml and .mli
files) for references to external compilation units, and outputs dependency
lines in a format suitable for the make utility. This ensures that make will
compile the source files in the correct order, and recompile those files that
need to when a source file is modified.
The typical usage is:
ocamldep options *.mli *.ml > .depend
where *.mli *.ml expands to all source files in the current directory and
.depend is the file that should contain the dependencies. (See below for a
typical Makefile.)
Dependencies are generated both for compiling with the bytecode compiler
ocamlc and with the native-code compiler ocamlopt.
11.1 Options
The following command-line option is recognized by ocamldep.
-I directory
Add the given directory to the list of directories searched for source
files. If a source file foo.ml mentions an external compilation unit
Bar, a dependency on that unit's interface bar.cmi is generated only if
the source for bar is found in the current directory or in one of the
directories specified with -I. Otherwise, Bar is assumed to be a module
form the standard library, and no dependencies are generated. For
programs that span multiple directories, it is recommended to pass
ocamldep the same -I options that are passed to the compiler.
11.2 A typical Makefile
Here is a template Makefile for a Objective Caml program.
CSLC=ocamlc
CSLOPT=ocamlopt
CSLDEP=ocamldep
INCLUDES= #all relevant -I options here
CSLFLAGS=$(INCLUDES) #add other options for ocamlc here
CSLOPTFLAGS=$(INCLUDES) #add other options for ocamlopt here
# prog1 should be compiled to bytecode, and is composed of three
# units: mod1, mod2 and mod3.
104
Chapter 11. Dependency generator (ocamldep) 105
# The list of object files for prog1
PROG1_OBJS=mod1.cmo mod2.cmo mod3.cmo
prog1: $(PROG1_OBJS)
$(CSLC) -o prog1 $(CSLFLAGS) $(PROG1_OBJS)
# prog2 should be compiled to native-code, and is composed of two
# units: mod4 and mod5.
# The list of object files for prog2
PROG2_OBJS=mod4.cmx mod5.cmx
prog2: $(PROG2_OBJS)
$(CSLOPT) -o prog2 $(CSLFLAGS) $(PROG2_OBJS)
# Common rules
.SUFFIXES: .ml .mli .cmo .cmi .cmx
.ml.cmo:
$(CSLC) $(CSLFLAGS) -c $<
.mli.cmi:
$(CSLC) $(CSLFLAGS) -c $<
.ml.cmx:
$(CSLOPT) $(CSLOPTFLAGS) -c $<
# Clean up
clean:
rm -f prog1 prog2
rm -f *.cm[iox]
# Dependencies
depend:
$(CSLDEP) $(INCLUDES) *.mli *.ml > .depend
include .depend
Chapter 12
The debugger (ocamldebug)
This chapter describes the Objective Caml source-level replay debugger
ocamldebug.
Unix:
The debugger is available on Unix systems that provides BSD sockets.
Windows:
The debugger is not available.
Mac:
The debugger is not available.
12.1 Compiling for debugging
Before the debugger can be used, the program must be compiled and linked with
the -g option: all .cmo and .cma files that are part of the program should
have been created with ocamlc -g, and they must be linked together with ocamlc
-g.
Compiling with -g entails no penalty on the running time of programs:
object files and bytecode executable files are bigger and take longer to
produce, but the executable files run at exactly the same speed as if they had
been compiled without -g.
12.2 Invocation
12.2.1 Starting the debugger
The Objective Caml debugger is invoked by running the program ocamldebug with
the name of the bytecode executable file as first argument:
ocamldebug [options] program [arguments]
The arguments following program are optional, and are passed as command-line
arguments to the program being debugged. (See also the set arguments
command.)
The following command-line options are recognized:
-I directory
Add directory to the list of directories searched for source files and
compiled files. (See also the directory command.)
106
Chapter 12. The debugger (ocamldebug) 107
-s socket
Use socket for communicating with the debugged program. See the
description of the command set socket (section 12.8.6) for the format of
socket.
-c count
Set the maximum number of simultaneously live checkpoints to count.
-cd directory
Run the debugger program from the working directory directory, instead of
the current directory. (See also the cd command.)
-emacs
Tell the debugger it is executed under Emacs. (See section 12.10 for
information on how to run the debugger under Emacs.)
12.2.2 Exiting the debugger
The command quit exits the debugger. You can also exit the debugger by typing
an end-of-file character (usually ctrl-D).
Typing an interrupt character (usually ctrl-C) will not exit the debugger,
but will terminate the action of any debugger command that is in progress and
return to the debugger command level.
12.3 Commands
A debugger command is a single line of input. It starts with a command name,
which is followed by arguments depending on this name. Examples:
run
goto 1000
set arguments arg1 arg2
A command name can be truncated as long as there is no ambiguity. For
instance, go 1000 is understood as goto 1000, since there are no other
commands whose name starts with go. For the most frequently used commands,
ambiguous abbreviations are allowed. For instance, r stands for run even
though there are others commands starting with r. You can test the validity
of an abbreviation using the help command.
If the previous command has been successful, a blank line (typing just RET)
will repeat it.
12.3.1 Getting help
The Objective Caml debugger has a simple on-line help system, which gives a
brief description of each command and variable.
help
Print the list of commands.
help command
Give help about the command command.
help set variable, help show variable
Give help about the variable variable. The list of all debugger
variables can be obtained with help set.
Chapter 12. The debugger (ocamldebug) 108
help info topic
Give help about topic. Use help info to get a list of known topics.
12.3.2 Accessing the debugger state
set variable value
Set the debugger variable variable to the value value.
show variable
Print the value of the debugger variable variable.
info subject
Give information about the given subject. For instance, info breakpoints
will print the list of all breakpoints.
12.4 Executing a program
12.4.1 Events
Events are ``interesting'' locations in the source code, corresponding to the
beginning or end of evaluation of ``interesting'' sub-expressions. Events are
the unit of single-stepping (stepping goes to the next or previous event
encountered in the program execution). Also, breakpoints can only be set at
events. Thus, events play the role of line numbers in debuggers for
conventional languages.
During program execution, a counter is incremented at each event
encountered. The value of this counter is referred as the current time.
Thanks to reverse execution, it is possible to jump back and forth to any time
of the execution.
Here is where the debugger events (written <) are located in the source
code:
- Following a function application:
(f arg)<
- On entrance to a function:
fun x y z -> < ...
- On each case of a pattern-matching definition (function, match...with
construct, try...with construct):
function pat1 -> < expr1
| ...
| patN -> < exprN
- Between subexpressions of a sequence:
expr1; < expr2; >< ...; >< exprN
Chapter 12. The debugger (ocamldebug) 109
- In the two branches of a conditional expression:
if cond then < expr1 else >< expr2
- At the beginning of each iteration of a loop:
while cond do < body done
for i = a to b do < body done
Exceptions: A function application followed by a function return is replaced
by the compiler by a jump (tail-call optimization). In this case, no event is
put after the function application.
12.4.2 Starting the debugged program
The debugger starts executing the debugged program only when needed. This
allows setting breapoints or assigning debugger variables before execution
starts. There are several ways to start execution:
run Run the program until a breakpoint is hit, or the program terminates.
step 0
Load the program and stop on the first event.
goto time
Load the program and execute it until the given time. Useful when you
already know approximately at what time the problem appears. Also useful
to set breakpoints on function values that have not been computed at time
0 (see section 12.5).
The execution of a program is affected by certain information it receives
when the debugger starts it, such as the command-line arguments to the program
and its working directory. The debugger provides commands to specify this
information (set arguments and cd). These commands must be used before
program execution starts. If you try to change the arguments or the working
directory after starting your program, the debugger will kill the program
(after asking for confirmation).
12.4.3 Running the program
The following commands execute the program forward or backward, starting at
the current time. The execution will stop either when specified by the
command or when a breakpoint is encountered.
run Execute the program forward from current time. Stops at next breakpoint
or when the program terminates.
reverse
Execute the program backward from current time. Mostly useful to go to
the last breakpoint encountered before the current time.
step [count]
Run the program and stop at the next event. With an argument, do it
Chapter 12. The debugger (ocamldebug) 110
count times.
backstep [count]
Run the program backward and stop at the previous event. With an
argument, do it count times.
next [count]
Run the program and stop at the next event, skipping over function calls.
With an argument, do it count times.
previous [count]
Run the program backward and stop at the previous event, skipping over
function calls. With an argument, do it count times.
finish
Run the program until the current function returns.
start
Run the program backward and stop at the first event before the current
function invocation.
12.4.4 Time travel
You can jump directly to a given time, without stopping on breakpoints, using
the goto command.
As you move through the program, the debugger maintains an history of the
successive times you stop at. The last command can be used to revisit these
times: each last command moves one step back through the history. That is
useful mainly to undo commands such as step and next.
goto time
Jump to the given time.
last [count]
Go back to the latest time recorded in the execution history. With an
argument, do it count times.
set history size
Set the size of the execution history.
12.4.5 Killing the program
kill
Kill the program being executed. This command is mainly useful if you
wish to recompile the program without leaving the debugger.
12.5 Breakpoints
A breakpoint causes the program to stop whenever a certain point in the
program is reached. It can be set in several ways using the break command.
Breakpoints are assigned numbers when set, for further reference. The most
comfortable way to set breakpoints is through the Emacs interface (see
section 12.10).
break
Set a breakpoint at the current position in the program execution. The
current position must be on an event (i.e., neither at the beginning, nor
Chapter 12. The debugger (ocamldebug) 111
at the end of the program).
break function
Set a breakpoint at the beginning of function. This works only when the
functional value of the identifier function has been computed and
assigned to the identifier. Hence this command cannot be used at the
very beginning of the program execution, when all identifiers are still
undefined; use goto time to advance execution until the functional value
is available.
break @ [module] line
Set a breakpoint in module module (or in the current module if module is
not given), at the first event of line line.
break @ [module] line column
Set a breakpoint in module module (or in the current module if module is
not given), at the event closest to line line, column column.
break @ [module] # character
Set a breakpoint in module module at the event closest to character
number character.
break address
Set a breakpoint at the code address address.
delete [breakpoint-numbers]
Delete the specified breakpoints. Without argument, all breakpoints are
deleted (after asking for confirmation).
info breakpoints
Print the list of all breakpoints.
12.6 The call stack
Each time the program performs a function application, it saves the location
of the application (the return address) in a block of data called a stack
frame. The frame also contains the local variables of the caller function.
All the frames are allocated in a region of memory called the call stack. The
command backtrace (or bt) displays parts of the call stack.
At any time, one of the stack frames is ``selected'' by the debugger;
several debugger commands refer implicitly to the selected frame. In
particular, whenever you ask the debugger for the value of a local variable,
the value is found in the selected frame. The commands frame, up and down
select whichever frame you are interested in.
When the program stops, the debugger automatically selects the currently
executing frame and describes it briefly as the frame command does.
frame
Describe the currently selected stack frame.
frame frame-number
Select a stack frame by number and describe it. The frame currently
executing when the program stopped has number 0; its caller has number 1;
and so on up the call stack.
backtrace [count], bt [count]
Print the call stack. This is useful to see which sequence of function
Chapter 12. The debugger (ocamldebug) 112
calls led to the currently executing frame. With a positive argument,
print only the innermost count frames. With a negative argument, print
only the outermost -count frames.
up [count]
Select and display the stack frame just ``above'' the selected frame,
that is, the frame that called the selected frame. An argument says how
many frames to go up.
down [count]
Select and display the stack frame just ``below'' the selected frame,
that is, the frame that was called by the selected frame. An argument
says how many frames to go down.
12.7 Examining variable values
The debugger can print the current value of simple expressions. The
expressions can involve program variables: all the identifiers that are in
scope at the selected program point can be accessed.
Expressions that can be printed are a subset of Objective Caml expressions,
as described by the following grammar:
expr ::= lowercase-ident
| {uppercase-ident .} lowercase-ident
| *
| $ integer
| expr . lowercase-ident
| expr .( integer )
| expr .[ integer ]
| ! expr
| ( expr )
The first two cases refer to a value identifier, either unqualified or
qualified by the path to the structure that define it. * refers to the result
just computed (typically, the value of a function application), and is valid
only if the selected event is an ``after'' event (typically, a function
application). $ integer refer to a previously printed value. The remaining
four forms select part of an expression: respectively, a record field, an
array element, a string element, and the current contents of a reference.
print variables
Print the values of the given variables. print can be abbreviated as p.
display variables
Same as print, but limit the depth of printing to 1. Useful to browse
large data structures without printing them in full. display can be
abbreviated as d.
When printing a complex expression, a name of the form $integer is
automatically assigned to its value. Such names are also assigned to parts of
the value that cannot be printed because the maximal printing depth is
exceeded. Named values can be printed later on with the commands p $integer
or d $integer. Named values are valid only as long as the program is stopped.
They are forgotten as soon as the program resumes execution.
set print_depth d
Limit the printing of values to a maximal depth of d.
Chapter 12. The debugger (ocamldebug) 113
set print_length l
Limit the printing of values to at most l nodes printed.
12.8 Controlling the debugger
12.8.1 Setting the program name and arguments
set program file
Set the program name to file.
set arguments arguments
Give arguments as command-line arguments for the program.
A shell is used to pass the arguments to the debugged program. You can
therefore use wildcards, shell variables, and file redirections inside the
arguments. To debug programs that read from standard input, it is recommended
to redirect their input from a file (using set arguments < input-file),
otherwise input to the program and input to the debugger are not properly
separated, and inputs are not properly replayed when running the program
backwards.
12.8.2 How programs are loaded
The loadingmode variable controls how the program is executed.
set loadingmode direct
The program is run directly by the debugger. This is the default mode.
set loadingmode runtime
The debugger execute the Objective Caml runtime camlrun on the program.
Rarely useful; moreover it prevents the debugging of programs compiled in
``custom runtime'' mode.
set loadingmode manual
The user starts manually the program, when asked by the debugger. Allows
remote debugging (see section 12.8.6).
12.8.3 Search path for files
The debugger searches for source files and compiled interface files in a list
of directories, the search path. The search path initially contains the
current directory . and the standard library directory. The directory
command adds directories to the path.
Whenever the search path is modified, the debugger will clear any
information it may have cached about the files.
directory directorynames
Add the given directories to the search path. These directories are
added at the front, and will therefore be searched first.
directory
Reset the search path. This requires confirmation.
12.8.4 Working directory
Each time a program is started in the debugger, it inherits its working
directory from the current working directory of the debugger. This working
Chapter 12. The debugger (ocamldebug) 114
directory is initially whatever it inherited from its parent process
(typically the shell), but you can specify a new working directory in the
debugger with the cd command or the -cd command-line option.
cd directory
Set the working directory for camldebug to directory.
pwd Print the working directory for camldebug.
12.8.5 Turning reverse execution on and off
In some cases, you may want to turn reverse execution off. This speeds up the
program execution, and is also sometimes useful for interactive programs.
Normally, the debugger takes checkpoints of the program state from time to
time. That is, it makes a copy of the current state of the program (using the
Unix system call fork). If the variable checkpoints is set to off, the
debugger will not take any checkpoints.
set checkpoints on/off
Select whether the debugger makes checkpoints or not.
12.8.6 Communication between the debugger and the program
The debugger communicate with the program being debugged through a Unix
socket. You may need to change the socket name, for example if you need to
run the debugger on a machine and your program on another.
set socket socket
Use socket for communication with the program. socket can be either a
file name, or an Internet port specification host:port, where host is a
host name or an Internet address in dot notation, and port is a port
number on the host.
On the debugged program side, the socket name is passed either by the -D
command line option to camlrun, or through the CAML_DEBUG_SOCKET environment
variable.
12.8.7 Fine-tuning the debugger
Several variables enables to fine-tune the debugger. Reasonable defaults are
provided, and you should normally not have to change them.
set processcount count
Set the maximum number of checkpoints to count. More checkpoints
facilitate going far back in time, but use more memory and create more
Unix processes.
As checkpointing is quite expensive, it must not be done too often. On the
other hand, backward execution is faster when checkpoints are taken more
often. In particular, backward single-stepping is more responsive when many
checkpoints have been taken just before the current time. To fine-tune the
checkpointing strategy, the debugger does not take checkpoints at the same
frequency for long displacements (e.g. run) and small ones (e.g. step). The
two variables bigstep and smallstep contain the number of events between two
checkpoints in each case.
Chapter 12. The debugger (ocamldebug) 115
set bigstep count
Set the number of events between two checkpoints for long displacements.
set smallstep count
Set the number of events between two checkpoints for small displacements.
The following commands display information on checkpoints and events:
info checkpoints
Print a list of checkpoints.
info events [module]
Print the list of events in the given module (the current module, by
default).
12.8.8 User-defined printers
Just as in the toplevel system (section 7.2), the user can register functions
for printing values of certain types. For technical reasons, the debugger
cannot call printing functions that reside in the program being debugged. The
code for the printing functions must therefore be loaded explicitly in the
debugger.
load_printer "file-name"
Load in the debugger the indicated .cmo or .cma object file. The file is
loaded in an environment consisting only of the Objective Caml standard
library plus the definitions provided by object files previously loaded
using load_printer. If this file depends on other object files not yet
loaded, the debugger automatically loads them if it is able to find them
in the search path. The loaded file does not have direct access to the
modules of the program being debugged.
install_printer printer-name
Register the function named printer-name (a value path) as a printer for
objects whose types match the argument type of the function. That is,
the debugger will call printer-name when it has such an object to print.
The printing function printer-name must use the Format library module to
produce its output, otherwise its output will not be correctly located in
the values printed by the toplevel loop.
The value path printer-name must refer to one of the functions defined by
the object files loaded using load_printer. It cannot reference the
functions of the program being debugged.
remove_printer printer-name
Remove the named function from the table of value printers.
12.9 Miscellaneous commands
list [module] [beginning] [end]
List the source of module module, from line number beginning to line
number end. By default, 20 lines of the current module are displayed,
starting 10 lines before the current position.
source filename
Read debugger commands from the script filename.
Chapter 12. The debugger (ocamldebug) 116
12.10 Running the debugger under Emacs
The most user-friendly way to use the debugger is to run it under Emacs. See
the file emacs/README in the distribution for information on how to load the
Emacs Lisp files for Caml support.
The Caml debugger is started under Emacs by the command M-x camldebug, with
argument the name of the executable file progname to debug. Communication
with the debugger takes place in an Emacs buffer named *camldebug-progname*.
The editing and history facilities of Shell mode are available for interacting
with the debugger.
In addition, Emacs displays the source files containing the current event
(the current position in the program execution) and highlights the location of
the event. This display is updated synchronously with the debugger action.
The following bindings for the most common debugger commands are available
in the *camldebug-progname* buffer:
C-c C-s
(command step): execute the program one step forward.
C-c C-k
(command backstep): execute the program one step backward.
C-c C-n
(command next): execute the program one step forward, skipping over
function calls.
Middle mouse button
(command display): display named value. $n under mouse cursor (support
incremental browsing of large data structures).
C-c C-p
(command print): print value of identifier at point.
C-c C-d
(command display): display value of identifier at point.
C-c C-r
(command run): execute the program forward to next breakpoint.
C-c C-v
(command reverse): execute the program backward to latest breakpoint.
C-c C-l
(command last): go back one step in the command history.
C-c C-t
(command backtrace): display backtrace of function calls.
C-c C-f
(command finish): run forward till the current function returns.
C-c <
(command up): select the stack frame below the current frame.
C-c >
(command down): select the stack frame above the current frame.
In all buffers in Caml editing mode, the following debugger commands are
Chapter 12. The debugger (ocamldebug) 117
also available:
C-x C-a C-b
(command break): set a breakpoint at event closest to point
C-x C-a C-p
(command print): print value of identifier at point
C-x C-a C-d
(command display): display value of identifier at point
Chapter 13
Profiling (ocamlprof)
This chapter describes how the execution of Objective Caml programs can be
profiled, by recording how many times functions are called, branches of
conditionals are taken, ...
13.1 Compiling for profiling
Before profiling an execution, the program must be compiled in profiling mode,
using the ocamlcp front-end to the ocamlc compiler (see chapter 6). When
compiling modules separately, ocamlcp must be used when compiling the modules
(production of .cmo files), and can also be used (though this is not strictly
necessary) when linking them together.
The amount of profiling information can be controlled through the -p option
to ocamlcp, followed by one or several letters indicating which parts of the
program should be profiled:
a all options
f function calls : a count point is set at the beginning of function
bodies
i if ...then ...else ... : count points are set in both then branch and
else branch
l while, for loops: a count point is set at the beginning of the loop body
m match branches: a count point is set at the beginning of the body of
each branch
t try ...with ... branches: a count point is set at the beginning of the
body of each branch
For instance, compiling with ocamlcp -pfilm profiles function calls,
if...then...else..., loops and pattern matching.
Calling ocamlcp without the -p option defaults to -p fm, meaning that only
function calls and pattern matching are profiled.
13.2 Profiling an execution
Running a bytecode executable file that has been compiled with ocamlcp records
the execution counts for the specified parts of the program and saves them in
a file called ocamlprof.dump in the current directory.
The ocamlprof.dump file is written only if the program terminates normally
118
Chapter 13. Profiling (ocamlprof) 119
(by calling exit or by falling through). It is not written if the program
terminates with an uncaught exception.
If a compatible dump file already exists in the current directory, then the
profiling information is accumulated in this dump file. This allows, for
instance, the profiling of several executions of a program on different
inputs.
13.3 Printing profiling information
The ocamlprof command produces a source listing of the program modules where
execution counts have been inserted as comments. For instance,
ocamlprof foo.ml
prints the source code for the foo module, with comments indicating how many
times the functions in this module have been called. Naturally, this
information is accurate only if the source file has not been modified since
the profiling execution took place.
The following options are recognized by ocamlprof:
-f dumpfile
Specifies an alternate dump file of profiling information
-F string
Specifies an additional string to be output with profiling information.
By default, ocamlprof will annotate programs with comments of the form (*
n *) where n is the counter value for a profiling point. With option -F
s, the annotation will be (* s n *).
13.4 Time profiling
Profiling with ocamlprof only records execution counts, not the actual time
spent into each function. There is currently no way to perform time profiling
on bytecode programs generated by ocamlc. On native-code programs generated
by ocamlopt, the standard Unix profiler prof can be used; just add the -ccopt
-p option when linking the program:
ocamlopt -o myprog -ccopt -p other-options files
./myprog
prof myprog
Function names in the output of prof have the following format:
Module-name_function-name_unique-number
Chapter 14
Interfacing C with Objective Caml
This chapter describes how user-defined primitives, written in C, can be
linked with Caml code and called from Caml functions.
14.1 Overview and compilation information
14.1.1 Declaring primitives
User primitives are declared in an implementation file or struct...end module
expression using the external keyword:
external name : type = C-function-name
This defines the value name name as a function with type type that executes by
calling the given C function. For instance, here is how the input primitive
is declared in the standard library module Pervasives:
external input : in_channel -> string -> int -> int -> int
= "input"
Primitives with several arguments are always curried. The C function does not
necessarily have the same name as the ML function.
External functions thus defined can be specified in interface files or
sig...end signatures either as regular values
val name : type
thus hiding their implementation as a C function, or explicitly as
``manifest'' external functions
external name : type = C-function-name
The latter is slightly more efficient, as it allows clients of the module to
call directly the C function instead of going through the corresponding Caml
function.
14.1.2 Implementing primitives
User primitives with arity n<5 are implemented by C functions that take n
arguments of type value, and return a result of type value. The type value is
the type of the representations for Caml values. It encodes objects of
several base types (integers, floating-point numbers, strings, ...), as well
as Caml data structures. The type value and the associated conversion
120
Chapter 14. Interfacing C with Objective Caml 121
functions and macros are described in details below. For instance, here is
the declaration for the C function implementing the input primitive:
value input(channel, buffer, offset, length)
value channel, buffer, offset, length;
{
...
}
When the primitive function is applied in a Caml program, the C function is
called with the values of the expressions to which the primitive is applied as
arguments. The value returned by the function is passed back to the Caml
program as the result of the function application.
User primitives with arity greater than 5 should be implemented by two C
functions. The first function, to be used in conjunction with the bytecode
compiler ocamlc, receives two arguments: a pointer to an array of Caml values
(the values for the arguments), and an integer which is the number of
arguments provided. The other function, to be used in conjunction with the
native-code compiler ocamlopt, takes its arguments directly. For instance,
here are the two C functions for the 7-argument primitive Nat.add_nat:
value add_nat_native(nat1, ofs1, len1, nat2, ofs2, len2, carry_in)
value nat1, ofs1, len1, nat2, ofs2, len2, carry_in;
{
...
}
value add_nat_bytecode(argv, argn)
value * argv;
int argn;
{
return add_nat_native(argv[0], argv[1], argv[2], argv[3],
argv[4], argv[5], argv[6]);
}
The names of the two C functions must be given in the primitive declaration,
as follows:
external name : type =
bytecode-C-function-name native-code-C-function-name
For instance, in the case of add_nat, the declaration is:
external add_nat: nat -> int -> int -> nat -> int -> int -> int -> int
= "add_nat_bytecode" "add_nat_native"
Implementing a user primitive is actually two separate tasks: on the one
hand, decoding the arguments to extract C values from the given Caml values,
and encoding the return value as a Caml value; on the other hand, actually
computing the result from the arguments. Except for very simple primitives,
it is often preferable to have two distinct C functions to implement these two
tasks. The first function actually implements the primitive, taking native C
values as arguments and returning a native C value. The second function,
often called the ``stub code'', is a simple wrapper around the first function
that converts its arguments from Caml values to C values, call the first
function, and convert the returned C value to Caml value. For instance, here
is the stub code for the input primitive:
value input(channel, buffer, offset, length)
Chapter 14. Interfacing C with Objective Caml 122
value channel, buffer, offset, length;
{
return Val_long(getblock((struct channel *) channel,
&Byte(buffer, Long_val(offset)),
Long_val(length)));
}
(Here, Val_long, Long_val and so on are conversion macros for the type value,
that will be described later.) The hard work is performed by the function
getblock, which is declared as:
long getblock(channel, p, n)
struct channel * channel;
char * p;
long n;
{
...
}
To write C code that operates on Objective Caml values, the following
include files are provided:
---------------------------------------------------------------------------
|Include file |Provides |
---------------------------------------------------------------------------
|caml/mlvalues.h |definition of the value type, and conversion macros |
|caml/alloc.h |allocation functions (to create structured Caml |
| |objects) |
|caml/memory.h |miscellaneous memory-related functions (for in-place |
| |modification of structures, etc). |
|caml/callback.h |callback from C to Caml (see section 14.6). |
---------------------------------------------------------------------------
These files reside in the caml/ subdirectory of the Objective Caml standard
library directory (usually /usr/local/lib/ocaml).
14.1.3 Linking C code with Caml code
The Objective Caml runtime system comprises three main parts: the bytecode
interpreter, the memory manager, and a set of C functions that implement the
primitive operations. Some bytecode instructions are provided to call these C
functions, designated by their offset in a table of functions (the table of
primitives).
In the default mode, the Caml linker produces bytecode for the standard
runtime system, with a standard set of primitives. References to primitives
that are not in this standard set result in the ``unavailable C primitive''
error.
In the ``custom runtime'' mode, the Caml linker scans the object files and
determines the set of required primitives. Then, it builds a suitable runtime
system, by calling the native code linker with:
- the table of the required primitives
- a library that provides the bytecode interpreter, the memory manager, and
the standard primitives
- libraries and object code files (.o files) mentioned on the command line
for the Caml linker, that provide implementations for the user's
primitives.
Chapter 14. Interfacing C with Objective Caml 123
This builds a runtime system with the required primitives. The Caml linker
generates bytecode for this custom runtime system. The bytecode is appended
to the end of the custom runtime system, so that it will be automatically
executed when the output file (custom runtime + bytecode) is launched.
To link in ``custom runtime'' mode, execute the ocamlc command with:
- the -custom option
- the names of the desired Caml object files (.cmo files)
- the names of the C object files and libraries (.o and .a files) that
implement the required primitives. (Under Unix, a library named
libname.a residing in one of the standard library directories can also be
specified as -cclib -lname.)
14.2 The value type
All Caml objects are represented by the C type value, defined in the include
file caml/mlvalues.h, along with macros to manipulate values of that type. An
object of type value is either:
- an unboxed integer
- a pointer to a block inside the heap (such as the blocks allocated
through one of the alloc_* functions below)
- a pointer to an object outside the heap (e.g., a pointer to a block
allocated by malloc, or to a C variable).
14.2.1 Integer values
Integer values encode 31-bit signed integers (63-bit on 64-bit architectures).
They are unboxed (unallocated).
14.2.2 Blocks
Blocks in the heap are garbage-collected, and therefore have strict structure
constraints. Each block includes a header containing the size of the block
(in words), and the tag of the block. The tag governs how the contents of the
blocks are structured. A tag lower than No_scan_tag indicates a structured
block, containing well-formed values, which is recursively traversed by the
garbage collector. A tag greater than or equal to No_scan_tag indicates a raw
block, whose contents are not scanned by the garbage collector. For the
benefits of ad-hoc polymorphic primitives such as equality and structured
input-output, structured and raw blocks are further classified according to
their tags as follows:
Chapter 14. Interfacing C with Objective Caml 124
---------------------------------------------------------------------
|Tag |Contents of the block |
---------------------------------------------------------------------
|0 to No_scan_tag- 1 |A structured block (an array of Caml objects). |
| |Each field is a value. |
|Closure_tag |A closure representing a functional value. |
| |The first word is a pointer to a piece |
| |of bytecode, the remaining words are value |
| |containing the environment. |
|String_tag |A character string. |
|Double_tag |A double-precision floating-point number. |
|Double_array_tag |An array of double-precision floating-point |
| |numbers (for the native-code compiler only). |
|Abstract_tag |A block representing an abstract datatype. |
|Final_tag |A block representing an abstract datatype with |
| |a ``finalization'' function, to be called when |
| |the block is deallocated. |
---------------------------------------------------------------------
14.2.3 Pointers to outside the heap
Any word-aligned pointer to outside the heap can be safely cast to and from
the type value. This includes pointers returned by malloc, and pointers to C
variables (of size at least one word) obtained with the & operator.
14.3 Representation of Caml data types
This section describes how Caml data types are encoded in the value type.
14.3.1 Atomic types
-------------------------------------------------
|Caml type |Encoding |
-------------------------------------------------
|int |Unboxed integer values. |
|char |Unboxed integer values (ASCII code). |
|float |Blocks with tag Double_tag. |
|string |Blocks with tag String_tag. |
-------------------------------------------------
14.3.2 Tuples and records
Tuples are represented by pointers to blocks, with tag 0.
Records are also represented by zero-tagged blocks. The ordering of labels
in the record type declaration determines the layout of the record fields:
the value associated to the label declared first is stored in field 0 of the
block, the value associated to the label declared next goes in field 1, and so
on.
The native-code compiler represents specially records whose fields all have
type float. These are represented as arrays of floating-point numbers, with
tag Double_array_tag. (See the section below on arrays.)
14.3.3 Arrays
In the bytecode compiler, all arrays are represented like tuples, that is, as
pointers to blocks tagged 0.
The native-code compiler has a special, unboxed, more efficient
representation for arrays of floating-point numbers (type float array). These
arrays are represented by pointers to blocks with tag Double_array_tag. They
should be accessed with the Double_field and Store_double_field macros.
Arrays of floating-point numbers in the bytecode compiler should be accessed
with Double_val(Field(v, n)) for reading and modify(&Field(v, n),
copy_double(d)) for writing.
Chapter 14. Interfacing C with Objective Caml 125
14.3.4 Concrete types
Constructed terms are represented either by unboxed integers (for constant
constructors) or by blocks whose tag encode the constructor (for non-constant
constructors). The constant constructors and the non-constant constructors
for a given concrete type are numbered separately, starting from 0, in the
order in which they appear in the concrete type declaration. Constant
constructors are represented by unboxed integers equal to the constructor
number. Non-constant constructors declared with a n-tuple as argument are
represented by a block of size n, tagged with the constructor number; the n
fields contain the components of its tuple argument. Other non-constant
constructors are represented by a block of size 1, tagged with the constructor
number; the field 0 contains the value of the constructor argument. Example:
---------------------------------------------------------
|Constructed term |Representation |
---------------------------------------------------------
|() |Val_int(0) |
|false |Val_int(0) |
|true |Val_int(1) |
|[] |Val_int(0) |
|h::t |Block with size = 2 and tag = 0; |
| |first field contains h, second field |
| |t |
---------------------------------------------------------
As a convenience, caml/mlvalues.h defines the macros Val_unit, Val_false and
Val_true to refer to (), false and true.
14.3.5 Objects
Objects are represented as zero-tagged blocks. The first field of the block
refers to the object class and associated method suite, in a format that
cannot easily be exploited from C. The remaining fields of the object contain
the values of the instance variables of the object. Instance variables are
stored in the order in which they appear in the class definition (taking
inherited classes into account).
14.4 Operations on values
14.4.1 Kind tests
- Is_long(v) is true if value v is an immediate integer, false otherwise
- Is_block(v) is true if value v is a pointer to a block, and false if it
is an immediate integer.
14.4.2 Operations on integers
- Val_long(l) returns the value encoding the long int l
- Long_val(v) returns the long int encoded in value v
- Val_int(i) returns the value encoding the int i
- Int_val(v) returns the int encoded in value v
Chapter 14. Interfacing C with Objective Caml 126
14.4.3 Accessing blocks
- Wosize_val(v) returns the size of value v, in words, excluding the
header.
- Tag_val(v) returns the tag of value v.
th
- Field(v, n) returns the value contained in the n field of the
structured block v. Fields are numbered from 0 to Wosize_val(v)-1.
- Code_val(v) returns the code part of the closure v.
- string_length(v) returns the length (number of characters) of the string
v.
th
- Byte(v, n) returns the n character of the string v, with type char.
Characters are numbered from 0 to string_length(v)-1.
th
- Byte_u(v, n) returns the n character of the string v, with type
unsigned char. Characters are numbered from 0 to string_length(v)-1.
- String_val(v) returns a pointer to the first byte of the string v, with
type char *. This pointer is a valid C string: there is a null
character after the last character in the string. However, Caml strings
can contain embedded null characters, that will confuse the usual C
functions over strings.
- Double_val(v) returns the floating-point number contained in value v,
with type double.
th
- (Native-code only.) Double_field(v, n) returns the n element of the
array of floating-point numbers v (a block tagged Double_array_tag).
- (Native-code only.) Store_double_field(v, n, d) stores the double
th
precision floating-point number d in the n element of the array of
floating-point numbers v.
The expressions Field(v, n), Byte(v, n) and Byte_u(v, n) are valid l-values.
Hence, they can be assigned to, resulting in an in-place modification of value
v. Assigning directly to Field(v, n) must be done with care to avoid
confusing the garbage collector (see below).
14.4.4 Allocating blocks
From the standpoint of the allocation functions, blocks are divided according
to their size as zero-sized blocks, small blocks (with size less than or equal
to Max_young_wosize), and large blocks (with size greater than to
Max_young_wosize). The constant Max_young_wosize is declared in the include
file mlvalues.h. It is guaranteed to be at least 64 (words), so that any
block with constant size less than or equal to 64 can be assumed to be small.
For blocks whose size is computed at run-time, the size must be compared
against Max_young_wosize to determine the correct allocation procedure.
Chapter 14. Interfacing C with Objective Caml 127
- Atom(t) returns an ``atom'' (zero-sized block) with tag t. Zero-sized
blocks are preallocated outside of the heap. It is incorrect to try and
allocate a zero-sized block using the functions below. For instance,
Atom(0) represents the empty array.
- alloc(n, t) returns a fresh small block of size n<Max_young_wosize
words, with tag t. If this block is a structured block (i.e. if
t<No_scan_tag), then the fields of the block (initially containing
garbage) must be initialized with legal values (using direct assignment
to the fields of the block) before the next allocation.
- alloc_tuple(n) returns a fresh small block of size n<Max_young_wosize
words, with tag 0. The fields of this block must be filled with legal
values before the next allocation or modification.
- alloc_shr(n, t) returns a fresh block of size n, with tag t. The size of
the block can be greater than Max_young_wosize. (It can also be smaller,
but in this case it is more efficient to call alloc instead of
alloc_shr.) If this block is a structured block (i.e. if
t<No_scan_tag), then the fields of the block (initially containing
garbage) must be initialized with legal values (using the initialize
function described below) before the next allocation.
- alloc_string(n) returns a string value of length n characters. The
string initially contains garbage.
- copy_string(s) returns a string value containing a copy of the
null-terminated C string s (a char *).
- copy_double(d) returns a floating-point value initialized with the double
d.
- alloc_array(f, a) allocates an array of values, calling function f over
each element of the input array a to transform it into a value. The
array a is an array of pointers terminated by the null pointer. The
function f receives each pointer as argument, and returns a value. The
zero-tagged block returned by alloc_array(f, a) is filled with the values
returned by the successive calls to f.
- copy_string_array(p) allocates an array of strings, copied from the
pointer to a string array p (a char **).
14.4.5 Raising exceptions
Two functions are provided to raise two standard exceptions:
- failwith(s), where s is a null-terminated C string (with type char *),
raises exception Failure with argument s.
- invalid_argument(s), where s is a null-terminated C string (with type
char *), raises exception Invalid_argument with argument s.
Raising arbitrary exceptions from C is more delicate: the exception
identifier is dynamically allocated by the Caml program, and therefore must be
communicated to the C function using the registration facility described below
in section 14.6.3. Once the exception identifier is recovered in C, the
following functions actually raise the exception:
Chapter 14. Interfacing C with Objective Caml 128
- raise_constant(id) raises the exception id with no argument;
- raise_with_arg(id, v) raises the exception id with the Caml value v as
argument;
- raise_with_string(id, s), where s is a null-terminated C string, raises
the exception id with a copy of the C string s as argument.
14.5 Living in harmony with the garbage collector
Unused blocks in the heap are automatically reclaimed by the garbage
collector. This requires some cooperation from C code that manipulates
heap-allocated blocks.
Rule 1 After a structured block (a block with tag less than No_scan_tag) is
allocated, all fields of this block must be filled with well-formed values
before the next allocation operation. If the block has been allocated with
alloc or alloc_tuple, filling is performed by direct assignment to the fields
of the block:
Field(v, n) = vn;
If the block has been allocated with alloc_shr, filling is performed through
the initialize function:
initialize(&Field(v, n), vn);
The next allocation can trigger a garbage collection. The garbage collector
assumes that all structured blocks contain well-formed values. Newly created
blocks contain random data, which generally do not represent well-formed
values.
If you really need to allocate before the fields can receive their final
value, first initialize with a constant value (e.g. Val_long(0)), then
allocate, then modify the fields with the correct value (see rule 3).
Rule 2 Local variables containing values must be registered with the garbage
collector (using the Begin_roots and End_roots macros), if they are to survive
a call to an allocation function.
Registration is performed with the Begin_roots set of macros.
Begin_roots1(v) registers variable v with the garbage collector. Generally, v
will be a local variable or a parameter of your function. It must be
initialized to a valid value (e.g. Val_unit) before the first allocation.
Likewise, Begin_roots2, ..., Begin_roots5 let you register up to 5 variables
at the same time. Begin_root is the same as Begin_roots1.
Begin_roots_block(ptr,size) allows you to register an array of roots. ptr is
a pointer to the first element, and size is the number of elements in the
array.
Once registered, each of your variables (or array element) has the following
properties: if it points to a heap-allocated block, this block (and its
contents) will not be reclaimed; moreover, if this block is relocated by the
garbage collector, the variable is updated to point to the new location for
the block.
Each of the Begin_roots macros open a C block that must be closed with a
matching End_roots at the same nesting level. The block must be exited
normally (i.e. not with return or goto). However, the roots are
automatically un-registered if a Caml exception is raised, so you can exit the
Chapter 14. Interfacing C with Objective Caml 129
block with failwith, invalid_argument, or one of the raise functions.
Note: The Begin_roots macros use a local variable and a structure tag named
caml__roots_block. Do not use this identifier in your programs.
Rule 3 Global variables containing values must be registered with the garbage
collector using the register_global_root function.
Registration of a global variable v is achieved by calling
register_global_root(&v) just before a valid value is stored in v for the
first time.
A registered global variable v can be un-registered by calling
remove_global_root(&v).
Rule 4 Direct assignment to a field of a block, as in
Field(v, n) = w;
is safe only if v is a block newly allocated by alloc or alloc_tuple; that is,
if no allocation took place between the allocation of v and the assignment to
the field. In all other cases, never assign directly. If the block has just
been allocated by alloc_shr, use initialize to assign a value to a field for
the first time:
initialize(&Field(v, n), w);
Otherwise, you are updating a field that previously contained a well-formed
value; then, call the modify function:
modify(&Field(v, n), w);
To illustrate the rules above, here is a C function that builds and returns
a list containing the two integers given as parameters:
value alloc_list_int(i1, i2)
int i1, i2;
{
value result = Val_unit;
value r = Val_unit;
Begin_roots2 (r, result);
r = alloc(2, 0); /* Allocate a cons cell */
Field(r, 0) = Val_int(i2); /* car = the integer i2 */
Field(r, 1) = Val_int(0); /* cdr = the empty list [] */
result = alloc(2, 0); /* Allocate the other cons cell */
Field(result, 0) = Val_int(i1); /* car = the integer i1 */
Field(result, 1) = r; /* cdr = the first cons cell */
End_roots ();
return result;
}
Here, the registering of result is not strictly needed, because no allocation
takes place after it gets its value, but it's easier and safer to simply
register all the local variables that have type value.
In the example above, the list is built bottom-up. Here is an alternate
way, that proceeds top-down. It is less efficient, but illustrates the use of
modify.
value alloc_list_int(i1, i2)
Chapter 14. Interfacing C with Objective Caml 130
int i1, i2;
{
value tail;
value r = Val_unit;
Begin_root (r);
r = alloc(2, 0); /* Allocate a cons cell */
Field(r, 0) = Val_int(i1); /* car = the integer i1 */
Field(r, 1) = Val_int(0); /* A dummy value
tail = alloc(2, 0); /* Allocate the other cons cell */
Field(tail, 0) = Val_int(i2); /* car = the integer i2 */
Field(tail, 1) = Val_int(0); /* cdr = the empty list [] */
modify(&Field(r, 1), tail); /* cdr of the result = tail */
End_roots ();
return r;
}
It would be incorrect to perform Field(r, 1) = tail directly, because the
allocation of tail has taken place since r was allocated. tail is not
registered as a root because there is no allocation between the assignment
where it takes its value and the modify statement that uses the value.
14.6 Callbacks from C to Caml
So far, we have described how to call C functions from Caml. In this section,
we show how C functions can call Caml functions, either as callbacks (Caml
calls C which calls Caml), or because the main program is written in C.
14.6.1 Applying Caml closures from C
C functions can apply Caml functional values (closures) to Caml values. The
following functions are provided to perform the applications:
- callback(f, a) applies the functional value f to the value a and return
the value returned by f.
- callback2(f, a, b) applies the functional value f (which is assumed to be
a curried Caml function with two arguments) to a and b.
- callback3(f, a, b, c) applies the functional value f (a curried Caml
function with three arguments) to a, b and c.
If the function f does not return, but raises an exception that escapes the
scope of the application, then this exception is propagated to the next
enclosing Caml code, skipping over the C code. That is, if a Caml function f
calls a C function g that calls back a Caml function h that raises a stray
exception, then the execution of g is interrupted and the exception is
propagated back into f.
14.6.2 Registering Caml closures for use in C functions
The main difficulty with the callback functions described above is obtaining a
closure to the Caml function to be called. For this purpose, Objective Caml
provides a simple registration mechanism, by which Caml code can register Caml
functions under some global name, and then C code can retrieve the
corresponding closure by this global name.
On the Caml side, registration is performed by evaluating Callback.register
Chapter 14. Interfacing C with Objective Caml 131
n v. Here, n is the global name (an arbitrary string) and v the Caml value.
For instance:
let f x = print_string "f is applied to "; print_int n; print_newline()
let _ = Callback.register "test function" f
On the C side, a pointer to the value registered under name n is obtained by
calling caml_named_value(n). The returned pointer must then be dereferenced
to recover the actual Caml value. If no value is registered under the name n,
the null pointer is returned. For example, here is a C wrapper that calls the
Caml function f above:
void call_caml_f(int arg)
{
callback(*caml_named_value("test function"), Val_int(arg));
}
The pointer returned by caml_named_value is constant and can safely be
cached in a C variable to avoid repeated name lookups. On the other hand, the
value pointed to can change during garbage collection and must always be
recomputed at the point of use. Here is a more efficient variant of
call_caml_f above that calls caml_named_value only once:
void call_caml_f(int arg)
{
static value * closure_f = NULL;
if (closure_f == NULL) {
/* First time around, look up by name */
closure_f = caml_named_value("test function");
}
callback(*closure_f, Val_int(arg));
}
14.6.3 Registering Caml exceptions for use in C functions
The registration mechanism described above can also be used to communicate
exception identifiers from Caml to C. The Caml code registers the exception by
evaluating Callback.register_exception n exn, where n is an arbitrary name and
exn is an exception value of the exception to register. For example:
exception Error of string
let _ = Callback.register_exception "test exception" (Error "any string")
The C code can then recover the exception identifier using caml_named_value
and pass it as first argument to the functions raise_constant, raise_with_arg,
and raise_with_string (described in section 14.4.5) to actually raise the
exception. For example, here is a C function that raises the Error exception
with the given argument:
void raise_error(char * msg)
{
raise_with_string(*caml_named_value("test exception"), msg);
}
Chapter 14. Interfacing C with Objective Caml 132
14.6.4 Main program in C
In normal operation, a mixed Caml/C program starts by executing the Caml
initialization code, which then can proceed to call C functions. We say that
the main program is the Caml code. In some applications, it is desirable that
the C code plays the role of the main program, calling Caml functions when
needed. This can be achieved as follows:
- The C part of the program must provide a main function, which will
override the default main function provided by the Caml runtime system.
Execution will start in the user-defined main function just like for a
regular C program.
- At some point, the C code must call caml_main(argv) to initialize the
Caml code. The argv argument is a C array of strings (type char **)
which represents the command-line arguments, as passed as second argument
to main. The Caml array Sys.argv will be initialized from this
parameter. For the bytecode compiler, argv[0] and argv[1] are also
consulted to find the file containing the bytecode.
- The call to caml_main initializes the Caml runtime system, loads the
bytecode (in the case of the bytecode compiler), and executes the
initialization code of the Caml program. Typically, this initialization
code registers callback functions using Callback.register. Once the Caml
initialization code is complete, control returns to the C code that
called caml_main.
- The C code can then invoke Caml functions using the callback mechanism
(see section 14.6.1).
14.6.5 Embedding the Caml code in the C code
The bytecode compiler in custom runtime mode (ocamlc -custom) normally appends
the bytecode to the executable file containing the custom runtime. This has
two consequences. First, the final linking step must be performed by ocamlc.
Second, the Caml runtime library must be able to find the name of the
executable file from the command-line arguments. When using caml_main(argv)
as in section 14.6.4, this means that argv[0] or argv[1] must contain the
executable file name.
An alternative is to embed the bytecode in the C code. The -output-obj
option to ocamlc is provided for this purpose. It causes the ocamlc compiler
to output a C object file (.o file) containing the bytecode for the Caml part
of the program, as well as a caml_startup function. The C object file
produced by ocamlc -output-obj can then be linked with C code using the
standard C compiler, or stored in a C library.
The caml_startup function must be called from the main C program in order to
initialize the Caml runtime and execute the Caml initialization code. Just
like caml_main, it takes one argv parameter containing the command-line
parameters. Unlike caml_main, this argv parameter is used only to initialize
Sys.argv, but not for finding the name of the executable file.
The native-code compiler ocamlopt also supports the -output-obj option,
causing it to output a C object file containing the native code for all Caml
modules on the command-line, as well as the Caml startup code. Initialization
is performed by calling caml_startup as in the case of the bytecode compiler.
Warning: On some ports, special options are required on the final linking
phase that links together the object file produced by the -output-obj option
and the remainder of the program. Those options are shows in the
Chapter 14. Interfacing C with Objective Caml 133
configuration file config/Makefile generated during compilation of Objective
Caml, as the variables BYTECCLINKOPTS (for object files produced by ocamlc
-output-obj) and NATIVECCLINKOPTS (for object files produced by ocamlopt
-output-obj). Currently, the only ports that require special attention are:
- Digital Unix on the Alpha: object files produced by ocamlc -output-obj
must be linked with the -taso option. This is not necessary for object
files produced by ocamlopt -output-obj.
- Windows NT: the object file produced by Objective Caml have been compiled
with the /MT flag, and therefore all other object files linked with it
should also be compiled with /MT.
14.7 A complete example
This section outlines how the functions from the Unix curses library can be
made available to Objective Caml programs. First of all, here is the
interface curses.mli that declares the curses primitives and data types:
type window (* The type "window" remains abstract *)
external initscr: unit -> window = "curses_initscr"
external endwin: unit -> unit = "curses_endwin"
external refresh: unit -> unit = "curses_refresh"
external wrefresh : window -> unit = "curses_wrefresh"
external newwin: int -> int -> int -> int -> window = "curses_newwin"
external mvwin: window -> int -> int -> unit = "curses_mvwin"
external addch: char -> unit = "curses_addch"
external mvwaddch: window -> int -> int -> char -> unit = "curses_mvwaddch"
external addstr: string -> unit = "curses_addstr"
external mvwaddstr: window -> int -> int -> string -
> unit = "curses_mvwaddstr"
(* lots more omitted *)
To compile this interface:
ocamlc -c curses.mli
To implement these functions, we just have to provide the stub code; the
core functions are already implemented in the curses library. The stub code
file, curses.o, looks like:
#include <curses.h>
#include <mlvalues.h>
value curses_initscr(unit)
value unit;
{
return (value) initscr(); /* OK to coerce directly from WIN-
DOW * to value
since that's a block created by malloc() */
}
value curses_wrefresh(win)
value win;
{
wrefresh((WINDOW *) win);
return Val_unit;
Chapter 14. Interfacing C with Objective Caml 134
}
value curses_newwin(nlines, ncols, x0, y0)
value nlines, ncols, x0, y0;
{
return (value) newwin(Int_val(nlines), Int_val(ncols),
Int_val(x0), Int_val(y0));
}
value curses_addch(c)
value c;
{
addch(Int_val(c)); /* Characters are encoded like integers */
return Val_unit;
}
value curses_addstr(s)
value s;
{
addstr(String_val(s));
return Val_unit;
}
/* This goes on for pages. */
(Actually, it would be better to create a library for the stub code, with each
stub code function in a separate file, so that linking would pick only those
functions from the curses library that are actually used.)
The file curses.c can be compiled with:
cc -c -I/usr/local/lib/ocaml curses.c
or, even simpler,
ocamlc -c curses.c
(When passed a .c file, the ocamlc command simply calls the C compiler on that
file, with the right -I option.)
Now, here is a sample Caml program test.ml that uses the curses module:
open Curses
let main_window = initscr () in
let small_window = newwin 10 5 20 10 in
mvwaddstr main_window 10 2 "Hello";
mvwaddstr small_window 4 3 "world";
refresh();
for i = 1 to 100000 do () done;
endwin()
To compile this program, run:
ocamlc -c test.ml
Finally, to link everything together:
ocamlc -custom -o test test.cmo curses.o -cclib -lcurses
Chapter 14. Interfacing C with Objective Caml 135
14.8 Advanced example with callbacks
This section illustrates the callback facilities described in section 14.6.
We are going to package some Caml functions in such a way that they can be
linked with C code and called from C just like any C functions. The Caml
functions are defined in the following mod.ml Caml source:
(* File mod.ml -- some ``useful'' Caml functions *)
let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2)
let format_result n = Printf.sprintf "Result is: %d\n" n
(* Export those two functions to C *)
let _ = Callback.register "fib" fib
let _ = Callback.register "format_result" format_result
Here is the C stub code for calling these functions from C:
/* File modwrap.c -- wrappers around the Caml functions */
#include <stdio.h>
#include <string.h>
#include <caml/mlvalues.h>
#include <caml/callback.h>
int fib(int n)
{
static value * fib_closure = NULL;
if (fib_closure == NULL) fib_closure = caml_named_value("fib");
return Int_val(callback(*fib_closure, Val_int(n)));
}
char * format_result(int n)
{
static value * format_result_closure = NULL;
if (format_result_closure == NULL)
format_result_closure = caml_named_value("format_result");
return strdup(String_val(callback(*format_result_closure, Val_int(n))));
/* We copy the C string returned by String_val to the C heap
so that it remains valid after garbage collection. */
}
We now compile the Caml code to a C object file and put it in a C library
along with the stub code in modwrap.c and the Caml runtime system:
ocamlc -custom -output-obj -o modcaml.o mod.ml
ocamlc -c modwrap.c
cp /usr/local/lib/ocaml/libcamlrun.a mod.a
ar r mod.a modcaml.o modwrap.o
Now, we can use the two fonctions fib and format_result in any C program,
just like regular C functions. Just remember to call caml_startup once
before.
/* File main.c -- a sample client for the Caml functions */
Chapter 14. Interfacing C with Objective Caml 136
#include <stdio.h>
int main(int argc, char ** argv)
{
int result;
/* Initialize Caml code */
caml_startup(argv);
/* Do some computation */
result = fib(10);
printf("fib(10) = %s\n", format_result(result));
return 0;
}
To build the whole program, just invoke the C compiler as follows:
cc -o prog main.c mod.a
Part IV
The Objective Caml library
137
Chapter 15
The core library
This chapter describes the functions provided by the Caml Light core library
module: module Pervasives. This module is special in two ways:
- It is automatically linked with the user's object code files by the
ocamlc command (chapter 6).
- It is automatically ``opened'' when a compilation starts, or when the
toplevel system is launched. Hence, it is possible to use unqualified
identifiers to refer to the functions provided by the Pervasives module,
without adding a open Pervasives directive.
Conventions
The declarations from the signature of the Pervasives module are printed one
by one in typewriter font, followed by a short comment. All modules and the
identifiers they export are indexed at the end of this report.
15.1 Module Pervasives: the initially opened module
This module provides the built-in types (numbers, booleans, strings,
exceptions, references, lists, arrays, input-output channels, ...) and
the basic operations over these types.
This module is automatically opened at the beginning of each compilation.
All components of this module can therefore be referred by their short
name, without prefixing them by Pervasives.
Predefined types
type int
The type of integer numbers.
type char
The type of characters.
type string
The type of character strings.
138
Chapter 15. The core library 139
type float
The type of floating-point numbers.
type bool
The type of booleans (truth values).
type unit = ()
The type of the unit value.
type exn
The type of exception values.
type 'a array
The type of arrays whose elements have type 'a.
type 'a list = [] | :: of 'a * 'a list
The type of lists whose elements have type 'a.
type 'a option = None | Some of 'a
The type of optional values.
type ('a, 'b, 'c) format
The type of format strings. 'a is the type of the parameters of the
format, 'c is the result type for the printf-style function, and 'b is
the type of the first argument given to %a and %t printing functions (see
module Printf).
Exceptions
val raise : exn -> 'a
Raise the given exception value
exception Match_failure of (string * int * int)
Exception raised when none of the cases of a pattern-matching apply. The
arguments are the location of the pattern-matching in the source code
(file name, position of first character, position of last character).
exception Assert_failure of (string * int * int)
Exception raised when an assertion fails. The arguments are the location
of the pattern-matching in the source code (file name, position of first
character, position of last character).
exception Invalid_argument of string
Exception raised by library functions to signal that the given arguments
do not make sense.
Chapter 15. The core library 140
exception Failure of string
Exception raised by library functions to signal that they are undefined
on the given arguments.
exception Not_found
Exception raised by search functions when the desired object could not be
found.
exception Out_of_memory
Exception raised by the garbage collector when there is insufficient
memory to complete the computation.
exception Stack_overflow
Exception raised by the bytecode interpreter when the evaluation stack
reaches its maximal size. This often indicates infinite or excessively
deep recursion in the user's program.
exception Sys_error of string
Exception raised by the input/output functions to report an operating
system error.
exception End_of_file
Exception raised by input functions to signal that the end of file has
been reached.
exception Division_by_zero
Exception raised by division and remainder operations when their second
argument is null.
exception Exit
This exception is not raised by any library function. It is provided for
use in your programs.
val invalid_arg: string -> 'a
Raise exception Invalid_argument with the given string.
val failwith: string -> 'a
Raise exception Failure with the given string.
Comparisons
val (=) : 'a -> 'a -> bool
e1 = e2 tests for structural equality of e1 and e2. Mutable structures
(e.g. references and arrays) are equal if and only if their current
contents are structurally equal, even if the two mutable objects are not
the same physical object. Equality between functional values raises
Chapter 15. The core library 141
Invalid_argument. Equality between cyclic data structures may not
terminate.
val (<>) : 'a -> 'a -> bool
Negation of (=).
val (<) : 'a -> 'a -> bool
val (>) : 'a -> 'a -> bool
val (<=) : 'a -> 'a -> bool
val (>=) : 'a -> 'a -> bool
Structural ordering functions. These functions coincide with the usual
orderings over integer, string and floating-point numbers, and extend
them to a total ordering over all types. The ordering is compatible with
(=). As in the case of (=), mutable structures are compared by contents.
Comparison between functional values raises Invalid_argument. Comparison
between cyclic structures may not terminate.
val compare: 'a -> 'a -> int
compare x y returns 0 if x=y, a negative integer if x<y, and a positive
integer if x>y. The same restrictions as for = apply. compare can be
used as the comparison function required by the Set and Map modules.
val min: 'a -> 'a -> 'a
Return the smaller of the two arguments.
val max: 'a -> 'a -> 'a
Return the greater of the two arguments.
val (==) : 'a -> 'a -> bool
e1 == e2 tests for physical equality of e1 and e2. On integers and
characters, it is the same as structural equality. On mutable
structures, e1 == e2 is true if and only if physical modification of e1
also affects e2. On non-mutable structures, the behavior of (==) is
implementation-dependent, except that e1 == e2 implies e1 = e2.
val (!=) : 'a -> 'a -> bool
Negation of (==).
Boolean operations
val not : bool -> bool
The boolean negation.
val (&) : bool -> bool -> bool
val (&&) : bool -> bool -> bool
The boolean ``and''. Evaluation is sequential, left-to-right: in
e1 & e2, e1 is evaluated first, and if it returns false, e2 is not
evaluated at all.
Chapter 15. The core library 142
val (or) : bool -> bool -> bool
val (||) : bool -> bool -> bool
The boolean ``or''. Evaluation is sequential, left-to-right: in
e1 or e2, e1 is evaluated first, and if it returns true, e2 is not
evaluated at all.
Integer arithmetic
Integers are 31 bits wide (or 63 bits on 64-bit processors). All
31 63
operations are taken modulo 2 (or 2 ). They do not fail on overflow.
val (~-) : int -> int
Unary negation. You can also write -e instead of ~-e.
val succ : int -> int
succ x is x+1.
val pred : int -> int
pred x is x-1.
val (+) : int -> int -> int
Integer addition.
val (-) : int -> int -> int
Integer subtraction.
val (*) : int -> int -> int
Integer multiplication.
val (/) : int -> int -> int
val (mod) : int -> int -> int
Integer division and remainder. Raise Division_by_zero if the second
argument is 0. If one of the arguments is negative, the result is
platform-dependent.
val abs : int -> int
Return the absolute value of the argument.
val max_int: int
val min_int: int
The greatest and smallest representable integers.
Chapter 15. The core library 143
Bitwise operations
val (land) : int -> int -> int
Bitwise logical and.
val (lor) : int -> int -> int
Bitwise logical or.
val (lxor) : int -> int -> int
Bitwise logical exclusive or.
val lnot: int -> int
Bitwise logical negation.
val (lsl) : int -> int -> int
n lsl m shifts n to the left by m bits.
val (lsr) : int -> int -> int
n lsr m shifts n to the right by m bits. This is a logical shift:
zeroes are inserted regardless of the sign of n.
val (asr) : int -> int -> int
n asr m shifts n to the right by m bits. This is an arithmetic shift:
the sign bit of n is replicated.
Floating-point arithmetic
On most platforms, Caml's floating-point numbers follow the IEEE 754
standard, using double precision (64 bits) numbers. Floating-point
operations do not fail on overflow or underflow, but return denormal
numbers.
val (~-.) : float -> float
Unary negation. You can also write -.e instead of ~-.e.
val (+.) : float -> float -> float
Floating-point addition
val (-.) : float -> float -> float
Floating-point subtraction
val (*.) : float -> float -> float
Floating-point multiplication
Chapter 15. The core library 144
val (/.) : float -> float -> float
Floating-point division.
val (**) : float -> float -> float
Exponentiation
val exp : float -> float
val acos : float -> float
val asin : float -> float
val atan : float -> float
val atan2 : float -> float -> float
val cos : float -> float
val cosh : float -> float
val log : float -> float
val log10 : float -> float
val sin : float -> float
val sinh : float -> float
val sqrt : float -> float
val tan : float -> float
val tanh : float -> float
Usual transcendental functions on floating-point numbers.
val ceil : float -> float
val floor : float -> float
Round the given float to an integer value. floor f returns the greatest
integer value less than or equal to f. ceil f returns the least integer
value greater than or equal to f.
val abs_float : float -> float
Return the absolute value of the argument.
val mod_float : float -> float -> float
fmod a b returns the remainder of a with respect to b.
val frexp : float -> float * int
frexp f returns the pair of the significant and the exponent of f (when f
is zero, the significant x and the exponent n of f are equal to zero;
when f is non-zero, they are defined by f = x *. 2 ** n).
val ldexp : float -> int -> float
ldexp x n returns x *. 2 ** n.
val modf : float -> float * float
modf f returns the pair of the fractional and integral part of f.
val float : int -> float
Convert an integer to floating-point.
Chapter 15. The core library 145
val truncate : float -> int
Truncate the given floating-point number to an integer. The result is
unspecified if it falls outside the range of representable integers.
String operations
More string operations are provided in module String.
val (^) : string -> string -> string
String concatenation.
String conversion functions
val string_of_bool : bool -> string
Return the string representation of a boolean.
val string_of_int : int -> string
Return the string representation of an integer, in decimal.
val int_of_string : string -> int
Convert the given string to an integer. The string is read in decimal
(by default) or in hexadecimal, octal or binary if the string begins with
0x, 0o or 0b respectively. Raise Failure "int_of_string" if the given
string is not a valid representation of an integer.
val string_of_float : float -> string
Return the string representation of a floating-point number.
val float_of_string : string -> float
Convert the given string to a float. The result is unspecified if the
given string is not a valid representation of a float.
Pair operations
val fst : 'a * 'b -> 'a
Return the first component of a pair.
val snd : 'a * 'b -> 'b
Return the second component of a pair.
List operations
More list operations are provided in module List.
Chapter 15. The core library 146
val (@) : 'a list -> 'a list -> 'a list
List concatenation.
Input/output
type in_channel
type out_channel
The types of input channels and output channels.
val stdin : in_channel
val stdout : out_channel
val stderr : out_channel
The standard input, standard output, and standard error output for the
process.
Output functions on standard output
val print_char : char -> unit
Print a character on standard output.
val print_string : string -> unit
Print a string on standard output.
val print_int : int -> unit
Print an integer, in decimal, on standard output.
val print_float : float -> unit
Print a floating-point number, in decimal, on standard output.
val print_endline : string -> unit
Print a string, followed by a newline character, on standard output.
val print_newline : unit -> unit
Print a newline character on standard output, and flush standard output.
This can be used to simulate line buffering of standard output.
Output functions on standard error
val prerr_char : char -> unit
Print a character on standard error.
val prerr_string : string -> unit
Print a string on standard error.
Chapter 15. The core library 147
val prerr_int : int -> unit
Print an integer, in decimal, on standard error.
val prerr_float : float -> unit
Print a floating-point number, in decimal, on standard error.
val prerr_endline : string -> unit
Print a string, followed by a newline character on standard error and
flush standard error.
val prerr_newline : unit -> unit
Print a newline character on standard error, and flush standard error.
Input functions on standard input
val read_line : unit -> string
Flush standard output, then read characters from standard input until a
newline character is encountered. Return the string of all characters
read, without the newline character at the end.
val read_int : unit -> int
Flush standard output, then read one line from standard input and convert
it to an integer. Raise Failure "int_of_string" if the line read is not
a valid representation of an integer.
val read_float : unit -> float
Flush standard output, then read one line from standard input and convert
it to a floating-point number. The result is unspecified if the line
read is not a valid representation of a floating-point number.
General output functions
type open_flag =
Open_rdonly | Open_wronly | Open_append
| Open_creat | Open_trunc | Open_excl
| Open_binary | Open_text | Open_nonblock
Opening modes for open_out_gen and open_in_gen.
Open_rdonly: open for reading.
Open_wronly: open for writing.
Open_append: open for appending.
Open_creat: create the file if it does not exist.
Open_trunc: empty the file if it already exists.
Open_excl: fail if the file already exists.
Open_binary: open in binary mode (no conversion).
Open_text: open in text mode (may perform conversions).
Open_nonblock: open in non-blocking mode.
Chapter 15. The core library 148
val open_out : string -> out_channel
Open the named file for writing, and return a new output channel on that
file, positionned at the beginning of the file. The file is truncated to
zero length if it already exists. It is created if it does not already
exists. Raise Sys_error if the file could not be opened.
val open_out_bin : string -> out_channel
Same as open_out, but the file is opened in binary mode, so that no
translation takes place during writes. On operating systems that do not
distinguish between text mode and binary mode, this function behaves like
open_out.
val open_out_gen : open_flag list -> int -> string -> out_channel
open_out_gen mode rights filename opens the file named filename for
writing, as above. The extra argument mode specify the opening mode.
The extra argument rights specifies the file permissions, in case the
file must be created. open_out and open_out_bin are special cases of
this function.
val flush : out_channel -> unit
Flush the buffer associated with the given output channel, performing all
pending writes on that channel. Interactive programs must be careful
about flushing standard output and standard error at the right time.
val output_char : out_channel -> char -> unit
Write the character on the given output channel.
val output_string : out_channel -> string -> unit
Write the string on the given output channel.
val output : out_channel -> string -> int -> int -> unit
output chan buff ofs len writes len characters from string buff, starting
at offset ofs, to the output channel chan. Raise
Invalid_argument "output" if ofs and len do not designate a valid
substring of buff.
val output_byte : out_channel -> int -> unit
Write one 8-bit integer (as the single character with that code) on the
given output channel. The given integer is taken modulo 256.
val output_binary_int : out_channel -> int -> unit
Write one integer in binary format on the given output channel. The only
reliable way to read it back is through the input_binary_int function.
The format is compatible across all machines for a given version of
Objective Caml.
Chapter 15. The core library 149
val output_value : out_channel -> 'a -> unit
Write the representation of a structured value of any type to a channel.
Circularities and sharing inside the value are detected and preserved.
The object can be read back, by the function input_value. See the
description of module Marshal for more information. output_value is
equivalent to Marshal.to_channel with an empty list of flags.
val seek_out : out_channel -> int -> unit
seek_out chan pos sets the current writing position to pos for channel
chan. This works only for regular files. On files of other kinds (such
as terminals, pipes and sockets), the behavior is unspecified.
val pos_out : out_channel -> int
Return the current writing position for the given channel.
val out_channel_length : out_channel -> int
Return the total length (number of characters) of the given channel.
This works only for regular files. On files of other kinds, the result
is meaningless.
val close_out : out_channel -> unit
Close the given channel, flushing all buffered write operations. The
behavior is unspecified if any of the functions above is called on a
closed channel.
General input functions
val open_in : string -> in_channel
Open the named file for reading, and return a new input channel on that
file, positionned at the beginning of the file. Raise Sys_error if the
file could not be opened.
val open_in_bin : string -> in_channel
Same as open_in, but the file is opened in binary mode, so that no
translation takes place during reads. On operating systems that do not
distinguish between text mode and binary mode, this function behaves like
open_in.
val open_in_gen : open_flag list -> int -> string -> in_channel
open_in_gen mode rights filename opens the file named filename for
reading, as above. The extra arguments mode and rights specify the
opening mode and file permissions. open_in and open_in_bin are special
cases of this function.
val input_char : in_channel -> char
Read one character from the given input channel. Raise End_of_file if
there are no more characters to read.
Chapter 15. The core library 150
val input_line : in_channel -> string
Read characters from the given input channel, until a newline character
is encountered. Return the string of all characters read, without the
newline character at the end. Raise End_of_file if the end of the file
is reached at the beginning of line.
val input : in_channel -> string -> int -> int -> int
input chan buff ofs len attempts to read len characters from channel
chan, storing them in string buff, starting at character number ofs. It
returns the actual number of characters read, between 0 and len
(inclusive). A return value of 0 means that the end of file was reached.
A return value between 0 and len exclusive means that no more characters
were available at that time; input must be called again to read the
remaining characters, if desired. Exception Invalid_argument "input" is
raised if ofs and len do not designate a valid substring of buff.
val really_input : in_channel -> string -> int -> int -> unit
really_input chan buff ofs len reads len characters from channel chan,
storing them in string buff, starting at character number ofs. Raise
End_of_file if the end of file is reached before len characters have been
read. Raise Invalid_argument "really_input" if ofs and len do not
designate a valid substring of buff.
val input_byte : in_channel -> int
Same as input_char, but return the 8-bit integer representing the
character. Raise End_of_file if an end of file was reached.
val input_binary_int : in_channel -> int
Read an integer encoded in binary format from the given input channel.
See output_binary_int. Raise End_of_file if an end of file was reached
while reading the integer.
val input_value : in_channel -> 'a
Read the representation of a structured value, as produced by
output_value, and return the corresponding value. This function is
identical to Marshal.from_channel; see the description of module Marshal
for more information, in particular concerning the lack of type safety.
val seek_in : in_channel -> int -> unit
seek_in chan pos sets the current reading position to pos for channel
chan. This works only for regular files. On files of other kinds, the
behavior is unspecified.
val pos_in : in_channel -> int
Return the current reading position for the given channel.
val in_channel_length : in_channel -> int
Chapter 15. The core library 151
Return the total length (number of characters) of the given channel.
This works only for regular files. On files of other kinds, the result
is meaningless.
val close_in : in_channel -> unit
Close the given channel. Anything can happen if any of the functions
above is called on a closed channel.
References
type 'a ref = { mutable contents: 'a }
The type of references (mutable indirection cells) containing a value of
type 'a.
val ref : 'a -> 'a ref
Return a fresh reference containing the given value.
val (!) : 'a ref -> 'a
!r returns the current contents of reference r. Could be defined as
fun r -> r.contents.
val (:=) : 'a ref -> 'a -> unit
r := a stores the value of a in reference r. Could be defined as
fun r v -> r.contents <- v.
val incr : int ref -> unit
Increment the integer contained in the given reference. Could be defined
as fun r -> r := succ !r.
val decr : int ref -> unit
Decrement the integer contained in the given reference. Could be defined
as fun r -> r := pred !r.
Program termination
val exit : int -> 'a
Flush all pending writes on stdout and stderr, and terminate the process,
returning the given status code to the operating system (usually 0 to
indicate no errors, and a small positive integer to indicate failure.)
An implicit exit 0 is performed each time a program terminates normally
(but not if it terminates because of an uncaught exception).
val at_exit: (unit -> unit) -> unit
Register the given function to be called at program termination time.
The functions registered with at_exit will be called in some unspecified
Chapter 15. The core library 152
order when the program executes exit. They will not be called if the
program terminates because of an uncaught exception.
Chapter 16
The standard library
This chapter describes the functions provided by the Caml Light standard
library. The modules from the standard library are automatically linked with
the user's object code files by the ocamlc command. Hence, these modules can
be used in standalone programs without having to add any .cmo file on the
command line for the linking phase. Similarly, in interactive use, these
globals can be used in toplevel phrases without having to load any .cmo file
in memory.
Unlike the Pervasive module from the core library, the modules from the
standard library are not automatically ``opened'' when a compilation starts,
or when the toplevel system is launched. Hence it is necessary to use
qualified identifiers to refer to the functions provided by these modules, or
to add open directives.
Conventions
For easy reference, the modules are listed below in alphabetical order of
module names. For each module, the declarations from its signature are
printed one by one in typewriter font, followed by a short comment. All
modules and the identifiers they export are indexed at the end of this report.
Overview
Here is a short listing, by theme, of the standard library modules.
Data structures:
Char p. 158 character operations
String p. 187 string operations
Array p. 155 array operations
List p. 173 list operations
Sort p. 184 sorting and merging lists
Hashtbl p. 170 hash tables and hash functions
Random p. 182 pseudo-random number generator
Set p. 182 sets over ordered types
Map p. 176 association tables over ordered types
Oo p. 179 useful functions on objects
Stack p. 185 last-in first-out stacks
Queue p. 181 first-in first-out queues
Lazy p. 172 delayed evaluation
Weak p. 191 references that don't prevent objects from being garbage-collected
153
Chapter 16. The standard library 154
Input/output:
Format p. 160 pretty printing
Marshal p. 177 marshaling of data structures
Printf p. 180 formatting printing functions
Digest p. 158 MD5 message digest
Parsing:
Genlex p. 169 a generic lexer over streams
Lexing p. 172 the run-time library for lexers generated by camllex
Parsing p. 179 the run-time library for parsers generated by camlyacc
Stream p. 185 basic functions over streams
System interface:
Arg p. 154 parsing of command line arguments
Callback p. 157 registering Caml functions to be called from C
Filename p. 159 operations on file names
Gc p. 166 memory management control and statistics
Printexc p. 180 a catch-all exception handler
Sys p. 189 system interface
16.1 Module Arg: parsing of command line arguments
This module provides a general mechanism for extracting options and
arguments from the command line to the program.
Syntax of command lines: A keyword is a character string starting with a
-. An option is a keyword alone or followed by an argument. There are
six types of keywords: Unit, Set, Clear, String, Int, and Float. Unit,
Set and Clear keywords take no argument. String, Int, and Float keywords
take the following word on the command line as an argument. Arguments
not preceded by a keyword are called anonymous arguments.
Examples (cmd is assumed to be the command name):
cmd -flag (a unit option)
cmd -int 1 (an int option with argument 1)
cmd -string foobar (a string option with argument "foobar")
cmd -float 12.34 (a float option with argument 12.34)
cmd a b c (three anonymous arguments: "a", "b", and "c")
type spec =
| Unit of (unit -> unit) (* Call the function with unit argument *)
| Set of bool ref (* Set the reference to true *)
| Clear of bool ref (* Set the reference to false *)
| String of (string -> unit) (* Call the function with a string argument *)
| Int of (int -> unit) (* Call the function with an int argument *)
| Float of (float -> unit) (* Call the function with a float argument *)
The concrete type describing the behavior associated with a keyword.
val parse : (string * spec * string) list -> (string -> unit) -> string -
> unit
Chapter 16. The standard library 155
parse speclist anonfun usage_msg parses the command line. speclist is a
list of triples (key, spec, doc). key is the option keyword, it must
start with a '-' character. spec gives the option type and the function
to call when this option is found on the command line. doc is a one-line
description of this option. anonfun is called on anonymous arguments.
The functions in spec and anonfun are called in the same order as their
arguments appear on the command line.
If an error occurs, parse exits the program, after printing an error
message as follows:
The reason for the error: unknown option, invalid or missing argument,
etc.
usage_msg
The list of options, each followed by the corresponding doc string.
For the user to be able to specify anonymous arguments starting with a -,
include for example ("--", String anonfun, doc) in speclist.
By default, parse recognizes a unit option -help, which will display
usage_msg and the list of options, and exit the program. You can
override this behaviour by specifying your own -help option in speclist.
exception Bad of string
Functions in spec or anonfun can raise Bad with an error message to
reject invalid arguments.
val usage: (string * spec * string) list -> string -> unit
usage speclist usage_msg speclist and usage_msg are the same as for
parse. usage prints the same error message that parse prints in case of
error.
val current: int ref;;
Position (in Sys.argv) of the argument being processed. You can change
this value, e.g. to force parse to skip some arguments.
16.2 Module Array: array operations
val length : 'a array -> int
Return the length (number of elements) of the given array.
val get: 'a array -> int -> 'a
Array.get a n returns the element number n of array a. The first element
has number 0. The last element has number Array.length a - 1. Raise
Invalid_argument "Array.get" if n is outside the range 0 to
(Array.length a - 1). You can also write a.(n) instead of Array.get a n.
val set: 'a array -> int -> 'a -> unit
Array.set a n x modifies array a in place, replacing element number n
with x. Raise Invalid_argument "Array.set" if n is outside the range 0
Chapter 16. The standard library 156
to Array.length a - 1. You can also write a.(n) <- x instead of
Array.set a n x.
val make: int -> 'a -> 'a array
val create: int -> 'a -> 'a array
Array.make n x returns a fresh array of length n, initialized with x.
All the elements of this new array are initially physically equal to x
(in the sense of the == predicate). Consequently, if x is mutable, it is
shared among all elements of the array, and modifying x through one of
the array entries will modify all other entries at the same time.
val init: int -> (int -> 'a) -> 'a array
Array.init n f returns a fresh array of length n, with element number i
equal to f i.
val make_matrix: int -> int -> 'a -> 'a array array
val create_matrix: int -> int -> 'a -> 'a array array
Array.make_matrix dimx dimy e returns a two-dimensional array (an array
of arrays) with first dimension dimx and second dimension dimy. All the
elements of this new matrix are initially physically equal to e. The
element (x,y) of a matrix m is accessed with the notation m.(x).(y).
val append: 'a array -> 'a array -> 'a array
Array.append v1 v2 returns a fresh array containing the concatenation of
arrays v1 and v2.
val concat: 'a array list -> 'a array
Same as Array.append, but catenates a list of arrays.
val sub: 'a array -> int -> int -> 'a array
Array.sub a start len returns a fresh array of length len, containing the
elements number start to start + len - 1 of array a. Raise
Invalid_argument "Array.sub" if start and len do not designate a valid
subarray of a; that is, if start < 0, or len < 0, or
start + len > Array.length a.
val copy: 'a array -> 'a array
Array.copy a returns a copy of a, that is, a fresh array containing the
same elements as a.
val fill: 'a array -> int -> int -> 'a -> unit
Array.fill a ofs len x modifies the array a in place, storing x in
elements number ofs to ofs + len - 1. Raise
Invalid_argument "Array.fill" if ofs and len do not designate a valid
subarray of a.
val blit: 'a array -> int -> 'a array -> int -> int -> unit
Array.blit v1 o1 v2 o2 len copies len elements from array v1, starting at
element number o1, to array v2, starting at element number o2. It works
Chapter 16. The standard library 157
correctly even if v1 and v2 are the same array, and the source and
destination chunks overlap. Raise Invalid_argument "Array.blit" if o1
and len do not designate a valid subarray of v1, or if o2 and len do not
designate a valid subarray of v2.
val to_list: 'a array -> 'a list
Array.to_list a returns the list of all the elements of a.
val of_list: 'a list -> 'a array
Array.of_list l returns a fresh array containing the elements of l.
val iter: ('a -> unit) -> 'a array -> unit
Array.iter f a applies function f in turn to all the elements of a. It
is equivalent to f a.(0); f a.(1); ...; f a.(Array.length a - 1); ().
val map: ('a -> 'b) -> 'a array -> 'b array
Array.map f a applies function f to all the elements of a, and builds an
array with the results returned by f:
[| f a.(0); f a.(1); ...; f a.(Array.length a - 1) |].
val iteri: (int -> 'a -> unit) -> 'a array -> unit
val mapi: (int -> 'a -> 'b) -> 'a array -> 'b array
Same as Array.iter and Array.map respectively, but the function is
applied to the index of the element as first argument, and the element
itself as second argument.
val fold_left: ('a -> 'b -> 'a) -> 'a -> 'b array -> 'a
Array.fold_left f x a computes f (... (f (f x a.(0)) a.(1)) ...) a.(n-1),
where n is the length of the array a.
val fold_right: ('b -> 'a -> 'a) -> 'b array -> 'a -> 'a
Array.fold_right f a x computes
f a.(0) (f a.(1) ( ... (f a.(n-1) x) ...)), where n is the length of the
array a.
16.3 Module Callback: registering Caml values with the C runtime
This module allows Caml values to be registered with the C runtime under
a symbolic name, so that C code can later call back registered Caml
functions, or raise registered Caml exceptions.
val register: string -> 'a -> unit
Callback.register n v registers the value v under the name n. C code can
later retrieve a handle to v by calling caml_named_value(n).
val register_exception: string -> exn -> unit
Chapter 16. The standard library 158
Callback.register_exception n exn registers the exception contained in
the exception value exn under the name n. C code can later retrieve a
handle to the exception by calling caml_named_value(n). The exception
value thus obtained is suitable for passign as first argument to
raise_constant or raise_with_arg.
16.4 Module Char: character operations
val code: char -> int
Return the ASCII code of the argument.
val chr: int -> char
Return the character with the given ASCII code. Raise
Invalid_argument "Char.chr" if the argument is outside the range 0--255.
val escaped : char -> string
Return a string representing the given character, with special characters
escaped following the lexical conventions of Objective Caml.
val lowercase: char -> char
val uppercase: char -> char
Convert the given character to its equivalent lowercase or uppercase
character, respectively.
16.5 Module Digest: MD5 message digest
This module provides functions to compute 128-bit ``digests'' of
arbitrary-length strings or files. The digests are cryptographic
quality: it is very hard, given a digest, to forge a string having that
digest. The algorithm used is MD5.
type t = string
The type of digests: 16-character strings.
val string: string -> t
Return the digest of the given string.
val substring: string -> int -> int -> t
Digest.substring s ofs len returns the digest of the substring of s
starting at character number ofs and containing len characters.
val channel: in_channel -> int -> t
Digest.channel ic len reads len characters from channel ic and returns
their digest.
Chapter 16. The standard library 159
val file: string -> t
Return the digest of the file whose name is given.
val output: out_channel -> t -> unit
Write a digest on the given output channel.
val input: in_channel -> t
Read a digest from the given input channel.
16.6 Module Filename: operations on file names
val current_dir_name : string
The conventional name for the current directory (e.g. . in Unix).
val concat : string -> string -> string
concat dir file returns a file name that designates file file in
directory dir.
val is_relative : string -> bool
Return true if the file name is relative to the current directory, false
if it is absolute (i.e. in Unix, starts with /.
val is_implicit : string -> bool
Return true if the file name is relative and does not start with an
explicit reference to the current directory (./ or ../ in Unix), false if
it starts with an explicit reference to the root directory or the current
directory.
val check_suffix : string -> string -> bool
check_suffix name suff returns true if the filename name ends with the
suffix suff.
val chop_suffix : string -> string -> string
chop_suffix name suff removes the suffix suff from the filename name.
The behavior is undefined if name does not end with the suffix suff.
val chop_extension : string -> string
Return the given file name without its extension. The extension is the
shortest suffix starting with a period, .xyz for instance. Raise
Invalid_argument if the given name does not contain a period.
val basename : string -> string
val dirname : string -> string
Split a file name into directory name / base file name.
concat (dirname name) (basename name) returns a file name which is
Chapter 16. The standard library 160
equivalent to name. Moreover, after setting the current directory to
dirname name (with Sys.chdir), references to basename name (which is a
relative file name) designate the same file as name before the call to
chdir.
val temp_file: string -> string -> string
temp_file prefix suffix returns the name of a non-existent temporary file
in the temporary directory. The base name of the temporary file is
formed by concatenating prefix, then a suitably chosen integer number,
then suffix. Under Unix, the temporary directory is /tmp by default; if
set, the value of the environment variable TMPDIR is used instead. Under
Windows, the name of the temporary directory is the value of the
environment variable TEMP, or C:\temp by default.
16.7 Module Format: pretty printing
This module implements a pretty-printing facility to format text within
``pretty-printing boxes''. The pretty-printer breaks lines at specified
break hints, and indents lines according to the box structure.
Rule of thumb for casual users:
use simple boxes (as obtained by open_box 0);
use simple break hints (as obtained by print_cut () that outputs a simple
break hint, or by print_space () that ouputs a space indicating a break
hint);
once a box is opened, display its material with basic printing functions
(e. g. print_int and print_string);
when the material for a box has been printed, call close_box () to close
the box;
at the end of your routine, evaluate print_newline () to close all
remaining boxes and flush the pretty-printer.
You may alternatively consider this module as providing an extension to
the printf facility: you can simply add pretty-printing annotations to
your regular printf formats, as explained below in the documentation of
the function fprintf.
The behaviour of pretty-printing commands is unspecified if there is no
opened pretty-printing box. Each box opened via one of the open_
functions below must be closed using close_box for proper formatting.
Otherwise, some of the material printed in the boxes may not be output,
or may be formatted incorrectly.
In case of interactive use, the system closes all opened boxes and
flushes all pending text (as with the print_newline function) after each
phrase. Each phrase is therefore executed in the initial state of the
pretty-printer.
Chapter 16. The standard library 161
Boxes
val open_box : int -> unit;;
open_box d opens a new pretty-printing box with offset d. This box is
the general purpose pretty-printing box. Material in this box is
displayed ``horizontal or vertical'': break hints inside the box may
lead to a new line, if there is no more room on the line to print the
remainder of the box, or if a new line may lead to a new indentation
(demonstrating the indentation of the box). When a new line is printed
in the box, d is added to the current indentation.
val close_box : unit -> unit;;
Close the most recently opened pretty-printing box.
Formatting functions
val print_string : string -> unit;;
print_string str prints str in the current box.
val print_as : int -> string -> unit;;
print_as len str prints str in the current box. The pretty-printer
formats str as if it were of length len.
val print_int : int -> unit;;
Print an integer in the current box.
val print_float : float -> unit;;
Print a floating point number in the current box.
val print_char : char -> unit;;
Print a character in the current box.
val print_bool : bool -> unit;;
Print an boolean in the current box.
Break hints
val print_space : unit -> unit;;
print_space () is used to separate items (typically to print a space
between two words). It indicates that the line may be split at this
point. It either prints one space or splits the line. It is equivalent
to print_break 1 0.
val print_cut : unit -> unit;;
Chapter 16. The standard library 162
print_cut () is used to mark a good break position. It indicates that
the line may be split at this point. It either prints nothing or splits
the line. This allows line splitting at the current point, without
printing spaces or adding indentation. It is equivalent to
print_break 0 0.
val print_break : int -> int -> unit;;
Insert a break hint in a pretty-printing box. print_break nspaces offset
indicates that the line may be split (a newline character is printed) at
this point, if the contents of the current box does not fit on one line.
If the line is split at that point, offset is added to the current
indentation. If the line is not split, nspaces spaces are printed.
val print_flush : unit -> unit;;
Flush the pretty printer: all opened boxes are closed, and all pending
text is displayed.
val print_newline : unit -> unit;;
Equivalent to print_flush followed by a new line.
val force_newline : unit -> unit;;
Force a newline in the current box. Not the normal way of
pretty-printing, you should prefer break hints.
val print_if_newline : unit -> unit;;
Execute the next formatting command if the preceding line has just been
split. Otherwise, ignore the next formatting command.
Margin
val set_margin : int -> unit;;
set_margin d sets the value of the right margin to d (in characters):
this value is used to detect line overflows that leads to split lines.
Nothing happens if d is smaller than 2 or bigger than 999999999.
val get_margin : unit -> int;;
Return the position of the right margin.
Maximum indentation limit
val set_max_indent : int -> unit;;
set_max_indent d sets the value of the maximum indentation limit to d (in
characters): once this limit is reached, boxes are rejected to the left,
if they do not fit on the current line. Nothing happens if d is smaller
than 2 or bigger than 999999999.
Chapter 16. The standard library 163
val get_max_indent : unit -> int;;
Return the value of the maximum indentation limit (in characters).
Formatting depth: maximum number of boxes allowed before ellipsis
val set_max_boxes : int -> unit;;
set_max_boxes max sets the maximum number of boxes simultaneously opened.
Material inside boxes nested deeper is printed as an ellipsis (more
precisely as the text returned by get_ellipsis_text ()). Nothing happens
if max is not greater than 1.
val get_max_boxes : unit -> int;;
Return the maximum number of boxes allowed before ellipsis.
val over_max_boxes : unit -> bool;;
Test the maximum number of boxes allowed have already been opened.
Advanced formatting
val open_hbox : unit -> unit;;
open_hbox () opens a new pretty-printing box. This box is
``horizontal'': the line is not split in this box (new lines may still
occur inside boxes nested deeper).
val open_vbox : int -> unit;;
open_vbox d opens a new pretty-printing box with offset d. This box is
``vertical'': every break hint inside this box leads to a new line.
When a new line is printed in the box, d is added to the current
indentation.
val open_hvbox : int -> unit;;
open_hvbox d opens a new pretty-printing box with offset d. This box is
``horizontal-vertical'': it behaves as an ``horizontal'' box if it fits
on a single line, otherwise it behaves as a ``vertical'' box. When a new
line is printed in the box, d is added to the current indentation.
val open_hovbox : int -> unit;;
open_hovbox d opens a new pretty-printing box with offset d. This box is
``horizontal or vertical'': break hints inside this box may lead to a
new line, if there is no more room on the line to print the remainder of
the box. When a new line is printed in the box, d is added to the
current indentation.
Tabulations
val open_tbox : unit -> unit;;
Chapter 16. The standard library 164
Open a tabulation box.
val close_tbox : unit -> unit;;
Close the most recently opened tabulation box.
val print_tbreak : int -> int -> unit;;
Break hint in a tabulation box. print_tbreak spaces offset moves the
insertion point to the next tabulation (spaces being added to this
position). Nothing occurs if insertion point is already on a tabulation
mark. If there is no next tabulation on the line, then a newline is
printed and the insertion point moves to the first tabulation of the box.
If a new line is printed, offset is added to the current indentation.
val set_tab : unit -> unit;;
Set a tabulation mark at the current insertion point.
val print_tab : unit -> unit;;
print_tab () is equivalent to print_tbreak (0,0).
Ellipsis
val set_ellipsis_text : string -> unit;;
Set the text of the ellipsis printed when too many boxes are opened (a
single dot, ., by default).
val get_ellipsis_text : unit -> string;;
Return the text of the ellipsis.
Redirecting formatter output
val set_formatter_out_channel : out_channel -> unit;;
Redirect the pretty-printer output to the given channel.
val set_formatter_output_functions :
(string -> int -> int -> unit) -> (unit -> unit) -> unit;;
set_formatter_output_functions out flush redirects the pretty-printer
output to the functions out and flush. The out function performs the
pretty-printer output. It is called with a string s, a start position p,
and a number of characters n; it is supposed to output characters p to
p+n-1 of s. The flush function is called whenever the pretty-printer is
flushed using print_flush or print_newline.
val get_formatter_output_functions :
unit -> (string -> int -> int -> unit) * (unit -> unit);;
Return the current output functions of the pretty-printer.
Chapter 16. The standard library 165
Multiple formatted output
type formatter;;
Abstract data type corresponding to a pretty-printer and all its
machinery. Defining new pretty-printers permits the output of material
in parallel on several channels. Parameters of the pretty-printer are
local to the pretty-printer: margin, maximum indentation limit, maximum
number of boxes simultaneously opened, ellipsis, and so on, are specific
to each pretty-printer and may be fixed independently. A new formatter
is obtained by calling the make_formatter function.
val std_formatter : formatter;;
The standard formatter used by the formatting functions above. It is
defined using make_formatter with output function output stdout and
flushing function fun () -> flush stdout.
val err_formatter : formatter;;
A formatter to use with formatting functions below for output to standard
error. It is defined using make_formatter with output function
output stderr and flushing function fun () -> flush stderr.
val make_formatter :
(string -> int -> int -> unit) -> (unit -> unit) -> formatter;;
make_formatter out flush returns a new formatter that writes according to
the output function out, and flushing function flush. Hence, a formatter
to out channel oc is returned by
make_formatter (output oc) (fun () -> flush oc).
val pp_open_hbox : formatter -> unit -> unit;;
val pp_open_vbox : formatter -> int -> unit;;
val pp_open_hvbox : formatter -> int -> unit;;
val pp_open_hovbox : formatter -> int -> unit;;
val pp_open_box : formatter -> int -> unit;;
val pp_close_box : formatter -> unit -> unit;;
val pp_print_string : formatter -> string -> unit;;
val pp_print_as : formatter -> int -> string -> unit;;
val pp_print_int : formatter -> int -> unit;;
val pp_print_float : formatter -> float -> unit;;
val pp_print_char : formatter -> char -> unit;;
val pp_print_bool : formatter -> bool -> unit;;
val pp_print_break : formatter -> int -> int -> unit;;
val pp_print_cut : formatter -> unit -> unit;;
val pp_print_space : formatter -> unit -> unit;;
val pp_force_newline : formatter -> unit -> unit;;
val pp_print_flush : formatter -> unit -> unit;;
val pp_print_newline : formatter -> unit -> unit;;
val pp_print_if_newline : formatter -> unit -> unit;;
val pp_open_tbox : formatter -> unit -> unit;;
val pp_close_tbox : formatter -> unit -> unit;;
val pp_print_tbreak : formatter -> int -> int -> unit;;
val pp_set_tab : formatter -> unit -> unit;;
val pp_print_tab : formatter -> unit -> unit;;
val pp_set_margin : formatter -> int -> unit;;
val pp_get_margin : formatter -> unit -> int;;
Chapter 16. The standard library 166
val pp_set_max_indent : formatter -> int -> unit;;
val pp_get_max_indent : formatter -> unit -> int;;
val pp_set_max_boxes : formatter -> int -> unit;;
val pp_get_max_boxes : formatter -> unit -> int;;
val pp_over_max_boxes : formatter -> unit -> bool;;
val pp_set_ellipsis_text : formatter -> string -> unit;;
val pp_get_ellipsis_text : formatter -> unit -> string;;
val pp_set_formatter_out_channel : formatter -> out_channel -> unit;;
val pp_set_formatter_output_functions : formatter ->
(string -> int -> int -> unit) -> (unit -> unit) -> unit;;
val pp_get_formatter_output_functions :
formatter -> unit -> (string -> int -> int -> unit) * (unit -> unit);;
The basic functions to use with formatters. These functions are the
basic ones: usual functions operating on the standard formatter are
defined via partial evaluation of these primitives. For instance,
print_string is equal to pp_print_string std_formatter.
val fprintf : formatter -> ('a, formatter, unit) format -> 'a;;
fprintf ff format arg1 ... argN formats the arguments arg1 to argN
according to the format string format, and outputs the resulting string
on the formatter ff. The format is a character string which contains
three types of objects: plain characters and conversion specifications
as specified in the printf module, and pretty-printing indications. The
pretty-printing indication characters are introduced by a @ character,
and their meanings are:
[: open a pretty-printing box. The type and offset of the box may be
optionally specified with the following syntax: the < character,
followed by an optional box type indication, then an optional integer
offset, and the closing > character. Box type is one of h, v, hv, or
hov, which stand respectively for an horizontal, vertical,
``horizontal-vertical'' and ``horizontal or vertical'' box.
]: close the most recently opened pretty-printing box.
,: output a good break as with print_cut ().
: output a space, as with print_space ().
\n: force a newline, as with force_newline ().
;: output a good break as with print_break. The nspaces and offset
parameters of the break may be optionally specified with the following
syntax: the < character, followed by an integer nspaces value, then an
integer offset, and a closing > character.
.: flush the pretty printer as with print_newline ().
@: a plain @ character.
val printf : ('a, formatter, unit) format -> 'a;;
Same as fprintf, but output on std_formatter.
val eprintf: ('a, formatter, unit) format -> 'a;;
Same as fprintf, but output on err_formatter.
16.8 Module Gc: memory management control and statistics
type stat = {
minor_words : int;
Chapter 16. The standard library 167
promoted_words : int;
major_words : int;
minor_collections : int;
major_collections : int;
heap_words : int;
heap_chunks : int;
live_words : int;
live_blocks : int;
free_words : int;
free_blocks : int;
largest_free : int;
fragments : int;
compactions : int
}
The memory management counters are returned in a stat record. All the
numbers are computed since the start of the program. The fields of this
record are:
minor_words Number of words allocated in the minor heap.
promoted_words Number of words allocated in the minor heap that survived
a minor collection and were moved to the major heap.
major_words Number of words allocated in the major heap, including the
promoted words.
minor_collections Number of minor collections.
major_collections Number of major collection cycles, not counting the
current cycle.
heap_words Total number of words in the major heap.
heap_chunks Number of times the major heap size was increased.
live_words Number of words of live data in the major heap, including the
header words.
live_blocks Number of live objects in the major heap.
free_words Number of words in the free list.
free_blocks Number of objects in the free list.
largest_free Size (in words) of the largest object in the free list.
fragments Number of wasted words due to fragmentation. These are 1-words
free blocks placed between two live objects. They cannot be inserted in
the free list, thus they are not available for allocation.
compactions Number of heap compactions.
The total amount of memory allocated by the program is (in words)
minor_words + major_words - promoted_words. Multiply by the word size (4
on a 32-bit machine, 8 on a 64-bit machine) to get the number of bytes.
type control = {
mutable minor_heap_size : int;
mutable major_heap_increment : int;
mutable space_overhead : int;
mutable verbose : bool;
mutable max_overhead : int;
mutable stack_limit : int
}
The GC parameters are given as a control record. The fields are:
minor_heap_size The size (in words) of the minor heap. Changing this
parameter will trigger a minor collection. Default: 32k.
major_heap_increment The minimum number of words to add to the major heap
when increasing it. Default: 62k.
Chapter 16. The standard library 168
space_overhead The major GC speed is computed from this parameter. This
is the memory that will be "wasted" because the GC does not immediatly
collect unreachable objects. It is expressed as a percentage of the
memory used for live data. The GC will work more (use more CPU time and
collect objects more eagerly) if space_overhead is smaller. The
computation of the GC speed assumes that the amount of live data is
constant. Default: 42.
max_overhead Heap compaction is triggered when the estimated amount of
free memory is more than max_overhead percent of the amount of live data.
If max_overhead is set to 0, heap compaction is triggered at the end of
each major GC cycle (this setting is intended for testing purposes only).
If max_overhead >= 1000000, compaction is never triggered. Default:
1000000.
verbose This flag controls the GC messages on standard error output.
Default: false.
stack_limit The maximum size of the stack (in words). This is only
relevant to the byte-code runtime, as the native code runtime uses the
operating system's stack. Default: 256k.
val stat : unit -> stat
Return the current values of the memory management counters in a stat
record.
val print_stat : out_channel -> unit
Print the current values of the memory management counters (in
human-readable form) into the channel argument.
val get : unit -> control
Return the current values of the GC parameters in a control record.
val set : control -> unit
set r changes the GC parameters according to the control record r. The
normal usage is:
let r = Gc.get () in (* Get the current parameters. *)
r.verbose <- true; (* Change some of them. *)
Gc.set r (* Set the new values. *)
val minor : unit -> unit
Trigger a minor collection.
val major : unit -> unit
Finish the current major collection cycle.
val full_major : unit -> unit
Finish the current major collection cycle and perform a complete new
cycle. This will collect all currently unreachable objects.
Chapter 16. The standard library 169
val compact : unit -> unit = "gc_compaction";;
Perform a full major collection and compact the heap. Note that heap
compaction is a lengthy operation.
16.9 Module Genlex: a generic lexical analyzer
This module implements a simple ``standard'' lexical analyzer, presented
as a function from character streams to token streams. It implements
roughly the lexical conventions of Caml, but is parameterized by the set
of keywords of your language.
type token =
Kwd of string
| Ident of string
| Int of int
| Float of float
| String of string
| Char of char
The type of tokens. The lexical classes are: Int and Float for integer
and floating-point numbers; String for string literals, enclosed in
double quotes; Char for character literals, enclosed in single quotes;
Ident for identifiers (either sequences of letters, digits, underscores
and quotes, or sequences of ``operator characters'' such as +, *, etc);
and Kwd for keywords (either identifiers or single ``special characters''
such as (, }, etc).
val make_lexer: string list -> (char Stream.t -> token Stream.t)
Construct the lexer function. The first argument is the list of
keywords. An identifier s is returned as Kwd s if s belongs to this
list, and as Ident s otherwise. A special character s is returned as
Kwd s if s belongs to this list, and cause a lexical error (exception
Parse_error) otherwise. Blanks and newlines are skipped. Comments
delimited by (* and *) are skipped as well, and can be nested.
Example: a lexer suitable for a desk calculator is obtained by
let lexer = make_lexer ["+";"-";"*";"/";"let";"="; "("; ")"]
The associated parser would be a function from token stream to, for
instance, int, and would have rules such as:
let parse_expr = parser
[< 'Int n >] -> n
| [< 'Kwd "("; n = parse_expr; 'Kwd ")" >] -> n
| [< n1 = parse_expr; n2 = parse_remainder n1 >] -> n2
and parse_remainder n1 = parser
[< 'Kwd "+"; n2 = parse_expr >] -> n1+n2
| ...
Chapter 16. The standard library 170
16.10 Module Hashtbl: hash tables and hash functions
Hash tables are hashed association tables, with in-place modification.
Generic interface
type ('a, 'b) t
The type of hash tables from type 'a to type 'b.
val create : int -> ('a,'b) t
Hashtbl.create n creates a new, empty hash table, with initial size n.
The table grows as needed, so n is just an initial guess. Better results
are said to be achieved when n is a prime number. Raise
Invalid_argument "hashtbl__new" if n is less than 1.
val clear : ('a, 'b) t -> unit
Empty a hash table.
val add : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.add tbl x y adds a binding of x to y in table tbl. Previous
bindings for x are not removed, but simply hidden. That is, after
performing remove tbl x, the previous binding for x, if any, is restored.
(This is the semantics of association lists.)
val find : ('a, 'b) t -> 'a -> 'b
Hashtbl.find tbl x returns the current binding of x in tbl, or raises
Not_found if no such binding exists.
val find_all : ('a, 'b) t -> 'a -> 'b list
Hashtbl.find_all tbl x returns the list of all data associated with x in
tbl. The current binding is returned first, then the previous bindings,
in reverse order of introduction in the table.
val remove : ('a, 'b) t -> 'a -> unit
Hashtbl.remove tbl x removes the current binding of x in tbl, restoring
the previous binding if it exists. It does nothing if x is not bound in
tbl.
val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
Hashtbl.iter f tbl applies f to all bindings in table tbl. f receives
the key as first argument, and the associated value as second argument.
The order in which the bindings are passed to f is unspecified. Each
binding is presented exactly once to f.
Functorial interface
module type HashedType =
Chapter 16. The standard library 171
sig
type t
val equal: t -> t -> bool
val hash: t -> int
end
The input signature of the functor Hashtbl.Make. t is the type of keys.
equal is the equality predicate used to compare keys. hash is a hashing
function on keys, returning a non-negative integer. It must be such that
if two keys are equal according to equal, then they must have identical
hash values as computed by hash. Examples: suitable (equal, hash) pairs
for arbitrary key types include ((=), Hashtbl.hash) for comparing objects
by structure, ((==), Hashtbl.hash) for comparing objects by addresses
(e.g. for mutable or cyclic keys).
module type S =
sig
type key
type 'a t
val create: int -> 'a t
val clear: 'a t -> unit
val add: 'a t -> key -> 'a -> unit
val remove: 'a t -> key -> unit
val find: 'a t -> key -> 'a
val find_all: 'a t -> key -> 'a list
val iter: (key -> 'a -> unit) -> 'a t -> unit
end
module Make(H: HashedType): (S with type key = H.t)
The functor Hashtbl.Make returns a structure containing a type key of
keys and a type 'a t of hash tables associating data of type 'a to keys
of type key. The operations perform similarly to those of the generic
interface, but use the hashing and equality functions specified in the
functor argument H instead of generic equality and hashing.
The polymorphic hash primitive
val hash : 'a -> int
Hashtbl.hash x associates a positive integer to any value of any type.
It is guaranteed that if x = y, then hash x = hash y. Moreover, hash
always terminates, even on cyclic structures.
val hash_param : int -> int -> 'a -> int
Hashtbl.hash_param n m x computes a hash value for x, with the same
properties as for hash. The two extra parameters n and m give more
precise control over hashing. Hashing performs a depth-first,
right-to-left traversal of the structure x, stopping after n meaningful
nodes were encountered, or m nodes, meaningful or not, were encountered.
Meaningful nodes are: integers; floating-point numbers; strings;
characters; booleans; and constant constructors. Larger values of m and
n means that more nodes are taken into account to compute the final hash
value, and therefore collisions are less likely to happen. However,
hashing takes longer. The parameters m and n govern the tradeoff between
accuracy and speed.
Chapter 16. The standard library 172
16.11 Module Lazy: deferred computations.
type 'a status =
| Delayed of (unit -> 'a)
| Value of 'a
| Exception of exn
;;
type 'a t = 'a status ref;;
A value of type 'a Lazy.t is a deferred computation (called a suspension)
that computes a result of type 'a. The expression lazy (expr) returns a
suspension that computes expr.
val force: 'a t -> 'a;;
Lazy.force x computes the suspension x and returns its result. If the
suspension was already computed, Lazy.force x returns the same value
again. If it raised an exception, the same exception is raised again.
16.12 Module Lexing: the run-time library for lexers generated by camllex
Lexer buffers
type lexbuf =
{ refill_buff : lexbuf -> unit;
mutable lex_buffer : string;
mutable lex_buffer_len : int;
mutable lex_abs_pos : int;
mutable lex_start_pos : int;
mutable lex_curr_pos : int;
mutable lex_last_pos : int;
mutable lex_last_action : int;
mutable lex_eof_reached : bool }
The type of lexer buffers. A lexer buffer is the argument passed to the
scanning functions defined by the generated scanners. The lexer buffer
holds the current state of the scanner, plus a function to refill the
buffer from the input.
val from_channel : in_channel -> lexbuf
Create a lexer buffer on the given input channel.
Lexing.from_channel inchan returns a lexer buffer which reads from the
input channel inchan, at the current reading position.
val from_string : string -> lexbuf
Create a lexer buffer which reads from the given string. Reading starts
from the first character in the string. An end-of-input condition is
generated when the end of the string is reached.
val from_function : (string -> int -> int) -> lexbuf
Create a lexer buffer with the given function as its reading method.
When the scanner needs more characters, it will call the given function,
giving it a character string s and a character count n. The function
Chapter 16. The standard library 173
should put n characters or less in s, starting at character number 0, and
return the number of characters provided. A return value of 0 means end
of input.
Functions for lexer semantic actions
The following functions can be called from the semantic actions of lexer
definitions (the ML code enclosed in braces that computes the value
returned by lexing functions). They give access to the character string
matched by the regular expression associated with the semantic action.
These functions must be applied to the argument lexbuf, which, in the
code generated by camllex, is bound to the lexer buffer passed to the
parsing function.
val lexeme : lexbuf -> string
Lexing.lexeme lexbuf returns the string matched by the regular
expression.
val lexeme_char : lexbuf -> int -> char
Lexing.lexeme_char lexbuf i returns character number i in the matched
string.
val lexeme_start : lexbuf -> int
Lexing.lexeme_start lexbuf returns the position in the input stream of
the first character of the matched string. The first character of the
stream has position 0.
val lexeme_end : lexbuf -> int
Lexing.lexeme_end lexbuf returns the position in the input stream of the
character following the last character of the matched string. The first
character of the stream has position 0.
16.13 Module List: list operations
val length : 'a list -> int
Return the length (number of elements) of the given list.
val hd : 'a list -> 'a
Return the first element of the given list. Raise Failure "hd" if the
list is empty.
val tl : 'a list -> 'a list
Return the given list without its first element. Raise Failure "tl" if
the list is empty.
val nth : 'a list -> int -> 'a
Chapter 16. The standard library 174
Return the n-th element of the given list. The first element (head of
the list) is at position 0. Raise Failure "nth" if the list is too
short.
val rev : 'a list -> 'a list
List reversal.
val concat : 'a list list -> 'a list
val flatten : 'a list list -> 'a list
Catenate (flatten) a list of lists.
Iterators
val iter : ('a -> unit) -> 'a list -> unit
List.iter f [a1; ...; an] applies function f in turn to a1; ...; an. It
is equivalent to begin f a1; f a2; ...; f an; () end.
val map : ('a -> 'b) -> 'a list -> 'b list
List.map f [a1; ...; an] applies function f to a1, ..., an, and builds
the list [f a1; ...; f an] with the results returned by f.
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a
List.fold_left f a [b1; ...; bn] is f (... (f (f a b1) b2) ...) bn.
val fold_right : ('a -> 'b -> 'b) -> 'a list -> 'b -> 'b
List.fold_right f [a1; ...; an] b is f a1 (f a2 (... (f an b) ...)).
Iterators on two lists
val iter2 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit
List.iter2 f [a1; ...; an] [b1; ...; bn] calls in turn
f a1 b1; ...; f an bn. Raise Invalid_argument if the two lists have
different lengths.
val map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
List.map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn].
Raise Invalid_argument if the two lists have different lengths.
val fold_left2 : ('a -> 'b -> 'c -> 'a) -> 'a -> 'b list -> 'c list -> 'a
List.fold_left2 f a [b1; ...; bn] [c1; ...; cn] is
f (... (f (f a b1 c1) b2 c2) ...) bn cn. Raise Invalid_argument if the
two lists have different lengths.
val fold_right2 : ('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> 'c -> 'c
Chapter 16. The standard library 175
List.fold_right2 f [a1; ...; an] [b1; ...; bn] c is
f a1 b1 (f a2 b2 (... (f an bn c) ...)). Raise Invalid_argument if the
two lists have different lengths.
List scanning
val for_all : ('a -> bool) -> 'a list -> bool
for_all p [a1; ...; an] checks if all elements of the list satisfy the
predicate p. That is, it returns (p a1) & (p a2) & ... & (p an).
val exists : ('a -> bool) -> 'a list -> bool
exists p [a1; ...; an] checks if at least one element of the list
satisfies the predicate p. That is, it returns
(p a1) or (p a2) or ... or (p an).
val for_all2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
val exists2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as for_all and exists, but for a two-argument predicate. Raise
Invalid_argument if the two lists have different lengths.
val mem : 'a -> 'a list -> bool
mem a l is true if and only if a is equal to an element of l.
val memq : 'a -> 'a list -> bool
Same as mem, but uses physical equality instead of structural equality to
compare list elements.
Association lists
val assoc : 'a -> ('a * 'b) list -> 'b
assoc a l returns the value associated with key a in the list of pairs l.
That is, assoc a [ ...; (a,b); ...] = b if (a,b) is the leftmost binding
of a in list l. Raise Not_found if there is no value associated with a
in the list l.
val mem_assoc : 'a -> ('a * 'b) list -> bool
Same as assoc, but simply return true if a binding exists, and false if
no bindings exist for the given key.
val assq : 'a -> ('a * 'b) list -> 'b
Same as assoc, but uses physical equality instead of structural equality
to compare keys.
Lists of pairs
val split : ('a * 'b) list -> 'a list * 'b list
Chapter 16. The standard library 176
Transform a list of pairs into a pair of lists:
split [(a1,b1); ...; (an,bn)] is ([a1; ...; an], [b1; ...; bn])
val combine : 'a list -> 'b list -> ('a * 'b) list
Transform a pair of lists into a list of pairs:
combine ([a1; ...; an], [b1; ...; bn]) is [(a1,b1); ...; (an,bn)]. Raise
Invalid_argument if the two lists have different lengths.
16.14 Module Map: association tables over ordered types
This module implements applicative association tables, also known as
finite maps or dictionaries, given a total ordering function over the
keys. All operations over maps are purely applicative (no side-effects).
The implementation uses balanced binary trees, and therefore searching
and insertion take time logarithmic in the size of the map.
module type OrderedType =
sig
type t
val compare: t -> t -> int
end
The input signature of the functor Map.Make. t is the type of the map
keys. compare is a total ordering function over the keys. This is a
two-argument function f such that f e1 e2 is zero if the keys e1 and e2
are equal, f e1 e2 is strictly negative if e1 is smaller than e2, and
f e1 e2 is strictly positive if e1 is greater than e2. Example: a
suitable ordering function is the generic structural comparison function
compare.
module type S =
sig
type key
The type of the map keys.
type 'a t
The type of maps from type key to type 'a.
val empty: 'a t
The empty map.
val add: key -> 'a -> 'a t -> 'a t
add x y m returns a map containing the same bindings as m, plus a binding
of x to y. If x was already bound in m, its previous binding disappears.
val find: key -> 'a t -> 'a
find x m returns the current binding of x in m, or raises Not_found if no
such binding exists.
Chapter 16. The standard library 177
val remove: key -> 'a t -> 'a t
remove x m returns a map containing the same bindings as m, except for x
which is unbound in the returned map.
val iter: (key -> 'a -> unit) -> 'a t -> unit
iter f m applies f to all bindings in map m. f receives the key as first
argument, and the associated value as second argument. The order in
which the bindings are passed to f is unspecified. Only current bindings
are presented to f: bindings hidden by more recent bindings are not
passed to f.
val map: ('a -> 'b) -> 'a t -> 'b t
map f m returns a map with same domain as m, where the associated value a
of all bindings of m has been replaced by the result of the application
of f to a. The order in which the associated values are passed to f is
unspecified.
val fold: (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
fold f m a computes (f kN dN ... (f k1 d1 a)...), where k1 ... kN are the
keys of all bindings in m, and d1 ... dN are the associated data. The
order in which the bindings are presented to f is unspecified.
end
module Make(Ord: OrderedType): (S with type key = Ord.t)
Functor building an implementation of the map structure given a totally
ordered type.
16.15 Module Marshal: marshaling of data structures
This module provides functions to encode arbitrary data structures as
sequences of bytes, which can then be written on a file or sent over a
pipe or network connection. The bytes can then be read back later,
possibly in another process, and decoded back into a data structure. The
format for the byte sequences is compatible across all machines for a
given version of Objective Caml.
Warning: marshaling is currently not type-safe. The type of marshaled
data is not transmitted along the value of the data, making it impossible
to check that the data read back possesses the type expected by the
context. In particular, the result type of the Marshal.from_* functions
is given as 'a, but this is misleading: the returned Caml value does not
possess type 'a for all 'a; it has one, unique type which cannot be
determined at compile-type. The programmer should explicitly give the
expected type of the returned value, using the following syntax:
(Marshal.from_channel chan : type). Anything can happen at run-time if
the object in the file does not belong to the given type.
The representation of marshaled values is not human-readable, and uses
bytes that are not printable characters. Therefore, input and output
channels used in conjunction with Marshal.to_channel and
Marshal.from_channel must be opened in binary mode, using e.g.
Chapter 16. The standard library 178
open_out_bin or open_in_bin; channels opened in text mode will cause
unmarshaling errors on platforms where text channels behave differently
than binary channels, e.g. Windows.
type extern_flags =
No_sharing (* Don't preserve sharing *)
| Closures (* Send function closures *)
The flags to the Marshal.to_* functions below.
val to_channel: out_channel -> 'a -> extern_flags list -> unit
Marshal.to_channel chan v flags writes the representation of v on channel
chan. The flags argument is a possibly empty list of flags that governs
the marshaling behavior with respect to sharing and functional values.
If flags does not contain Marshal.No_sharing, circularities and sharing
inside the value v are detected and preserved in the sequence of bytes
produced. In particular, this guarantees that marshaling always
terminates. Sharing between values marshaled by successive calls to
Marshal.to_channel is not detected, though. If flags contains
Marshal.No_sharing, sharing is ignored. This results in faster
marshaling if v contains no shared substructures, but may cause slower
marshaling and larger byte representations if v actually contains
sharing, or even non-termination if v contains cycles.
If flags does not contain Marshal.Closures, marshaling fails when it
encounters a functional value inside v: only ``pure'' data structures,
containing neither functions nor objects, can safely be transmitted
between different programs. If flags contains Marshal.Closures,
functional values will be marshaled as a position in the code of the
program. In this case, the output of marshaling can only be read back in
processes that run exactly the same program, with exactly the same
compiled code. (This is checked at un-marshaling time, using an MD5
digest of the code transmitted along with the code position.)
val to_string: 'a -> extern_flags list -> string
Marshal.to_string v flags returns a string containing the representation
of v as a sequence of bytes. The flags argument has the same meaning as
for Marshal.to_channel.
val to_buffer: string -> int -> int -> 'a -> extern_flags list -> int
Marshal.to_buffer buff ofs len v flags marshals the value v, storing its
byte representation in the string buff, starting at character number ofs,
and writing at most len characters. It returns the number of characters
actually written to the string. If the byte representation of v does not
fit in len characters, the exception Failure is raised.
val from_channel: in_channel -> 'a
Marshal.from_channel chan reads from channel chan the byte representation
of a structured value, as produced by one of the Marshal.to_* functions,
and reconstructs and returns the corresponding value.
Chapter 16. The standard library 179
val from_string: string -> int -> 'a
Marshal.from_string buff ofs unmarshals a structured value like
Marshal.from_channel does, except that the byte representation is not
read from a channel, but taken from the string buff, starting at position
ofs.
val header_size : int
val data_size : string -> int -> int
val total_size : string -> int -> int
The bytes representing a marshaled value are composed of a fixed-size
header and a variable-sized data part, whose size can be determined from
the header. Marshal.header_size is the size, in characters, of the
header. Marshal.data_size buff ofs is the size, in characters, of the
data part, assuming a valid header is stored in buff starting at position
ofs. Finally, Marshal.total_size buff ofs is the total size, in
characters, of the marshaled value. Both Marshal.data_size and
Marshal.total_size raise Failure if buff, ofs does not contain a valid
header.
To read the byte representation of a marshaled value into a string
buffer, the program needs to read first Marshal.header_size characters
into the buffer, then determine the length of the remainder of the
representation using Marshal.data_size, make sure the buffer is large
enough to hold the variable size, then read it, and finally call
Marshal.from_string to unmarshal the value.
16.16 Module Oo: object-oriented extension
val copy : (< .. > as 'a) -> 'a
Oo.copy o returns a copy of object o, that is a fresh object with the
same methods and instance variables as o
16.17 Module Parsing: the run-time library for parsers generated by camlyacc
val symbol_start : unit -> int
val symbol_end : unit -> int
symbol_start and symbol_end are to be called in the action part of a
grammar rule only. They return the position of the string that matches
the left-hand side of the rule: symbol_start() returns the position of
the first character; symbol_end() returns the position of the last
character, plus one. The first character in a file is at position 0.
val rhs_start: int -> int
val rhs_end: int -> int
Same as symbol_start and symbol_end, but return the position of the
string matching the nth item on the right-hand side of the rule, where n
is the integer parameter to lhs_start and lhs_end. n is 1 for the
leftmost item.
val clear_parser : unit -> unit
Chapter 16. The standard library 180
Empty the parser stack. Call it just after a parsing function has
returned, to remove all pointers from the parser stack to structures that
were built by semantic actions during parsing. This is optional, but
lowers the memory requirements of the programs.
exception Parse_error
Raised when a parser encounters a syntax error. Can also be raised from
the action part of a grammar rule, to initiate error recovery.
16.18 Module Printexc: a catch-all exception handler
val catch: ('a -> 'b) -> 'a -> 'b
Printexc.catch fn x applies fn to x and returns the result. If the
evaluation of fn x raises any exception, the name of the exception is
printed on standard error output, and the programs aborts with exit code
2. Typical use is Printexc.catch main (), where main, with type
unit->unit, is the entry point of a standalone program. This catches and
reports any exception that escapes the program.
val print: ('a -> 'b) -> 'a -> 'b
Same as catch, but re-raise the stray exception after printing it,
instead of aborting the program.
val to_string : exn -> string
Printexc.to_string e returns a string representation of e.
16.19 Module Printf: formatting printing functions
val fprintf: out_channel -> ('a, out_channel, unit) format -> 'a
fprintf outchan format arg1 ... argN formats the arguments arg1 to argN
according to the format string format, and outputs the resulting string
on the channel outchan.
The format is a character string which contains two types of objects:
plain characters, which are simply copied to the output channel, and
conversion specifications, each of which causes conversion and printing
of one argument.
Conversion specifications consist in the % character, followed by
optional flags and field widths, followed by one conversion character.
The conversion characters and their meanings are:
d or i: convert an integer argument to signed decimal
u: convert an integer argument to unsigned decimal
x: convert an integer argument to unsigned hexadecimal, using lowercase
letters.
X: convert an integer argument to unsigned hexadecimal, using uppercase
letters.
s: insert a string argument
c: insert a character argument
Chapter 16. The standard library 181
f: convert a floating-point argument to decimal notation, in the style
dddd.ddd
e or E: convert a floating-point argument to decimal notation, in the
style d.ddd e+-dd (mantissa and exponent)
g or G: convert a floating-point argument to decimal notation, in style f
or e, E (whichever is more compact)
b: convert a boolean argument to the string true or false
a: user-defined printer. Takes two arguments and apply the first one to
outchan (the current output channel) and to the second argument. The
first argument must therefore have type out_channel -> 'b -> unit and the
second 'b. The output produced by the function is therefore inserted in
the output of fprintf at the current point.
t: same as %a, but takes only one argument (with type
out_channel -> unit) and apply it to outchan.
Refer to the C library printf function for the meaning of flags and field
width specifiers.
If too few arguments are provided, printing stops just before converting
the first missing argument.
val printf: ('a, out_channel, unit) format -> 'a
Same as fprintf, but output on stdout.
val eprintf: ('a, out_channel, unit) format -> 'a
Same as fprintf, but output on stderr.
val sprintf: ('a, unit, string) format -> 'a
Same as printf, but return the result of formatting in a string.
16.20 Module Queue: first-in first-out queues
This module implements queues (FIFOs), with in-place modification.
type 'a t
The type of queues containing elements of type 'a.
exception Empty
Raised when take is applied to an empty queue.
val create: unit -> 'a t
Return a new queue, initially empty.
val add: 'a -> 'a t -> unit
add x q adds the element x at the end of the queue q.
val take: 'a t -> 'a
take q removes and returns the first element in queue q, or raises Empty
if the queue is empty.
Chapter 16. The standard library 182
val peek: 'a t -> 'a
peek q returns the first element in queue q, without removing it from the
queue, or raises Empty if the queue is empty.
val clear : 'a t -> unit
Discard all elements from a queue.
val length: 'a t -> int
Return the number of elements in a queue.
val iter: ('a -> unit) -> 'a t -> unit
iter f q applies f in turn to all elements of q, from the least recently
entered to the most recently entered. The queue itself is unchanged.
16.21 Module Random: pseudo-random number generator
val init : int -> unit
Initialize the generator, using the argument as a seed. The same seed
will always yield the same sequence of numbers.
val full_init : int array -> unit
Same as init but takes more data as seed.
val bits : unit -> int
Return 30 random bits in a nonnegative integer.
val int : int -> int
Random.int bound returns a random integer between 0 (inclusive) and bound
30
(exclusive). bound must be more than 0 and less than 2 .
val float : float -> float
Random.float bound returns a random floating-point number between 0
(inclusive) and bound (exclusive). If bound is negative, the result is
negative. If bound is 0, the result is 0.
16.22 Module Set: sets over ordered types
This module implements the set data structure, given a total ordering
function over the set elements. All operations over sets are purely
applicative (no side-effects). The implementation uses balanced binary
trees, and is therefore reasonably efficient: insertion and membership
take time logarithmic in the size of the set, for instance.
Chapter 16. The standard library 183
module type OrderedType =
sig
type t
val compare: t -> t -> int
end
The input signature of the functor Set.Make. t is the type of the set
elements. compare is a total ordering function over the set elements.
This is a two-argument function f such that f e1 e2 is zero if the
elements e1 and e2 are equal, f e1 e2 is strictly negative if e1 is
smaller than e2, and f e1 e2 is strictly positive if e1 is greater than
e2. Example: a suitable ordering function is the generic structural
comparison function compare.
module type S =
sig
type elt
The type of the set elements.
type t
The type of sets.
val empty: t
The empty set.
val is_empty: t -> bool
Test whether a set is empty or not.
val mem: elt -> t -> bool
mem x s tests whether x belongs to the set s.
val add: elt -> t -> t
add x s returns a set containing all elements of s, plus x. If x was
already in s, s is returned unchanged.
val remove: elt -> t -> t
remove x s returns a set containing all elements of s, except x. If x
was not in s, s is returned unchanged.
val union: t -> t -> t
val inter: t -> t -> t
val diff: t -> t -> t
Union, intersection and set difference.
val compare: t -> t -> int
Total ordering between sets. Can be used as the ordering function for
doing sets of sets.
Chapter 16. The standard library 184
val equal: t -> t -> bool
equal s1 s2 tests whether the sets s1 and s2 are equal, that is, contain
the same elements.
val subset: t -> t -> bool
subset s1 s2 tests whether the set s1 is a subset of the set s2.
val iter: (elt -> unit) -> t -> unit
iter f s applies f in turn to all elements of s. The order in which the
elements of s are presented to f is unspecified.
val fold: (elt -> 'a -> 'a) -> t -> 'a -> 'a
fold f s a computes (f xN ... (f x2 (f x1 a))...), where x1 ... xN are
the elements of s. The order in which elements of s are presented to f
is unspecified.
val cardinal: t -> int
Return the number of elements of a set.
val elements: t -> elt list
Return the list of all elements of the given set. The elements appear in
the list in some unspecified order.
val choose: t -> elt
Return one element of the given set, or raise Not_found if the set is
empty. Which element is chosen is unspecified, but equal elements will
be chosen for equal sets.
end
module Make(Ord: OrderedType): (S with type elt = Ord.t)
Functor building an implementation of the set structure given a totally
ordered type.
16.23 Module Sort: sorting and merging lists
val list : ('a -> 'a -> bool) -> 'a list -> 'a list
Sort a list in increasing order according to an ordering predicate. The
predicate should return true if its first argument is less than or equal
to its second argument.
val merge : ('a -> 'a -> bool) -> 'a list -> 'a list -> 'a list
Merge two lists according to the given predicate. Assuming the two
argument lists are sorted according to the predicate, merge returns a
sorted list containing the elements from the two lists. The behavior is
undefined if the two argument lists were not sorted.
Chapter 16. The standard library 185
16.24 Module Stack: last-in first-out stacks
This module implements stacks (LIFOs), with in-place modification.
type 'a t
The type of stacks containing elements of type 'a.
exception Empty
Raised when pop is applied to an empty stack.
val create: unit -> 'a t
Return a new stack, initially empty.
val push: 'a -> 'a t -> unit
push x s adds the element x at the top of stack s.
val pop: 'a t -> 'a
pop s removes and returns the topmost element in stack s, or raises Empty
if the stack is empty.
val clear : 'a t -> unit
Discard all elements from a stack.
val length: 'a t -> int
Return the number of elements in a stack.
val iter: ('a -> unit) -> 'a t -> unit
iter f s applies f in turn to all elements of s, from the element at the
top of the stack to the element at the bottom of the stack. The stack
itself is unchanged.
16.25 Module Stream: streams and parsers
type 'a t
The type of streams holding values of type 'a.
exception Failure;;
Raised by parsers when none of the first components of the stream
patterns is accepted.
exception Error of string;;
Raised by parsers when the first component of a stream pattern is
accepted, but one of the following components is rejected.
Chapter 16. The standard library 186
Stream builders
Warning: these functions create streams with fast access; it is illegal
to mix them with streams built with [< >]; would raise Failure when
accessing such mixed streams.
val from : (int -> 'a option) -> 'a t;;
Stream.from f returns a stream built from the function f. To create a
new stream element, the function f is called with the current stream
count. The user function f must return either Some <value> for a value
or None to specify the end of the stream.
val of_list : 'a list -> 'a t;;
Return the stream holding the elements of the list in the same order.
val of_string : string -> char t;;
Return the stream of the characters of the string parameter.
val of_channel : in_channel -> char t;;
Return the stream of the characters read from the input channel.
Stream iterator
val iter : ('a -> unit) -> 'a t -> unit;;
Stream.iter f s scans the whole stream s, applying function f in turn to
each stream element encountered.
Predefined parsers
val next : 'a t -> 'a;;
Return the first element of the stream and remove it from the stream.
Raise Stream.Failure if the stream is empty.
val empty : 'a t -> unit;;
Return () if the stream is empty, else raise Stream.Failure.
Useful functions
val peek : 'a t -> 'a option;;
Return Some of "the first element" of the stream, or None if the stream
is empty.
val junk : 'a t -> unit;;
Remove the first element of the stream, possibly unfreezing it before.
Chapter 16. The standard library 187
val count : 'a t -> int = "%field0";;
Return the current count of the stream elements, i.e. the number of the
stream elements discarded.
val npeek : int -> 'a t -> 'a list;;
npeek n returns the list of the n first elements of the stream, or all
its remaining elements if less than n elements are available.
16.26 Module String: string operations
val length : string -> int
Return the length (number of characters) of the given string.
val get : string -> int -> char
String.get s n returns character number n in string s. The first
character is character number 0. The last character is character number
String.length s - 1. Raise Invalid_argument if n is ouside the range 0
to (String.length s - 1). You can also write s.[n] instead of
String.get s n.
val set : string -> int -> char -> unit
String.set s n c modifies string s in place, replacing the character
number n by c. Raise Invalid_argument if n is ouside the range 0 to
(String.length s - 1). You can also write s.[n] <- c instead of
String.set s n c.
val create : int -> string
String.create n returns a fresh string of length n. The string initially
contains arbitrary characters.
val make : int -> char -> string
String.make n c returns a fresh string of length n, filled with the
character c.
val copy : string -> string
Return a copy of the given string.
val sub : string -> int -> int -> string
String.sub s start len returns a fresh string of length len, containing
the characters number start to start + len - 1 of string s. Raise
Invalid_argument if start and len do not designate a valid substring of
s; that is, if start < 0, or len < 0, or start + len > String.length s.
val fill : string -> int -> int -> char -> unit
Chapter 16. The standard library 188
String.fill s start len c modifies string s in place, replacing the
characters number start to start + len - 1 by c. Raise Invalid_argument
if start and len do not designate a valid substring of s.
val blit : string -> int -> string -> int -> int -> unit
String.blit src srcoff dst dstoff len copies len characters from string
src, starting at character number srcoff, to string dst, starting at
character number dstoff. It works correctly even if src and dst are the
same string, and the source and destination chunks overlap. Raise
Invalid_argument if srcoff and len do not designate a valid substring of
src, or if dstoff and len do not designate a valid substring of dst.
val concat : string -> string list -> string
String.concat sep sl catenates the list of strings sl, inserting the
separator string sep between each.
val escaped: string -> string
Return a copy of the argument, with special characters represented by
escape sequences, following the lexical conventions of Objective Caml.
val index: string -> char -> int
index s c returns the position of the leftmost occurrence of character c
in string s. Raise Not_found if c does not occur in s.
val rindex: string -> char -> int
rindex s c returns the position of the rightmost occurrence of character
c in string s. Raise Not_found if c does not occur in s.
val index_from: string -> int -> char -> int
val rindex_from: string -> int -> char -> int
Same as index and rindex, but start searching at the character position
given as second argument. index s c is equivalent to index_from s 0 c,
and rindex s c to rindex_from s (String.length s - 1) c.
val uppercase: string -> string
Return a copy of the argument, with all lowercase letters translated to
uppercase, including accented letters of the ISO Latin-1 (8859-1)
character set.
val lowercase: string -> string
Return a copy of the argument, with all uppercase letters translated to
lowercase, including accented letters of the ISO Latin-1 (8859-1)
character set.
val capitalize: string -> string
Return a copy of the argument, with the first letter set to uppercase.
Chapter 16. The standard library 189
val uncapitalize: string -> string
Return a copy of the argument, with the first letter set to lowercase.
16.27 Module Sys: system interface
val argv: string array
The command line arguments given to the process. The first element is
the command name used to invoke the program. The following elements are
the arguments given to the program.
val file_exists: string -> bool
Test if a file with the given name exists.
val remove: string -> unit
Remove the given file name from the file system.
val rename : string -> string -> unit
Rename a file. The first argument is the old name and the second is the
new name.
val getenv: string -> string
Return the value associated to a variable in the process environment.
Raise Not_found if the variable is unbound.
val command: string -> int
Execute the given shell command and return its exit code.
val chdir: string -> unit
Change the current working directory of the process.
val getcwd: unit -> string
Return the current working directory of the process.
val interactive: bool ref
This reference is initially set to false in standalone programs and to
true if the code is being executed under the interactive toplevel csltop.
val os_type: string
Operating system currently executing the Caml program. One of "Unix",
"Win32", or "MacOS".
val word_size: int
Size of one word on the machine currently executing the Caml program, in
bits: 32 or 64.
Chapter 16. The standard library 190
val max_string_length: int
Maximum length of a string.
val max_array_length: int
Maximum length of an array.
Signal handling
type signal_behavior =
Signal_default
| Signal_ignore
| Signal_handle of (int -> unit)
What to do when receiving a signal:
Signal_default: take the default behavior
Signal_ignore: ignore the signal
Signal_handle f: call function f, giving it the signal number as
argument.
val signal: int -> signal_behavior -> unit
Set the behavior of the system on receipt of a given signal. The first
argument is the signal number.
val sigabrt: int (* Abnormal termination *)
val sigalrm: int (* Timeout *)
val sigfpe: int (* Arithmetic exception *)
val sighup: int (* Hangup on controlling terminal *)
val sigill: int (* Invalid hardware instruction *)
val sigint: int (* Interactive interrupt (ctrl-C) *)
val sigkill: int (* Termination (cannot be ignored) *)
val sigpipe: int (* Broken pipe *)
val sigquit: int (* Interactive termination *)
val sigsegv: int (* Invalid memory reference *)
val sigterm: int (* Termination *)
val sigusr1: int (* Application-defined signal 1 *)
val sigusr2: int (* Application-defined signal 2 *)
val sigchld: int (* Child process terminated *)
val sigcont: int (* Continue *)
val sigstop: int (* Stop *)
val sigtstp: int (* Interactive stop *)
val sigttin: int (* Terminal read from background process *)
val sigttou: int (* Terminal write from background process *)
val sigvtalrm: int (* Timeout in virtual time *)
val sigprof: int (* Profiling interrupt *)
Signal numbers for the standard POSIX signals.
exception Break
Exception raised on interactive interrupt if catch_break is on.
val catch_break: bool -> unit
Chapter 16. The standard library 191
catch_break governs whether interactive interrupt (ctrl-C) terminates the
program or raises the Break exception. Call catch_break true to enable
raising Break, and catch_break false to let the system terminate the
program on user interrupt.
16.28 Module Weak: arrays of weak pointers
type 'a t;;
The type of arrays of weak pointers (weak arrays). A weak pointer is an
object that the garbage collector may erase at any time. A weak pointer
is said to be full if it points to an object, empty if the object was
erased by the GC.
val create : int -> 'a t;;
Weak.create n returns a new weak array of length n. All the pointers are
initially empty.
val length : 'a t -> int;;
Weak.length ar returns the length (number of elements) of ar.
val set : 'a t -> int -> 'a option -> unit;;
Weak.set ar n (Some el) sets the nth cell of ar to be a (full) pointer to
el; Weak.set ar n None sets the nth cell of ar to empty. Raise
Invalid_argument "Weak.set" if n is not in the range 0 to
Weak.length a - 1.
val get : 'a t -> int -> 'a option;;
Weak.get ar n returns None if the nth cell of ar is empty, Some x (where
x is the object) if it is full. Raise Invalid_argument "Weak.get" if n
is not in the range 0 to Weak.length a - 1.
val fill: 'a t -> int -> int -> 'a option -> unit;;
Weak.fill ar ofs len el sets to el all pointers of ar from ofs to
ofs + len - 1. Raise Invalid_argument "Weak.fill" if ofs and len do not
designate a valid subarray of a.
val blit : 'a t -> int -> 'a t -> int -> int -> unit;;
Weak.blit ar1 off1 ar2 off2 len copies len weak pointers from ar1
(starting at off1) to ar2 (starting at off2). It works correctly even if
ar1 and ar2 are the same. Raise Invalid_argument "Weak.blit" if off1 and
len do not designate a valid subarray of ar1, or if off2 and len do not
designate a valid subarray of ar2.
Chapter 17
The unix library: Unix system calls
The unix library makes many Unix system calls and system-related library
functions available to Objective Caml programs. This chapter describes
briefly the functions provided. Refer to sections 2 and 3 of the Unix manual
for more details on the behavior of these functions.
Not all functions are provided by all Unix variants. If some functions are
not available, they will raise Invalid_arg when called.
Unix:
Programs that use the unix library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options unix.cma other files -cclib -lunix
ocamlopt other options unix.cmxa other files -cclib -lunix
For interactive use of the unix library, do:
ocamlmktop -custom -o mytop unix.cma -cclib -lunix
./mytop
Windows:
A fairly complete emulation of the Unix system calls is provided in the
Windows version of Objective Caml. The end of this chapter gives more
information on the functions that are not supported under Windows.
Programs that use the unix library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options unix.cma other files \
%CAMLLIB%/libunix.lib wsock32.lib
ocamlopt other options unix.cmxa other files \
%CAMLLIB%/libunix.lib wsock32.lib
For interactive use of the unix library, do:
ocamlmktop -custom -o mytop.exe unix.cma \
%CAMLLIB%/libunix.lib wsock32.lib
192
Chapter 17. The unix library: Unix system calls 193
mytop.exe
17.1 Module Unix: interface to the Unix system
Error report
type error =
Errors defined in the POSIX standard
E2BIG (* Argument list too long *)
| EACCES (* Permission denied *)
| EAGAIN (* Resource temporarily unavailable; try again *)
| EBADF (* Bad file descriptor *)
| EBUSY (* Resource unavailable *)
| ECHILD (* No child process *)
| EDEADLK (* Resource deadlock would occur *)
| EDOM (* Domain error for math functions, etc. *)
| EEXIST (* File exists *)
| EFAULT (* Bad address *)
| EFBIG (* File too large *)
| EINTR (* Function interrupted by signal *)
| EINVAL (* Invalid argument *)
| EIO (* Hardware I/O error *)
| EISDIR (* Is a directory *)
| EMFILE (* Too many open files by the process *)
| EMLINK (* Too many links *)
| ENAMETOOLONG (* Filename too long *)
| ENFILE (* Too many open files in the system *)
| ENODEV (* No such device *)
| ENOENT (* No such file or directory *)
| ENOEXEC (* Not an executable file *)
| ENOLCK (* No locks available *)
| ENOMEM (* Not enough memory *)
| ENOSPC (* No space left on device *)
| ENOSYS (* Function not supported *)
| ENOTDIR (* Not a directory *)
| ENOTEMPTY (* Directory not empty *)
| ENOTTY (* Inappropriate I/O control operation *)
| ENXIO (* No such device or address *)
| EPERM (* Operation not permitted *)
| EPIPE (* Broken pipe *)
| ERANGE (* Result too large *)
| EROFS (* Read-only file system *)
| ESPIPE (* Invalid seek e.g. on a pipe *)
| ESRCH (* No such process *)
| EXDEV (* Invalid link *)
Additional errors, mostly BSD
| EWOULDBLOCK (* Operation would block *)
| EINPROGRESS (* Operation now in progress *)
| EALREADY (* Operation already in progress *)
| ENOTSOCK (* Socket operation on non-socket *)
| EDESTADDRREQ (* Destination address required *)
| EMSGSIZE (* Message too long *)
Chapter 17. The unix library: Unix system calls 194
| EPROTOTYPE (* Protocol wrong type for socket *)
| ENOPROTOOPT (* Protocol not available *)
| EPROTONOSUPPORT (* Protocol not supported *)
| ESOCKTNOSUPPORT (* Socket type not supported *)
| EOPNOTSUPP (* Operation not supported on socket *)
| EPFNOSUPPORT (* Protocol family not supported *)
| EAFNOSUPPORT (* Address family not supported by protocol family *)
| EADDRINUSE (* Address already in use *)
| EADDRNOTAVAIL (* Can't assign requested address *)
| ENETDOWN (* Network is down *)
| ENETUNREACH (* Network is unreachable *)
| ENETRESET (* Network dropped connection on reset *)
| ECONNABORTED (* Software caused connection abort *)
| ECONNRESET (* Connection reset by peer *)
| ENOBUFS (* No buffer space available *)
| EISCONN (* Socket is already connected *)
| ENOTCONN (* Socket is not connected *)
| ESHUTDOWN (* Can't send after socket shutdown *)
| ETOOMANYREFS (* Too many references: can't splice *)
| ETIMEDOUT (* Connection timed out *)
| ECONNREFUSED (* Connection refused *)
| EHOSTDOWN (* Host is down *)
| EHOSTUNREACH (* No route to host *)
| ELOOP (* Too many levels of symbolic links *)
All other errors are mapped to EUNKNOWNERR
| EUNKNOWNERR (* Unknown error *)
The type of error codes.
exception Unix_error of error * string * string
Raised by the system calls below when an error is encountered. The first
component is the error code; the second component is the function name;
the third component is the string parameter to the function, if it has
one, or the empty string otherwise.
val error_message : error -> string
Return a string describing the given error code.
val handle_unix_error : ('a -> 'b) -> 'a -> 'b
handle_unix_error f x applies f to x and returns the result. If the
exception Unix_error is raised, it prints a message describing the error
and exits with code 2.
Interface with the parent process
val environment : unit -> string array
Return the process environment, as an array of strings with the format
``variable=value''. See also Sys.getenv.
Chapter 17. The unix library: Unix system calls 195
Process handling
type process_status =
WEXITED of int
| WSIGNALED of int
| WSTOPPED of int
The termination status of a process. WEXITED means that the process
terminated normally by exit; the argument is the return code. WSIGNALED
means that the process was killed by a signal; the argument is the signal
number. WSTOPPED means that the process was stopped by a signal; the
argument is the signal number.
type wait_flag =
WNOHANG
| WUNTRACED
Flags for waitopt and waitpid. WNOHANG means do not block if no child
has died yet, but immediately return with a pid equal to 0. WUNTRACED
means report also the children that receive stop signals.
val execv : string -> string array -> unit
execv prog args execute the program in file prog, with the arguments
args, and the current process environment.
val execve : string -> string array -> string array -> unit
Same as execv, except that the third argument provides the environment to
the program executed.
val execvp : string -> string array -> unit
val execvpe : string -> string array -> string array -> unit
Same as execv and execvp respectively, except that the program is
searched in the path.
val fork : unit -> int
Fork a new process. The returned integer is 0 for the child process, the
pid of the child process for the parent process.
val wait : unit -> int * process_status
Wait until one of the children processes die, and return its pid and
termination status.
val waitpid : wait_flag list -> int -> int * process_status
Same as wait, but waits for the process whose pid is given. A pid of -1
means wait for any child. A pid of 0 means wait for any child in the
same process group as the current process. Negative pid arguments
represent process groups. The list of options indicates whether waitpid
should return immediately without waiting, or also report stopped
children.
Chapter 17. The unix library: Unix system calls 196
val system : string -> process_status
Execute the given command, wait until it terminates, and return its
termination status. The string is interpreted by the shell /bin/sh and
therefore can contain redirections, quotes, variables, etc. The result
WEXITED 127 indicates that the shell couldn't be executed.
val getpid : unit -> int
Return the pid of the process.
val getppid : unit -> int
Return the pid of the parent process.
val nice : int -> int
Change the process priority. The integer argument is added to the
``nice'' value. (Higher values of the ``nice'' value mean lower
priorities.) Return the new nice value.
Basic file input/output
type file_descr
The abstract type of file descriptors.
val stdin : file_descr
val stdout : file_descr
val stderr : file_descr
File descriptors for standard input, standard output and standard error.
type open_flag =
O_RDONLY (* Open for reading *)
| O_WRONLY (* Open for writing *)
| O_RDWR (* Open for reading and writing *)
| O_NONBLOCK (* Open in non-blocking mode *)
| O_APPEND (* Open for append *)
| O_CREAT (* Create if nonexistent *)
| O_TRUNC (* Truncate to 0 length if existing *)
| O_EXCL (* Fail if existing *)
The flags to open.
type file_perm = int
The type of file access rights.
val openfile : string -> open_flag list -> file_perm -> file_descr
Open the named file with the given flags. Third argument is the
permissions to give to the file if it is created. Return a file
descriptor on the named file.
Chapter 17. The unix library: Unix system calls 197
val close : file_descr -> unit
Close a file descriptor.
val read : file_descr -> string -> int -> int -> int
read fd buff ofs len reads len characters from descriptor fd, storing
them in string buff, starting at position ofs in string buff. Return the
number of characters actually read.
val write : file_descr -> string -> int -> int -> int
write fd buff ofs len writes len characters to descriptor fd, taking them
from string buff, starting at position ofs in string buff. Return the
number of characters actually written.
Interfacing with the standard input/output library.
val in_channel_of_descr : file_descr -> in_channel
Create an input channel reading from the given descriptor.
val out_channel_of_descr : file_descr -> out_channel
Create an output channel writing on the given descriptor.
val descr_of_in_channel : in_channel -> file_descr
Return the descriptor corresponding to an input channel.
val descr_of_out_channel : out_channel -> file_descr
Return the descriptor corresponding to an output channel.
Seeking and truncating
type seek_command =
SEEK_SET
| SEEK_CUR
| SEEK_END
Positioning modes for lseek. SEEK_SET indicates positions relative to
the beginning of the file, SEEK_CUR relative to the current position,
SEEK_END relative to the end of the file.
val lseek : file_descr -> int -> seek_command -> int
Set the current position for a file descriptor
val truncate : string -> int -> unit
Truncates the named file to the given size.
Chapter 17. The unix library: Unix system calls 198
val ftruncate : file_descr -> int -> unit
Truncates the file corresponding to the given descriptor to the given
size.
File statistics
type file_kind =
S_REG (* Regular file *)
| S_DIR (* Directory *)
| S_CHR (* Character device *)
| S_BLK (* Block device *)
| S_LNK (* Symbolic link *)
| S_FIFO (* Named pipe *)
| S_SOCK (* Socket *)
type stats =
{ st_dev : int; (* Device number *)
st_ino : int; (* Inode number *)
st_kind : file_kind; (* Kind of the file *)
st_perm : file_perm; (* Access rights *)
st_nlink : int; (* Number of links *)
st_uid : int; (* User id of the owner *)
st_gid : int; (* Group id of the owner *)
st_rdev : int; (* Device minor number *)
st_size : int; (* Size in bytes *)
st_atime : int; (* Last access time *)
st_mtime : int; (* Last modification time *)
st_ctime : int } (* Last status change time *)
The informations returned by the stat calls.
val stat : string -> stats
Return the information for the named file.
val lstat : string -> stats
Same as stat, but in case the file is a symbolic link, return the
information for the link itself.
val fstat : file_descr -> stats
Return the information for the file associated with the given descriptor.
Operations on file names
val unlink : string -> unit
Removes the named file
val rename : string -> string -> unit
rename old new changes the name of a file from old to new.
val link : string -> string -> unit
Chapter 17. The unix library: Unix system calls 199
link source dest creates a hard link named dest to the file named new.
File permissions and ownership
type access_permission =
R_OK (* Read permission *)
| W_OK (* Write permission *)
| X_OK (* Execution permission *)
| F_OK (* File exists *)
Flags for the access call.
val chmod : string -> file_perm -> unit
Change the permissions of the named file.
val fchmod : file_descr -> file_perm -> unit
Change the permissions of an opened file.
val chown : string -> int -> int -> unit
Change the owner uid and owner gid of the named file.
val fchown : file_descr -> int -> int -> unit
Change the owner uid and owner gid of an opened file.
val umask : int -> int
Set the process creation mask, and return the previous mask.
val access : string -> access_permission list -> unit
Check that the process has the given permissions over the named file.
Raise Unix_error otherwise.
Operations on file descriptors
val dup : file_descr -> file_descr
Return a new file descriptor referencing the same file as the given
descriptor.
val dup2 : file_descr -> file_descr -> unit
dup2 fd1 fd2 duplicates fd1 to fd2, closing fd2 if already opened.
val set_nonblock : file_descr -> unit
val clear_nonblock : file_descr -> unit
Set or clear the ``non-blocking'' flag on the given descriptor. When the
non-blocking flag is set, reading on a descriptor on which there is
temporarily no data available raises the EAGAIN or EWOULDBLOCK error
Chapter 17. The unix library: Unix system calls 200
instead of blocking; writing on a descriptor on which there is
temporarily no room for writing also raises EAGAIN or EWOULDBLOCK.
val set_close_on_exec : file_descr -> unit
val clear_close_on_exec : file_descr -> unit
Set or clear the ``close-on-exec'' flag on the given descriptor. A
descriptor with the close-on-exec flag is automatically closed when the
current process starts another program with one of the exec functions.
Directories
val mkdir : string -> file_perm -> unit
Create a directory with the given permissions.
val rmdir : string -> unit
Remove an empty directory.
val chdir : string -> unit
Change the process working directory.
val getcwd : unit -> string
Return the name of the current working directory.
type dir_handle
The type of descriptors over opened directories.
val opendir : string -> dir_handle
Open a descriptor on a directory
val readdir : dir_handle -> string
Return the next entry in a directory. Raise End_of_file when the end of
the directory has been reached.
val rewinddir : dir_handle -> unit
Reposition the descriptor to the beginning of the directory
val closedir : dir_handle -> unit
Close a directory descriptor.
Pipes and redirections
val pipe : unit -> file_descr * file_descr
Chapter 17. The unix library: Unix system calls 201
Create a pipe. The first component of the result is opened for reading,
that's the exit to the pipe. The second component is opened for writing,
that's the entrance to the pipe.
val mkfifo : string -> file_perm -> unit
Create a named pipe with the given permissions.
High-level process and redirection management
val create_process :
string -> string array -> file_descr -> file_descr -> file_descr -> int
create_process prog args new_stdin new_stdout new_stderr forks a new
process that executes the program in file prog, with arguments args. The
pid of the new process is returned immediately; the new process executes
concurrently with the current process. The standard input and outputs of
the new process are connected to the descriptors new_stdin, new_stdout
and new_stderr. Passing e.g. stdout for new_stdout prevents the
redirection and causes the new process to have the same standard output
as the current process. The executable file prog is searched in the
path. The new process has the same environment as the current process.
All file descriptors of the current process are closed in the new
process, except those redirected to standard input and outputs.
val create_process_env :
string -> string array -> string array ->
file_descr -> file_descr -> file_descr -> int
create_process_env prog args env new_stdin new_stdout new_stderr works as
create_process, except that the extra argument env specifies the
environment passed to the program.
val open_process_in: string -> in_channel
val open_process_out: string -> out_channel
val open_process: string -> in_channel * out_channel
High-level pipe and process management. These functions run the given
command in parallel with the program, and return channels connected to
the standard input and/or the standard output of the command. The
command is interpreted by the shell /bin/sh (cf. system). Warning:
writes on channels are buffered, hence be careful to call flush at the
right times to ensure correct synchronization.
val close_process_in: in_channel -> process_status
val close_process_out: out_channel -> process_status
val close_process: in_channel * out_channel -> process_status
Close channels opened by open_process_in, open_process_out and
open_process, respectively, wait for the associated command to terminate,
and return its termination status.
Symbolic links
val symlink : string -> string -> unit
Chapter 17. The unix library: Unix system calls 202
symlink source dest creates the file dest as a symbolic link to the file
source.
val readlink : string -> string
Read the contents of a link.
Polling
val select :
file_descr list -> file_descr list -> file_descr list -> float ->
file_descr list * file_descr list * file_descr list
Wait until some input/output operations become possible on some channels.
The three list arguments are, respectively, a set of descriptors to check
for reading (first argument), for writing (second argument), or for
exceptional conditions (third argument). The fourth argument is the
maximal timeout, in seconds; a negative fourth argument means no timeout
(unbounded wait). The result is composed of three sets of descriptors:
those ready for reading (first component), ready for writing (second
component), and over which an exceptional condition is pending (third
component).
Locking
type lock_command =
F_ULOCK (* Unlock a region *)
| F_LOCK (* Lock a region, and block if already locked *)
| F_TLOCK (* Lock a region, or fail if already locked *)
| F_TEST (* Test a region for other process' locks *)
Commands for lockf.
val lockf : file_descr -> lock_command -> int -> unit
lockf fd cmd size puts a lock on a region of the file opened as fd. The
region starts at the current read/write position for fd (as set by
lseek), and extends size bytes forward if size is positive, size bytes
backwards if size is negative, or to the end of the file if size is zero.
Signals
val kill : int -> int -> unit
kill pid sig sends signal number sig to the process with id pid.
val pause : unit -> unit
Wait until a non-ignored signal is delivered.
Time functions
type process_times =
Chapter 17. The unix library: Unix system calls 203
{ tms_utime : float; (* User time for the process *)
tms_stime : float; (* System time for the process *)
tms_cutime : float; (* User time for the children processes *)
tms_cstime : float } (* System time for the children processes *)
The execution times (CPU times) of a process.
type tm =
{ tm_sec : int; (* Seconds 0..59 *)
tm_min : int; (* Minutes 0..59 *)
tm_hour : int; (* Hours 0..23 *)
tm_mday : int; (* Day of month 1..31 *)
tm_mon : int; (* Month of year 0..11 *)
tm_year : int; (* Year - 1900 *)
tm_wday : int; (* Day of week (Sunday is 0) *)
tm_yday : int; (* Day of year 0..365 *)
tm_isdst : bool } (* Daylight time savings in effect *)
The type representing wallclock time and calendar date.
val time : unit -> int
Return the current time since 00:00:00 GMT, Jan. 1, 1970, in seconds.
val gettimeofday : unit -> float
Same as time, but with resolution better than 1 second.
val gmtime : int -> tm
Convert a time in seconds, as returned by time, into a date and a time.
Assumes Greenwich meridian time zone.
val localtime : int -> tm
Convert a time in seconds, as returned by time, into a date and a time.
Assumes the local time zone.
val mktime : tm -> int * tm
Convert a date and time, specified by the tm argument, into a time in
seconds, as returned by time. Also return a normalized copy of the given
tm record, with the tm_wday and tm_yday recomputed from the other fields.
val alarm : int -> int
Schedule a SIGALRM signals after the given number of seconds.
val sleep : int -> unit
Stop execution for the given number of seconds.
val times : unit -> process_times =
"unix_times_bytecode" "unix_times_native"
Return the execution times of the process.
Chapter 17. The unix library: Unix system calls 204
val utimes : string -> int -> int -> unit
Set the last access time (second arg) and last modification time (third
arg) for a file. Times are expressed in seconds from 00:00:00 GMT, Jan.
1, 1970.
type interval_timer =
ITIMER_REAL
| ITIMER_VIRTUAL
| ITIMER_PROF
The three kinds of interval timers. ITIMER_REAL decrements in real time,
and sends the signal SIGALRM when expired. ITIMER_VIRTUAL decrements in
process virtual time, and sends SIGVTALRM when expired. ITIMER_PROF (for
profiling) decrements both when the process is running and when the
system is running on behalf of the process; it sends SIGPROF when
expired.
type interval_timer_status =
{ it_interval: float; (* Period *)
it_value: float } (* Current value of the timer *)
The type describing the status of an interval timer
val getitimer: interval_timer -> interval_timer_status
Return the current status of the given interval timer.
val setitimer:
interval_timer -> interval_timer_status -> interval_timer_status
setitimer t s sets the interval timer t and returns its previous status.
The s argument is interpreted as follows: s.it_value, if nonzero, is the
time to the next timer expiration; s.it_interval, if nonzero, specifies a
value to be used in reloading it_value when the timer expires. Setting
s.it_value to zero disable the timer. Setting s.it_interval to zero
causes the timer to be disabled after its next expiration.
User id, group id
val getuid : unit -> int
Return the user id of the user executing the process.
val geteuid : unit -> int
Return the effective user id under which the process runs.
val setuid : int -> unit
Set the real user id and effective user id for the process.
val getgid : unit -> int
Return the group id of the user executing the process.
Chapter 17. The unix library: Unix system calls 205
val getegid : unit -> int
Return the effective group id under which the process runs.
val setgid : int -> unit
Set the real group id and effective group id for the process.
val getgroups : unit -> int array
Return the list of groups to which the user executing the process
belongs.
type passwd_entry =
{ pw_name : string;
pw_passwd : string;
pw_uid : int;
pw_gid : int;
pw_gecos : string;
pw_dir : string;
pw_shell : string }
Structure of entries in the passwd database.
type group_entry =
{ gr_name : string;
gr_passwd : string;
gr_gid : int;
gr_mem : string array }
Structure of entries in the groups database.
val getlogin : unit -> string
Return the login name of the user executing the process.
val getpwnam : string -> passwd_entry
Find an entry in passwd with the given name, or raise Not_found.
val getgrnam : string -> group_entry
Find an entry in group with the given name, or raise Not_found.
val getpwuid : int -> passwd_entry
Find an entry in passwd with the given user id, or raise Not_found.
val getgrgid : int -> group_entry
Find an entry in group with the given group id, or raise Not_found.
Internet addresses
type inet_addr
The abstract type of Internet addresses.
Chapter 17. The unix library: Unix system calls 206
val inet_addr_of_string : string -> inet_addr
val string_of_inet_addr : inet_addr -> string
Conversions between string with the format XXX.YYY.ZZZ.TTT and Internet
addresses. inet_addr_of_string raises Failure when given a string that
does not match this format.
val inet_addr_any : inet_addr
A special Internet address, for use only with bind, representing all the
Internet addresses that the host machine possesses.
Sockets
type socket_domain =
PF_UNIX (* Unix domain *)
| PF_INET (* Internet domain *)
The type of socket domains.
type socket_type =
SOCK_STREAM (* Stream socket *)
| SOCK_DGRAM (* Datagram socket *)
| SOCK_RAW (* Raw socket *)
| SOCK_SEQPACKET (* Sequenced packets socket *)
The type of socket kinds, specifying the semantics of communications.
type sockaddr =
ADDR_UNIX of string
| ADDR_INET of inet_addr * int
The type of socket addresses. ADDR_UNIX name is a socket address in the
Unix domain; name is a file name in the file system.
ADDR_INET(addr,port) is a socket address in the Internet domain; addr is
the Internet address of the machine, and port is the port number.
val socket : socket_domain -> socket_type -> int -> file_descr
Create a new socket in the given domain, and with the given kind. The
third argument is the protocol type; 0 selects the default protocol for
that kind of sockets.
val socketpair :
socket_domain -> socket_type -> int -> file_descr * file_descr
Create a pair of unnamed sockets, connected together.
val accept : file_descr -> file_descr * sockaddr
Accept connections on the given socket. The returned descriptor is a
socket connected to the client; the returned address is the address of
the connecting client.
Chapter 17. The unix library: Unix system calls 207
val bind : file_descr -> sockaddr -> unit
Bind a socket to an address.
val connect : file_descr -> sockaddr -> unit
Connect a socket to an address.
val listen : file_descr -> int -> unit
Set up a socket for receiving connection requests. The integer argument
is the maximal number of pending requests.
type shutdown_command =
SHUTDOWN_RECEIVE (* Close for receiving *)
| SHUTDOWN_SEND (* Close for sending *)
| SHUTDOWN_ALL (* Close both *)
The type of commands for shutdown.
val shutdown : file_descr -> shutdown_command -> unit
Shutdown a socket connection. SHUTDOWN_SEND as second argument causes
reads on the other end of the connection to return an end-of-file
condition. SHUTDOWN_RECEIVE causes writes on the other end of the
connection to return a closed pipe condition (SIGPIPE signal).
val getsockname : file_descr -> sockaddr
Return the address of the given socket.
val getpeername : file_descr -> sockaddr
Return the address of the host connected to the given socket.
type msg_flag =
MSG_OOB
| MSG_DONTROUTE
| MSG_PEEK
The flags for recv, recvfrom, send and sendto.
val recv : file_descr -> string -> int -> int -> msg_flag list -> int
val recvfrom :
file_descr -> string -> int -> int -> msg_flag list -> int * sockaddr
Receive data from an unconnected socket.
val send : file_descr -> string -> int -> int -> msg_flag list -> int
val sendto :
file_descr -> string -> int -> int -> msg_flag list -> sockaddr -> int
Send data over an unconnected socket.
type socket_option =
SO_DEBUG (* Record debugging information *)
| SO_BROADCAST (* Permit sending of broadcast messages *)
| SO_REUSEADDR (* Allow reuse of local addresses for bind *)
Chapter 17. The unix library: Unix system calls 208
| SO_KEEPALIVE (* Keep connection active *)
| SO_DONTROUTE (* Bypass the standard routing algorithms *)
| SO_OOBINLINE (* Leave out-of-band data in line *)
The socket options settable with setsockopt.
val getsockopt : file_descr -> socket_option -> bool
Return the current status of an option in the given socket.
val setsockopt : file_descr -> socket_option -> bool -> unit
Set or clear an option in the given socket.
High-level network connection functions
val open_connection : sockaddr -> in_channel * out_channel
Connect to a server at the given address. Return a pair of buffered
channels connected to the server. Remember to call flush on the output
channel at the right times to ensure correct synchronization.
val shutdown_connection : in_channel -> unit
``Shut down'' a connection established with open_connection; that is,
transmit an end-of-file condition to the server reading on the other side
of the connection.
val establish_server : (in_channel -> out_channel -> 'a) -> sockaddr -> unit
Establish a server on the given address. The function given as first
argument is called for each connection with two buffered channels
connected to the client. A new process is created for each connection.
The function establish_server never returns normally.
Host and protocol databases
type host_entry =
{ h_name : string;
h_aliases : string array;
h_addrtype : socket_domain;
h_addr_list : inet_addr array }
Structure of entries in the hosts database.
type protocol_entry =
{ p_name : string;
p_aliases : string array;
p_proto : int }
Structure of entries in the protocols database.
type service_entry =
{ s_name : string;
Chapter 17. The unix library: Unix system calls 209
s_aliases : string array;
s_port : int;
s_proto : string }
Structure of entries in the services database.
val gethostname : unit -> string
Return the name of the local host.
val gethostbyname : string -> host_entry
Find an entry in hosts with the given name, or raise Not_found.
val gethostbyaddr : inet_addr -> host_entry
Find an entry in hosts with the given address, or raise Not_found.
val getprotobyname : string -> protocol_entry
Find an entry in protocols with the given name, or raise Not_found.
val getprotobynumber : int -> protocol_entry
Find an entry in protocols with the given protocol number, or raise
Not_found.
val getservbyname : string -> string -> service_entry
Find an entry in services with the given name, or raise Not_found.
val getservbyport : int -> string -> service_entry
Find an entry in services with the given service number, or raise
Not_found.
Terminal interface
The following functions implement the POSIX standard terminal interface.
They provide control over asynchronous communication ports and
pseudo-terminals. Refer to the termios man page for a complete
description.
type terminal_io = {
Input modes:
mutable c_ignbrk: bool; (* Ignore the break condition. *)
mutable c_brkint: bool; (* Signal interrupt on break condition. *)
mutable c_ignpar: bool; (* Ignore characters with parity errors. *)
mutable c_parmrk: bool; (* Mark parity errors. *)
mutable c_inpck: bool; (* Enable parity check on input. *)
mutable c_istrip: bool; (* Strip 8th bit on input characters. *)
Chapter 17. The unix library: Unix system calls 210
mutable c_inlcr: bool; (* Map NL to CR on input. *)
mutable c_igncr: bool; (* Ignore CR on input. *)
mutable c_icrnl: bool; (* Map CR to NL on input. *)
mutable c_ixon: bool; (* Recognize XON/XOFF characters on input. *)
mutable c_ixoff: bool; (* Emit XON/XOFF chars to control input flow. *)
Output modes:
mutable c_opost: bool; (* Enable output processing. *)
Control modes:
mutable c_obaud: int; (* Output baud rate (0 means close connection).*)
mutable c_ibaud: int; (* Input baud rate. *)
mutable c_csize: int; (* Number of bits per character (5-8). *)
mutable c_cstopb: int; (* Number of stop bits (1-2). *)
mutable c_cread: bool; (* Reception is enabled. *)
mutable c_parenb: bool; (* Enable parity generation and detection. *)
mutable c_parodd: bool; (* Specify odd parity instead of even. *)
mutable c_hupcl: bool; (* Hang up on last close. *)
mutable c_clocal: bool; (* Ignore modem status lines. *)
Local modes:
mutable c_isig: bool; (* Generate signal on INTR, QUIT, SUSP. *)
mutable c_icanon: bool; (* Enable canonical processing
(line buffering and editing) *)
mutable c_noflsh: bool; (* Disable flush after INTR, QUIT, SUSP. *)
mutable c_echo: bool; (* Echo input characters. *)
mutable c_echoe: bool; (* Echo ERASE (to erase previous character). *)
mutable c_echok: bool; (* Echo KILL (to erase the current line). *)
mutable c_echonl: bool; (* Echo NL even if c_echo is not set. *)
Control characters:
mutable c_vintr: char; (* Interrupt character (usually ctrl-C). *)
mutable c_vquit: char; (* Quit character (usually ctrl-\). *)
mutable c_verase: char; (* Erase character (usually DEL or ctrl-H). *)
mutable c_vkill: char; (* Kill line character (usually ctrl-U). *)
mutable c_veof: char; (* End-of-file character (usually ctrl-D). *)
mutable c_veol: char; (* Alternate end-of-line char. (usually none). *)
mutable c_vmin: int; (* Minimum number of characters to read
before the read request is satisfied. *)
mutable c_vtime: int; (* Maximum read wait (in 0.1s units). *)
mutable c_vstart: char; (* Start character (usually ctrl-Q). *)
mutable c_vstop: char (* Stop character (usually ctrl-S). *)
}
val tcgetattr: file_descr -> terminal_io
Return the status of the terminal referred to by the given file
descriptor.
type setattr_when = TCSANOW | TCSADRAIN | TCSAFLUSH
val tcsetattr: file_descr -> setattr_when -> terminal_io -> unit
Set the status of the terminal referred to by the given file descriptor.
The second argument indicates when the status change takes place:
Chapter 17. The unix library: Unix system calls 211
immediately (TCSANOW), when all pending output has been transmitted
(TCSADRAIN), or after flushing all input that has been received but not
read (TCSAFLUSH). TCSADRAIN is recommended when changing the output
parameters; TCSAFLUSH, when changing the input parameters.
val tcsendbreak: file_descr -> int -> unit
Send a break condition on the given file descriptor. The second argument
is the duration of the break, in 0.1s units; 0 means standard duration
(0.25s).
val tcdrain: file_descr -> unit
Waits until all output written on the given file descriptor has been
transmitted.
type flush_queue = TCIFLUSH | TCOFLUSH | TCIOFLUSH
val tcflush: file_descr -> flush_queue -> unit
Discard data written on the given file descriptor but not yet
transmitted, or data received but not yet read, depending on the second
argument: TCIFLUSH flushes data received but not read, TCOFLUSH flushes
data written but not transmitted, and TCIOFLUSH flushes both.
type flow_action = TCOOFF | TCOON | TCIOFF | TCION
val tcflow: file_descr -> flow_action -> unit
Suspend or restart reception or transmission of data on the given file
descriptor, depending on the second argument: TCOOFF suspends output,
TCOON restarts output, TCIOFF transmits a STOP character to suspend
input, and TCION transmits a START character to restart input.
val setsid : unit -> int
Put the calling process in a new session and detach it from its
controlling terminal.
Windows:
Below is a list of the functions that are not implemented, or only
partially implemented, under Windows. Functions not mentioned are fully
implemented and behave as described previously in this chapter.
Chapter 17. The unix library: Unix system calls 212
---------------------------------------------------------------------
|Functions |Comment |
---------------------------------------------------------------------
|fork |not implemented, use create_process |
| |or threads |
|wait |not implemented, use waitpid |
|waitpid |can only wait for a given PID, not |
| |any child process |
|getppid |not implemented (meaningless under |
| |Windows) |
|nice |not implemented |
|truncate, ftruncate |not implemented |
|lstat, fstat |not implemented |
|link, symlink, readlink |not implemented (no links under |
| |Windows) |
|chmod, fchmod |not implemented |
|chown, fchown |not implemented (make no sense on a |
| |DOS file system) |
|umask |not implemented |
|set_nonblock, clear_nonblock |implemented as dummy functions; use |
| |threads instead of non-blocking I/O |
|rewinddir |not implemented; re-open the direc- |
| |tory instead |
|mkfifo |not implemented |
|ioctl_int, ioctl_ptr |not implemented (no ioctl in |
| |Windows) |
|select |implemented, but works only for |
| |sockets; use threads if you need |
| |to wait on other kinds of file |
| |descriptors |
|lockf |not implemented |
|kill, pause |not implemented (no inter-process |
| |signals in Windows) |
|alarm, times |not implemented |
|getitimer, setitimer |not implemented |
|getuid, getgid |always return 1 |
|getgid, getegid, getgroups |not implemented |
|setuid, setgid |not implemented |
|getpwnam, getpwuid |always raise Not_found |
|getgrnam, getgrgid |always raise Not_found |
|type socket_domain |the domain PF_UNIX is not supported; |
| |PF_INET is fully supported |
|establish_server |not implemented; use threads |
|terminal functions (tc*) |not implemented |
---------------------------------------------------------------------
Chapter 18
The num library: arbitrary-precision rational arithmetic
The num library implements exact-precision rational arithmetic. It is built
upon the state-of-the-art BigNum arbitrary-precision integer arithmetic
package, and therefore achieves very high performance.
The functions provided in this library are fully documented in The CAML
Numbers Reference Manual by Valerie Menissier-Morain, technical report 141,
INRIA, july 1992 (available by anonymous FTP from ftp.inria.fr, directory
INRIA/publications/RT, file RT-0141.ps.Z). A summary of the functions is given
below.
Unix:
Programs that use the num library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options nums.cma other files -cclib -lnums
ocamlopt other options nums.cmxa other files -cclib -lnums
For interactive use of the nums library, do:
ocamlmktop -custom -o mytop nums.cma -cclib -lnums
./mytop
Windows:
Programs that use the num library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options nums.cma other files %CAMLLIB%/libnums.lib
ocamlopt other options nums.cmxa other files %CAMLLIB%/libnums.lib
For interactive use of the nums library, do:
ocamlmktop -custom -o mytop.exe nums.cma %CAMLLIB%/libnums.lib
./mytop.exe
213
Chapter 18. The num library: arbitrary-precision rational arithmetic 214
18.1 Module Num: operation on arbitrary-precision numbers
open Nat
open Big_int
open Ratio
Numbers (type num) are arbitrary-precision rational numbers, plus the
special elements 1/0 (infinity) and 0/0 (undefined).
type num = Int of int | Big_int of big_int | Ratio of ratio
The type of numbers.
Arithmetic operations
val (+/) : num -> num -> num
val add_num : num -> num -> num
Addition
val minus_num : num -> num
Unary negation.
val (-/) : num -> num -> num
val sub_num : num -> num -> num
Subtraction
val (*/) : num -> num -> num
val mult_num : num -> num -> num
Multiplication
val square_num : num -> num
Squaring
val (//) : num -> num -> num
val div_num : num -> num -> num
Division
val quo_num : num -> num -> num
val mod_num : num -> num -> num
Euclidean division: quotient and remainder
val (**/) : num -> num -> num
val power_num : num -> num -> num
Exponentiation
val is_integer_num : num -> bool
Test if a number is an integer
Chapter 18. The num library: arbitrary-precision rational arithmetic 215
val integer_num : num -> num
val floor_num : num -> num
val round_num : num -> num
val ceiling_num : num -> num
Approximate a number by an integer. floor_num n returns the largest
integer smaller or equal to n. ceiling_num n returns the smallest
integer bigger or equal to n. integer_num n returns the integer closest
to n. In case of ties, rounds towards zero. round_num n returns the
integer closest to n. In case of ties, rounds off zero.
val sign_num : num -> int
Return -1, 0 or 1 according to the sign of the argument.
val (=/) : num -> num -> bool
val (</) : num -> num -> bool
val (>/) : num -> num -> bool
val (<=/) : num -> num -> bool
val (>=/) : num -> num -> bool
val (<>/) : num -> num -> bool
val eq_num : num -> num -> bool
val lt_num : num -> num -> bool
val le_num : num -> num -> bool
val gt_num : num -> num -> bool
val ge_num : num -> num -> bool
Usual comparisons between numbers
val compare_num : num -> num -> int
Return -1, 0 or 1 if the first argument is less than, equal to, or
greater than the second argument.
val max_num : num -> num -> num
val min_num : num -> num -> num
Return the greater (resp. the smaller) of the two arguments.
val abs_num : num -> num
Absolute value.
val succ_num: num -> num
succ n is n+1
val pred_num: num -> num
pred n is n-1
val incr_num: num ref -> unit
incr r is r:=!r+1, where r is a reference to a number.
val decr_num: num ref -> unit
decr r is r:=!r-1, where r is a reference to a number.
Chapter 18. The num library: arbitrary-precision rational arithmetic 216
Coercions with strings
val string_of_num : num -> string
Convert a number to a string, using fractional notation.
val approx_num_fix : int -> num -> string
val approx_num_exp : int -> num -> string
Approximate a number by a decimal. The first argument is the required
precision. The second argument is the number to approximate. approx_fix
uses decimal notation; the first argument is the number of digits after
the decimal point. approx_exp uses scientific (exponential) notation;
the first argument is the number of digits in the mantissa.
val num_of_string : string -> num
Convert a string to a number.
Coercions between numerical types
val int_of_num : num -> int
val num_of_int : int -> num
val nat_of_num : num -> nat
val num_of_nat : nat -> num
val num_of_big_int : big_int -> num
val big_int_of_num : num -> big_int
val ratio_of_num : num -> ratio
val num_of_ratio : ratio -> num
val float_of_num : num -> float
18.2 Module Arith_status: flags that control rational arithmetic
val arith_status: unit -> unit
Print the current status of the arithmetic flags.
val get_error_when_null_denominator : unit -> bool
val set_error_when_null_denominator : bool -> unit
Get or set the flag null_denominator. When on, attempting to create a
rational with a null denominator raises an exception. When off,
rationals with null denominators are accepted. Initially: on.
val get_normalize_ratio : unit -> bool
val set_normalize_ratio : bool -> unit
Get or set the flag normalize_ratio. When on, rational numbers are
normalized after each operation. When off, rational numbers are not
normalized until printed. Initially: off.
val get_normalize_ratio_when_printing : unit -> bool
val set_normalize_ratio_when_printing : bool -> unit
Get or set the flag normalize_ratio_when_printing. When on, rational
numbers are normalized before being printed. When off, rational numbers
Chapter 18. The num library: arbitrary-precision rational arithmetic 217
are printed as is, without normalization. Initially: on.
val get_approx_printing : unit -> bool
val set_approx_printing : bool -> unit
Get or set the flag approx_printing. When on, rational numbers are
printed as a decimal approximation. When off, rational numbers are
printed as a fraction. Initially: off.
val get_floating_precision : unit -> int
val set_floating_precision : int -> unit
Get or set the parameter floating_precision. This parameter is the
number of digits displayed when approx_printing is on. Initially: 12.
Chapter 19
The str library: regular expressions and string processing
The str library provides high-level string processing functions, some based on
regular expressions. It is intended to support the kind of file processing
that is usually performed with scripting languages such as awk, perl or sed.
Unix:
Programs that use the str library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options str.cma other files -cclib -lstr
ocamlopt other options str.cmxa other files -cclib -lstr
For interactive use of the str library, do:
ocamlmktop -custom -o mytop str.cma -cclib -lstr
./mytop
Windows:
Programs that use the str library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options str.cma other files %CAMLLIB%/libstr.lib
ocamlopt other options str.cmxa other files %CAMLLIB%/libstr.lib
For interactive use of the str library, do:
ocamlmktop -custom -o mytop.exe str.cma %CAMLLIB%/libstr.lib
./mytop.exe
19.1 Module Str: regular expressions and high-level string processing
Regular expressions
type regexp
218
Chapter 19. The str library: regular expressions and string processing 219
The type of compiled regular expressions.
val regexp: string -> regexp
Compile a regular expression. The syntax for regular expressions is the
same as in Gnu Emacs. The special characters are
$^.*+?[]]. The following constructs are recognized:
\\[\smallskipamount]\verb. ` matches any character except newline
* (postfix) matches the previous expression zero, one or several
times
+ (postfix) matches the previous expression one or several times
? (postfix) matches the previous expression once or not at all
[..] character set; ranges are denoted with -, as in a-z; an initial ^,
as in ^0-9, complements the set
^ matches at beginning of line
$ matches at end of line
\| (infix) alternative between two expressions
\(..\) grouping and naming of the enclosed expression
\1 the text matched by the first \(...\) expression (\2 for the
second expression, etc)
\b matches word boundaries
\ quotes special characters.
val regexp_case_fold: string -> regexp
Same as regexp, but the compiled expression will match text in a
case-insensitive way: uppercase and lowercase letters will be considered
equivalent.
String matching and searching
val string_match: regexp -> string -> int -> bool
string_match r s start tests whether the characters in s starting at
position start match the regular expression r. The first character of a
string has position 0, as usual.
val search_forward: regexp -> string -> int -> int
search_forward r s start searchs the string s for a substring matching
the regular expression r. The search starts at position start and
proceeds towards the end of the string. Return the position of the first
character of the matched substring, or raise Not_found if no substring
matches.
val search_backward: regexp -> string -> int -> int
Same as search_forward, but the search proceeds towards the beginning of
the string.
val matched_string: string -> string
matched_string s returns the substring of s that was matched by the
latest string_match, search_forward or search_backward. The user must
make sure that the parameter s is the same string that was passed to the
matching or searching function.
Chapter 19. The str library: regular expressions and string processing 220
val match_beginning: unit -> int
val match_end: unit -> int
match_beginning() returns the position of the first character of the
substring that was matched by string_match, search_forward or
search_backward. match_end() returns the position of the character
following the last character of the matched substring.
val matched_group: int -> string -> string
matched_group n s returns the substring of s that was matched by the nth
group \(...\) of the regular expression during the latest string_match,
search_forward or search_backward. The user must make sure that the
parameter s is the same string that was passed to the matching or
searching function.
val group_beginning: int -> int
val group_end: int -> int
group_beginning n returns the position of the first character of the
substring that was matched by the nth group of the regular expression.
group_end n returns the position of the character following the last
character of the matched substring.
Replacement
val global_replace: regexp -> string -> string -> string
global_replace regexp repl s returns a string identical to s, except that
all substrings of s that match regexp have been replaced by repl. The
replacement text repl can contain \1, \2, etc; these sequences will be
replaced by the text matched by the corresponding group in the regular
expression. \0 stands for the text matched by the whole regular
expression.
val replace_first: regexp -> string -> string -> string
Same as global_replace, except that only the first substring matching the
regular expression is replaced.
val global_substitute: regexp -> (string -> string) -> string -> string
global_substitute regexp subst s returns a string identical to s, except
that all substrings of s that match regexp have been replaced by the
result of function subst. The function subst is called once for each
matching substring, and receives s (the whole text) as argument.
val substitute_first: regexp -> (string -> string) -> string -> string
Same as global_substitute, except that only the first substring matching
the regular expression is replaced.
Splitting
val split: regexp -> string -> string list
Chapter 19. The str library: regular expressions and string processing 221
split r s splits s into substrings, taking as delimiters the substrings
that match r, and returns the list of substrings. For instance,
split (regexp "[ \t]+") s splits s into blank-separated words.
val bounded_split: regexp -> string -> int -> string list
Same as split, but splits into at most n substrings, where n is the extra
integer parameter.
Extracting substrings
val string_before: string -> int -> string
string_before s n returns the substring of all characters of s that
precede position n (excluding the character at position n).
val string_after: string -> int -> string
string_after s n returns the substring of all characters of s that follow
position n (including the character at position n).
val first_chars: string -> int -> string
first_chars s n returns the first n characters of s. This is the same
function as string_before.
val last_chars: string -> int -> string
last_chars s n returns the last n characters of s.
Chapter 20
The threads library
The threads library allows concurrent programming in Objective Caml. It
provides multiple threads of control (also called lightweight processes) that
execute concurrently in the same memory space. Threads communicate by
in-place modification of shared data structures, or by sending and receiving
data on communication channels.
The threads library is implemented by time-sharing on a single processor.
It will not take advantage of multi-processor machines. Using this library
will therefore never make programs run faster. However, many programs are
easier to write when structured as several communicating processes.
Unix:
Programs that use the threads library must be linked as follows:
ocamlc -thread -custom other options threads.cma other files -
cclib -lthreads
All object files on the command line must also have been compiled with
the -thread option, which selects a special, thread-safe version of the
standard library (see chapter 6).
The default thread implementation cannot be used in native-code programs
compiled with ocamlopt. If your operating system provides POSIX 1003.1c
compliant threads, you can select an alternate implementation when
configuring Objective Caml (use the -with-pthread option to configure)
which also supports native-code programs. Programs that use this
alternate implementation of the threads library must be linked as
follows:
ocamlc -thread -custom other options threads.cma other files \
-cclib -lthreads -cclib -lunix -cclib -lpthread
ocamlopt -thread other options threads.cmxa other files \
-cclib -lthreadsnat -cclib -lunix -cclib -lpthread
Depending on the operating system, extra system libraries can be
necessary. For instance, under Solaris 2.5, add -cclib -lposix4 at the
end of the command line.
Windows:
Programs that use the threads library must be linked as follows:
222
Chapter 20. The threads library 223
ocamlc -thread -custom other options threads.cma other files \
%CAMLLIB%/libthreads.lib %CAMLLIB%/libunix.lib
ocamlopt -thread -custom other options threads.cmxa other files \
%CAMLLIB%/libthreadsnat.lib %CAMLLIB%/libunix.lib
All object files on the command line must also have been compiled with
the -thread option, which selects a special, thread-safe version of the
standard library (see chapter 6).
20.1 Module Thread: lightweight threads
type t
The type of thread handles.
Thread creation and termination
val create : ('a -> 'b) -> 'a -> t
Thread.create funct arg creates a new thread of control, in which the
function application funct arg is executed concurrently with the other
threads of the program. The application of Thread.create returns the
handle of the newly created thread. The new thread terminates when the
application funct arg returns, either normally or by raising an uncaught
exception. In the latter case, the exception is printed on standard
error, but not propagated back to the parent thread. Similarly, the
result of the application funct arg is discarded and not directly
accessible to the parent thread.
val self : unit -> t
Return the thread currently executing.
val id : t -> int
Return the identifier of the given thread. A thread identifier is an
integer that identifies uniquely the thread. It can be used to build
data structures indexed by threads.
val exit : unit -> unit
Terminate prematurely the currently executing thread.
val kill : t -> unit
Terminate prematurely the thread whose handle is given. This
functionality is available only with bytecode-level threads.
Suspending threads
val delay: float -> unit
delay d suspends the execution of the calling thread for d seconds. The
other program threads continue to run during this time.
Chapter 20. The threads library 224
val join : t -> unit
join th suspends the execution of the calling thread until the thread th
has terminated.
val wait_read : Unix.file_descr -> unit
val wait_write : Unix.file_descr -> unit
Suspend the execution of the calling thread until at least one character
is available for reading (wait_read) or one character can be written
without blocking (wait_write) on the given Unix file descriptor.
val wait_timed_read : Unix.file_descr -> float -> bool
val wait_timed_write : Unix.file_descr -> float -> bool
Same as wait_read and wait_write, but wait for at most the amount of time
given as second argument (in seconds). Return true if the file
descriptor is ready for input/output and false if the timeout expired.
val select :
Unix.file_descr list -> Unix.file_descr list ->
Unix.file_descr list -> float ->
Unix.file_descr list * Unix.file_descr list * Unix.file_descr list
Suspend the execution of the calling thead until input/output becomes
possible on the given Unix file descriptors. The arguments and results
have the same meaning as for Unix.select.
val wait_pid : int -> int * Unix.process_status
wait_pid p suspends the execution of the calling thread until the Unix
process specified by the process identifier p terminates. A pid p of -1
means wait for any child. A pid of 0 means wait for any child in the
same process group as the current process. Negative pid arguments
represent process groups. Returns the pid of the child caught and its
termination status, as per Unix.wait.
20.2 Module Mutex: locks for mutual exclusion
Mutexes (mutual-exclusion locks) are used to implement critical sections
and protect shared mutable data structures against concurrent accesses.
The typical use is (if m is the mutex associated with the data structure
D):
Mutex.lock m;
(* Critical section that operates over D *);
Mutex.unlock m
type t
The type of mutexes.
val create: unit -> t
Chapter 20. The threads library 225
Return a new mutex.
val lock: t -> unit
Lock the given mutex. Only one thread can have the mutex locked at any
time. A thread that attempts to lock a mutex already locked by another
thread will suspend until the other thread unlocks the mutex.
val try_lock: t -> bool
Same as try_lock, but does not suspend the calling thread if the mutex is
already locked: just return false immediately in that case. If the
mutex is unlocked, lock it and return true.
val unlock: t -> unit
Unlock the given mutex. Other threads suspended trying to lock the mutex
will restart.
20.3 Module Condition: condition variables to synchronize between threads
Condition variables are used when one thread wants to wait until another
thread has finished doing something: the former thread ``waits'' on the
condition variable, the latter thread ``signals'' the condition when it
is done. Condition variables should always be protected by a mutex. The
typical use is (if D is a shared data structure, m its mutex, and c is a
condition variable):
Mutex.lock m;
while (* some predicate P over D is not satisfied *) do
Condition.wait c m
done;
(* Modify D *)
if (* the predicate P over D is now satified *) then Condi-
tion.signal c;
Mutex.unlock m
type t
The type of condition variables.
val create: unit -> t
Return a new condition variable.
val wait: t -> Mutex.t -> unit
wait c m atomically unlocks the mutex m and suspends the calling process
on the condition variable c. The process will restart after the
condition variable c has been signalled. The mutex m is locked again
before wait returns.
val signal: t -> unit
Chapter 20. The threads library 226
signal c restarts one of the processes waiting on the condition variable
c.
val broadcast: t -> unit
broadcast c restarts all processes waiting on the condition variable c.
20.4 Module Event: first-class synchronous communication
This module implements synchronous interprocess communications over
channels. As in John Reppy's Concurrent ML system, the communication
events are first-class values: they can be built and combined
independently before being offered for communication.
type 'a channel
The type of communication channels carrying values of type 'a.
val new_channel: unit -> 'a channel
Return a new channel.
type 'a event
The type of communication events returning a result of type 'a.
val send: 'a channel -> 'a -> unit event
send ch v returns the event consisting in sending the value v over the
channel ch. The result value of this event is ().
val receive: 'a channel -> 'a event
receive ch returns the event consisting in receiving a value from the
channel ch. The result value of this event is the value received.
val always: 'a -> 'a event
always v returns an event that is always ready for synchronization. The
result value of this event is v.
val choose: 'a event list -> 'a event
choose evl returns the event that is the alternative of all the events in
the list evl.
val wrap: 'a event -> ('a -> 'b) -> 'b event
wrap ev fn returns the event that performs the same communications as ev,
then applies the post-processing function fn on the return value.
val guard: (unit -> 'a event) -> 'a event
guard fn returns the event that, when synchronized, computes fn() and
behaves as the resulting event. This allows to compute events with
side-effects at the time of the synchronization operation.
Chapter 20. The threads library 227
val sync: 'a event -> 'a
``Synchronize'' on an event: offer all the communication possibilities
specified in the event to the outside world, and block until one of the
communications succeed. The result value of that communication is
returned.
val select: 'a event list -> 'a
``Synchronize'' on an alternative of events. select evl is shorthand for
sync(choose evl).
val poll: 'a event -> 'a option
Non-blocking version of sync: offer all the communication possibilities
specified in the event to the outside world, and if one can take place
immediately, perform it and return Some r where r is the result value of
that communication. Otherwise, return None without blocking.
20.5 Module ThreadUnix: thread-compatible system calls
This module reimplements some of the functions from Unix so that they
only block the calling thread, not all threads in the program, if they
cannot complete immediately. See the documentation of the Unix module
for more precise descriptions of the functions below.
Process handling
val execv : string -> string array -> unit
val execve : string -> string array -> string array -> unit
val execvp : string -> string array -> unit
val wait : unit -> int * Unix.process_status
val waitpid : Unix.wait_flag list -> int -> int * Unix.process_status
val system : string -> Unix.process_status
Basic input/output
val read : Unix.file_descr -> string -> int -> int -> int
val write : Unix.file_descr -> string -> int -> int -> int
Input/output with timeout
val timed_read : Unix.file_descr -> string -> int -> int -> float -> int
val timed_write : Unix.file_descr -> string -> int -> int -> float -> int
Behave as read and write, except that Unix_error(ETIMEDOUT,_,_) is raised
if no data is available for reading or ready for writing after d seconds.
The delay d is given in the fifth argument, in seconds.
Polling
val select :
Unix.file_descr list -> Unix.file_descr list ->
Unix.file_descr list -> float ->
Chapter 20. The threads library 228
Unix.file_descr list * Unix.file_descr list * Unix.file_descr list
Pipes and redirections
val pipe : unit -> Unix.file_descr * Unix.file_descr
val open_process_out: string -> out_channel
val open_process: string -> in_channel * out_channel
Time
val sleep : int -> unit
Sockets
val socket : Unix.socket_domain -> Unix.socket_type -> int -> Unix.file_descr
val socketpair : Unix.socket_domain -> Unix.socket_type -> int ->
Unix.file_descr * Unix.file_descr
val accept : Unix.file_descr -> Unix.file_descr * Unix.sockaddr
val connect : Unix.file_descr -> Unix.sockaddr -> unit
val recv : Unix.file_descr -> string -> int -> int -> Unix.msg_flag list -
> int
val recvfrom : Unix.file_descr -> string -> int -> int ->
Unix.msg_flag list -> int * Unix.sockaddr
val send : Unix.file_descr -> string -> int -> int ->
Unix.msg_flag list -> int
val sendto : Unix.file_descr -> string -> int -> int ->
Unix.msg_flag list -> Unix.sockaddr -> int
val open_connection : Unix.sockaddr -> in_channel * out_channel
val establish_server :
(in_channel -> out_channel -> 'a) -> Unix.sockaddr -> unit
Chapter 21
The graphics library
The graphics library provides a set of portable drawing primitives. Drawing
takes place in a separate window that is created when open_graph is called.
Unix:
This library is implemented under the X11 windows system. Programs that
use the graphics library must be linked as follows:
ocamlc -custom other options graphics.cma other files \
-cclib -lgraphics -cclib -lX11
For interactive use of the graphics library, do:
ocamlmktop -custom -o mytop graphics.cma -cclib -lgraphics -
cclib -lX11
./mytop
Here are the graphics mode specifications supported by open_graph on the
X11 implementation of this library: the argument to open_graph has the
format "display-name geometry", where display-name is the name of the
X-windows display to connect to, and geometry is a standard X-windows
geometry specification. The two components are separated by a space.
Either can be omitted, or both. Examples:
open_graph "foo:0"
connects to the display foo:0 and creates a window with the default
geometry
open_graph "foo:0 300x100+50-0"
connects to the display foo:0 and creates a window 300 pixels wide by
100 pixels tall, at location (50,0)
open_graph " 300x100+50-0"
connects to the default display and creates a window 300 pixels wide
by 100 pixels tall, at location (50,0)
open_graph ""
connects to the default display and creates a window with the default
geometry.
229
Chapter 21. The graphics library 230
Windows:
This library is available only under the toplevel application
ocamlwin.exe. Before using it, the Caml part of this library must be
loaded in-core, either by typing
#load graphics.cmo;;
in the input windows or by using the ``Load'' entry of the ``File'' menu.
The screen coordinates are interpreted as shown in the figure below. Notice
that the coordinate system used is the same as in mathematics: y increases
from the bottom of the screen to the top of the screen, and angles are
measured counterclockwisey(in degrees). Drawing is clipped to the screen.
|
-------------------------
size_y() | |
| Screen |
| |
| |
| |pixel at (x,y) |
y ---------
| | |
| | |
| | |
| | |
| |
------------------------------
| | | x
| x size_x()
21.1 Module Graphics: machine-independent graphics primitives
exception Graphic_failure of string
Raised by the functions below when they encounter an error.
Initializations
val open_graph: string -> unit
Show the graphics window or switch the screen to graphic mode. The
graphics window is cleared. The string argument is used to pass optional
information on the desired graphics mode, the graphics window size, and
so on. Its interpretation is implementation-dependent. If the empty
string is given, a sensible default is selected.
val close_graph: unit -> unit
Delete the graphics window or switch the screen back to text mode.
val clear_graph : unit -> unit
Erase the graphics window.
val size_x : unit -> int
val size_y : unit -> int
Chapter 21. The graphics library 231
Return the size of the graphics window. Coordinates of the screen pixels
range over 0 .. size_x()-1 and 0 .. size_y()-1. Drawings outside of this
rectangle are clipped, without causing an error. The origin (0,0) is at
the lower left corner.
Colors
type color = int
A color is specified by its R, G, B components. Each component is in the
range 0..255. The three components are packed in an int: 0xRRGGBB,
where RR are the two hexadecimal digits for the red component, GG for the
green component, BB for the blue component.
val rgb: int -> int -> int -> color
rgb r g b returns the integer encoding the color with red component r,
green component g, and blue component b. r, g and b are in the range
0..255.
val set_color : color -> unit
Set the current drawing color.
val black : color
val white : color
val red : color
val green : color
val blue : color
val yellow : color
val cyan : color
val magenta : color
Some predefined colors.
val background: color
val foreground: color
Default background and foreground colors (usually, either black
foreground on a white background or white foreground on a black
background). clear_graph fills the screen with the background color.
The initial drawing color is foreground.
Point and line drawing
val plot : int -> int -> unit
Plot the given point with the current drawing color.
val point_color : int -> int -> color
Return the color of the given point.
val moveto : int -> int -> unit
Chapter 21. The graphics library 232
Position the current point.
val current_point : unit -> int * int
Return the position of the current point.
val lineto : int -> int -> unit
Draw a line with endpoints the current point and the given point, and
move the current point to the given point.
val draw_arc : int -> int -> int -> int -> int -> int -> unit
draw_arc x y rx ry a1 a2 draws an elliptical arc with center x,y,
horizontal radius rx, vertical radius ry, from angle a1 to angle a2 (in
degrees). The current point is unchanged.
val draw_ellipse : int -> int -> int -> int -> unit
draw_ellipse x y rx ry draws an ellipse with center x,y, horizontal
radius rx and vertical radius ry. The current point is unchanged.
val draw_circle : int -> int -> int -> unit
draw_circle x y r draws a circle with center x,y and radius r. The
current point is unchanged.
val set_line_width : int -> unit
Set the width of points and lines drawn with the functions above. Under
X Windows, set_line_width 0 selects a width of 1 pixel and a faster, but
less precise drawing algorithm than the one used when set_line_width 1 is
specified.
Text drawing
val draw_char : char -> unit
val draw_string : string -> unit
Draw a character or a character string with lower left corner at current
position. After drawing, the current position is set to the lower right
corner of the text drawn.
val set_font : string -> unit
val set_text_size : int -> unit
Set the font and character size used for drawing text. The
interpretation of the arguments to set_font and set_text_size is
implementation-dependent.
val text_size : string -> int * int
Return the dimensions of the given text, if it were drawn with the
current font and size.
Chapter 21. The graphics library 233
Filling
val fill_rect : int -> int -> int -> int -> unit
fill_rect x y w h fills the rectangle with lower left corner at x,y,
width w and heigth h, with the current color.
val fill_poly : (int * int) array -> unit
Fill the given polygon with the current color. The array contains the
coordinates of the vertices of the polygon.
val fill_arc : int -> int -> int -> int -> int -> int -> unit
Fill an elliptical pie slice with the current color. The parameters are
the same as for draw_arc.
val fill_ellipse : int -> int -> int -> int -> unit
Fill an ellipse with the current color. The parameters are the same as
for draw_ellipse.
val fill_circle : int -> int -> int -> unit
Fill a circle with the current color. The parameters are the same as for
draw_circle.
Images
type image
The abstract type for images, in internal representation. Externally,
images are represented as matrices of colors.
val transp : color
In matrices of colors, this color represent a ``transparent'' point:
when drawing the corresponding image, all pixels on the screen
corresponding to a transparent pixel in the image will not be modified,
while other points will be set to the color of the corresponding point in
the image. This allows superimposing an image over an existing
background.
val make_image : color array array -> image
Convert the given color matrix to an image. Each sub-array represents
one horizontal line. All sub-arrays must have the same length;
otherwise, exception Graphic_failure is raised.
val dump_image : image -> color array array
Convert an image to a color matrix.
val draw_image : image -> int -> int -> unit
Draw the given image with lower left corner at the given point.
Chapter 21. The graphics library 234
val get_image : int -> int -> int -> int -> image
Capture the contents of a rectangle on the screen as an image. The
parameters are the same as for fill_rect.
val create_image : int -> int -> image
create_image w h returns a new image w pixels wide and h pixels tall, to
be used in conjunction with blit_image. The initial image contents are
random.
val blit_image : image -> int -> int -> unit
blit_image img x y copies screen pixels into the image img, modifying img
in-place. The pixels copied are those inside the rectangle with lower
left corner at x,y, and width and height equal to those of the image.
Mouse and keyboard events
type status =
{ mouse_x : int; (* X coordinate of the mouse *)
mouse_y : int; (* Y coordinate of the mouse *)
button : bool; (* true if a mouse button is pressed *)
keypressed : bool; (* true if a key has been pressed *)
key : char } (* the character for the key pressed *)
To report events.
type event =
Button_down (* A mouse button is pressed *)
| Button_up (* A mouse button is released *)
| Key_pressed (* A key is pressed *)
| Mouse_motion (* The mouse is moved *)
| Poll (* Don't wait; return immediately *)
To specify events to wait for.
val wait_next_event : event list -> status
Wait until one of the events specified in the given event list occurs,
and return the status of the mouse and keyboard at that time. If Poll is
given in the event list, return immediately with the current status. If
the mouse cursor is outside of the graphics window, the mouse_x and
mouse_y fields of the event are outside the range
0..size_x()-1, 0..size_y()-1. Keypresses are queued, and dequeued one by
one when the Key_pressed event is specified.
Mouse and keyboard polling
val mouse_pos : unit -> int * int
Return the position of the mouse cursor, relative to the graphics window.
If the mouse cursor is outside of the graphics window, mouse_pos()
returns a point outside of the range 0..size_x()-1, 0..size_y()-1.
Chapter 21. The graphics library 235
val button_down : unit -> bool
Return true if the mouse button is pressed, false otherwise.
val read_key : unit -> char
Wait for a key to be pressed, and return the corresponding character.
Keypresses are queued.
val key_pressed : unit -> bool
Return true if a keypress is available; that is, if read_key would not
block.
Sound
val sound : int -> int -> unit
sound freq dur plays a sound at frequency freq (in hertz) for a duration
dur (in milliseconds).
Chapter 22
The dbm library: access to NDBM databases
The dbm library provides access to NDBM databases under Unix. NDBM databases
maintain key/data associations, where both the key and the data are arbitrary
strings. They support fairly large databases (several gigabytes) and can
retrieve a keyed item in one or two file system accesses. Refer to the Unix
manual pages for more information.
Unix:
Programs that use the dbm library must be linked in ``custom runtime''
mode, as follows:
ocamlc -custom other options dbm.cma other files -cclib -lmldbm -
cclib -lndbm
ocamlopt other options dbm.cmxa other files -cclib -lmldbm -
cclib -lndbm
For interactive use of the dbm library, do:
ocamlmktop -custom -o mytop dbm.cma -cclib -lmldbm -cclib -lndbm
./mytop
Depending on the Unix system used, the -cclib -lndbm option is not always
necessary, or the library may have another name than -lndbm.
Windows:
This library is not available.
22.1 Module Dbm: interface to the NDBM databases
type t
The type of file descriptors opened on NDBM databases.
type open_flag =
Dbm_rdonly | Dbm_wronly | Dbm_rdwr | Dbm_create
Flags for opening a database (see opendbm).
exception Dbm_error of string
236
Chapter 22. The dbm library: access to NDBM databases 237
Raised by the following functions when an error is encountered.
val opendbm : string -> open_flag list -> int -> t
Open a descriptor on an NDBM database. The first argument is the name of
the database (without the .dir and .pag suffixes). The second argument
is a list of flags: Dbm_rdonly opens the database for reading only,
Dbm_wronly for writing only, Dbm_rdwr for reading and writing; Dbm_create
causes the database to be created if it does not already exist. The
third argument is the permissions to give to the database files, if the
database is created.
val close : t -> unit
Close the given descriptor.
val find : t -> string -> string
find db key returns the data associated with the given key in the
database opened for the descriptor db. Raise Not_found if the key has no
associated data.
val add : t -> string -> string -> unit
add db key data inserts the pair (key, data) in the database db. If the
database already contains data associated with key, raise
Dbm_error "Entry already exists".
val replace : t -> string -> string -> unit
replace db key data inserts the pair (key, data) in the database db. If
the database already contains data associated with key, that data is
discarded and silently replaced by the new data.
val remove : t -> string -> unit
remove db key data removes the data associated with key in db. If key
has no associated data, raise Dbm_error "dbm_delete".
val firstkey : t -> string
val nextkey : t -> string
Enumerate all keys in the given database, in an unspecified order.
firstkey db returns the first key, and repeated calls to nextkey db
return the remaining keys. Not_found is raised when all keys have been
enumerated.
val iter : (string -> string -> 'a) -> t -> unit
iter f db applies f to each (key, data) pair in the database db. f
receives key as first argument and data as second argument.
Part V
Appendix
238
Index to the library
! (operator), 151 alarm, 203
!= (operator), 141 always, 226
& (operator), 141 append, 156
&& (operator), 141 approx_num_exp, 216
* (operator), 142 approx_num_fix, 216
** (operator), 144 Arg (module), 154
**/ (operator), 214 argv, 189
*. (operator), 143 arith_status, 216
*/ (operator), 214 Arith_status (module), 216
+ (operator), 142 Array (module), 155
+. (operator), 143 asin, 144
+/ (operator), 214 asr (operator), 143
- (operator), 142 Assert_failure (exception), 73, 139
-. (operator), 143 assoc, 175
-/ (operator), 214 assq, 175
/ (operator), 142 at_exit, 151
/. (operator), 144 atan, 144
// (operator), 214 atan2, 144
:= (operator), 151 background, 231
< (operator), 141 Bad (exception), 155
</ (operator), 215 basename, 159
<= (operator), 141 big_int_of_num, 216
<=/ (operator), 215 bind, 207
<> (operator), 141 bits, 182
<>/ (operator), 215 black, 231
= (operator), 140 blit, 156, 188, 191
=/ (operator), 215 blit_image, 234
== (operator), 141 blue, 231
> (operator), 141 bounded_split, 221
>/ (operator), 215 Break (exception), 190
>= (operator), 141 broadcast, 226
>=/ (operator), 215 button_down, 235
@ (operator), 146
^ (operator), 145 Callback (module), 157
|| (operator), 142 capitalize, 188
~- (operator), 142 catch, 180
~-. (operator), 143 catch_break, 190
abs, 142 ceil, 144
abs_float, 144 ceiling_num, 215
abs_num, 215 channel, 158
accept, 206, 228 Char (module), 158
access, 199 chdir, 189, 200
acos, 144 check_suffix, 159
add, 170, 181, 237 chmod, 199
add_num, 214 choose, 226
239
Index to the library 240
chop_extension, 159 draw_ellipse, 232
chop_suffix, 159 draw_image, 233
chown, 199 draw_string, 232
chr, 158 dump_image, 233
clear, 170, 182, 185 dup, 199
clear_close_on_exec, 200 dup2, 199
clear_graph, 230 empty, 186
clear_nonblock, 199 Empty (exception), 181, 185
clear_parser, 180 End_of_file (exception), 140
close, 197, 237 environment, 194
close_box, 161 eprintf, 166, 181
close_graph, 230 eq_num, 215
close_in, 151 err_formatter, 165
close_out, 149 Error (exception), 185
close_process, 201 error_message, 194
close_process_in, 201 escaped, 158, 188
close_process_out, 201 establish_server, 208, 228
close_tbox, 164 Event (module), 226
closedir, 200 execv, 195, 227
code, 158 execve, 195, 227
combine, 176 execvp, 195, 227
command, 189 execvpe, 195
compact, 169 exists, 175
compare, 141 exists2, 175
compare_num, 215 exit, 151, 223
concat, 156, 159, 174, 188 Exit (exception), 140
Condition (module), 225 exp, 144
connect, 207, 228
copy, 156, 179, 187 Failure (exception), 140, 185
cos, 144 failwith, 140
cosh, 144 fchmod, 199
count, 187 fchown, 199
create, 156, 170, 181, 185, 187, 191, file, 159
223--225 file_exists, 189
create_image, 234 Filename (module), 159
create_matrix, 156 fill, 156, 187, 191
create_process, 201 fill_arc, 233
create_process_env, 201 fill_circle, 233
current, 155 fill_ellipse, 233
current_dir_name, 159 fill_poly, 233
current_point, 232 fill_rect, 233
cyan, 231 find, 170, 237
data_size, 179 find_all, 170
Dbm (module), 236 first_chars, 221
Dbm_error (exception), 236 firstkey, 237
decr, 151 flatten, 174
decr_num, 215 float, 144, 182
delay, 223 float_of_num, 216
descr_of_in_channel, 197 float_of_string, 145
descr_of_out_channel, 197 floor, 144
Digest (module), 158 floor_num, 215
dirname, 159 flush, 148
div_num, 214 fold_left, 157, 174
Division_by_zero (exception), 140 fold_left2, 174
draw_arc, 232 fold_right, 157, 174
draw_char, 232 fold_right2, 174
draw_circle, 232 for_all, 175
Index to the library 241
for_all2, 175 getuid, 204
force, 74, 172 global_replace, 220
force_newline, 162 global_substitute, 220
foreground, 231 gmtime, 203
fork, 195 Graphic_failure (exception), 230
Format (module), 160 Graphics (module), 230
fprintf, 166, 180 green, 231
frexp, 144 group_beginning, 220
from, 186 group_end, 220
from_channel, 172, 178 gt_num, 215
from_function, 172 guard, 226
from_string, 172, 179 handle_unix_error, 194
fst, 145 hash, 171
fstat, 198 hash_param, 171
ftruncate, 198 HashedType (module type), 171
full_init, 182 Hashtbl (module), 170
full_major, 168 hd, 173
Gc (module), 166 header_size, 179
ge_num, 215 id, 223
Genlex (module), 169 in_channel_length, 150
get, 155, 168, 187, 191 in_channel_of_descr, 197
get_approx_printing, 217 incr, 151
get_ellipsis_text, 164 incr_num, 215
get_error_when_null_denominator, 216 index, 188
get_floating_precision, 217 index_from, 188
get_formatter_output_functions, 164 inet_addr_any, 206
get_image, 234 inet_addr_of_string, 206
get_margin, 162 init, 156, 182
get_max_boxes, 163 input, 150, 159
get_max_indent, 163 input_binary_int, 150
get_normalize_ratio, 216 input_byte, 150
get_normalize_ratio_when_printing, 216 input_char, 149
getcwd, 189, 200 input_line, 150
getegid, 205 input_value, 150
getenv, 189 int, 182
geteuid, 204 int_of_num, 216
getgid, 204 int_of_string, 145
getgrgid, 205 integer_num, 215
getgrnam, 205 interactive, 189
getgroups, 205 invalid_arg, 140
gethostbyaddr, 209 Invalid_argument (exception), 139
gethostbyname, 209 is_implicit, 159
gethostname, 209 is_integer_num, 214
getitimer, 204 is_relative, 159
getlogin, 205 iter, 157, 170, 174, 182, 185, 186,
getpeername, 207 237
getpid, 196 iter2, 174
getppid, 196 iteri, 157
getprotobyname, 209
getprotobynumber, 209 join, 224
getpwnam, 205 junk, 186
getpwuid, 205
getservbyname, 209 key_pressed, 235
getservbyport, 209 kill, 202, 223
getsockname, 207
getsockopt, 208 land (operator), 143
gettimeofday, 203 last_chars, 221
Index to the library 242
Lazy (module), 74, 172 min_num, 215
ldexp, 144 minor, 168
le_num, 215 minus_num, 214
length, 155, 173, 182, 185, 187, 191 mkdir, 200
lexeme, 173 mkfifo, 201
lexeme_char, 173 mktime, 203
lexeme_end, 173 mod (operator), 142
lexeme_start, 173 mod_float, 144
Lexing (module), 172 mod_num, 214
lineto, 232 modf, 144
link, 198 mouse_pos, 234
list, 184 moveto, 231
List (module), 173 mult_num, 214
listen, 207 Mutex (module), 224
lnot, 143 nat_of_num, 216
localtime, 203 new_channel, 226
lock, 225 next, 186
lockf, 202 nextkey, 237
log, 144 nice, 196
log10, 144 not, 141
lor (operator), 143 Not_found (exception), 140
lowercase, 158, 188 npeek, 187
lseek, 197 nth, 173
lsl (operator), 143 Num (module), 214
lsr (operator), 143 num_of_big_int, 216
lstat, 198 num_of_int, 216
lt_num, 215 num_of_nat, 216
lxor (operator), 143 num_of_ratio, 216
magenta, 231 num_of_string, 216
major, 168 of_channel, 186
make, 156, 187 of_list, 157, 186
Make (functor), 171, 177, 184 of_string, 186
make_formatter, 165 Oo (module), 179
make_image, 233 open_box, 161
make_lexer, 169 open_connection, 208, 228
make_matrix, 156 open_graph, 230
map, 157, 174 open_hbox, 163
Map (module), 176 open_hovbox, 163
map2, 174 open_hvbox, 163
mapi, 157 open_in, 149
Marshal (module), 177 open_in_bin, 149
match_beginning, 220 open_in_gen, 149
match_end, 220 open_out, 148
Match_failure (exception), 55--57, 139 open_out_bin, 148
matched_group, 220 open_out_gen, 148
matched_string, 219 open_process, 201, 228
max, 141 open_process_in, 201
max_array_length, 190 open_process_out, 201, 228
max_int, 142 open_tbox, 163
max_num, 215 open_vbox, 163
max_string_length, 190 opendbm, 237
mem, 175
mem_assoc, 175 opendir, 200
memq, 175 openfile, 196
merge, 184 or (operator), 142
min, 141 OrderedType (module type), 176, 183
min_int, 142 os_type, 189
Index to the library 243
out_channel_length, 149 pp_set_max_boxes, 166
out_channel_of_descr, 197 pp_set_max_indent, 166
Out_of_memory (exception), 140 pp_set_tab, 166
output, 148, 159 pred, 142
output_binary_int, 148 pred_num, 215
output_byte, 148 prerr_char, 146
output_char, 148 prerr_endline, 147
output_string, 148 prerr_float, 147
output_value, 149 prerr_int, 147
over_max_boxes, 163 prerr_newline, 147
parse, 154 prerr_string, 146
Parse_error (exception), 180 print, 180
Parsing (module), 179 print_as, 161
pause, 202 print_bool, 161
peek, 182, 186 print_break, 162
Pervasives (module), 138 print_char, 146, 161
pipe, 200, 228 print_cut, 161
plot, 231 print_endline, 146
point_color, 231 print_float, 146, 161
poll, 227 print_flush, 162
pop, 185 print_if_newline, 162
pos_in, 150 print_int, 146, 161
pos_out, 149 print_newline, 146, 162
power_num, 214 print_space, 161
pp_close_box, 166 print_stat, 168
pp_close_tbox, 166 print_string, 146, 161
pp_force_newline, 166 print_tab, 164
pp_get_ellipsis_text, 166 print_tbreak, 164
pp_get_formatter_output_functions, 166 Printexc (module), 180
pp_get_margin, 166 printf, 166, 181
pp_get_max_boxes, 166 Printf (module), 180
pp_get_max_indent, 166 push, 185
pp_open_box, 166 Queue (module), 181
pp_open_hbox, 166 quo_num, 214
pp_open_hovbox, 166 raise, 139
pp_open_hvbox, 166 Random (module), 182
pp_open_tbox, 166 ratio_of_num, 216
pp_open_vbox, 166 read, 197, 227
pp_over_max_boxes, 166 read_float, 147
pp_print_as, 166 read_int, 147
pp_print_bool, 166 read_key, 235
pp_print_break, 166 read_line, 147
pp_print_char, 166 readdir, 200
pp_print_cut, 166 readlink, 202
pp_print_float, 166 really_input, 150
pp_print_flush, 166 receive, 226
pp_print_if_newline, 166 recv, 207, 228
pp_print_int, 166 recvfrom, 207, 228
pp_print_newline, 166 red, 231
pp_print_space, 166 ref, 151
pp_print_string, 166 regexp, 219
pp_print_tab, 166
pp_print_tbreak, 166 regexp_case_fold, 219
pp_set_ellipsis_text, 166 register, 157
pp_set_formatter_out_channel, 166 register_exception, 157
pp_set_formatter_output_functions, 166 remove, 170, 189, 237
pp_set_margin, 166 rename, 189, 198
Index to the library 244
replace, 237 sigpipe, 190
replace_first, 220 sigprof, 190
rev, 174 sigquit, 190
rewinddir, 200 sigsegv, 190
rgb, 231 sigstop, 190
rhs_end, 179 sigterm, 190
rhs_start, 179 sigtstp, 190
rindex, 188 sigttin, 190
rindex_from, 188 sigttou, 190
rmdir, 200 sigusr1, 190
round_num, 215 sigusr2, 190
search_backward, 219 sigvtalrm, 190
search_forward, 219 sin, 144
seek_in, 150 sinh, 144
seek_out, 149 size_x, 230
select, 202, 224, 227, 228 size_y, 230
self, 223 sleep, 203, 228
send, 207, 226, 228 snd, 145
sendto, 207, 228 socket, 206, 228
set, 155, 168, 187, 191 socketpair, 206, 228
Set (module), 182 Sort (module), 184
set_approx_printing, 217 sound, 235
set_close_on_exec, 200 split, 175, 220
set_color, 231 sprintf, 181
set_ellipsis_text, 164 sqrt, 144
set_error_when_null_denominator, 216 square_num, 214
set_floating_precision, 217 Stack (module), 185
set_font, 232 Stack_overflow (exception), 140
set_formatter_out_channel, 164 stat, 168, 198
set_formatter_output_functions, 164 std_formatter, 165
set_line_width, 232 stderr, 146, 196
set_margin, 162 stdin, 146, 196
set_max_boxes, 163 stdout, 146, 196
set_max_indent, 162 Str (module), 218
set_nonblock, 199 Stream (module), 185
set_normalize_ratio, 216 string, 158
set_normalize_ratio_when_printing, 216 String (module), 187
set_tab, 164 string_after, 221
set_text_size, 232 string_before, 221
setgid, 205 string_match, 219
setitimer, 204 string_of_bool, 145
setsid, 211 string_of_float, 145
setsockopt, 208 string_of_inet_addr, 206
setuid, 204 string_of_int, 145
shutdown, 207 string_of_num, 216
shutdown_connection, 208 sub, 156, 187
sigabrt, 190 sub_num, 214
sigalrm, 190 substitute_first, 220
sigchld, 190 substring, 158
sigcont, 190 succ, 142
sigfpe, 190 succ_num, 215
sighup, 190 symbol_end, 179
sigill, 190 symbol_start, 179
sigint, 190 symlink, 201
sigkill, 190 sync, 227
Sys (module), 189
sign_num, 215 Sys_error (exception), 140
signal, 190, 225 system, 196, 227
Index to the library 245
take, 181 umask, 199
tan, 144 uncapitalize, 189
tanh, 144 Unix (module), 193
tcdrain, 211 Unix_error (exception), 194
tcflow, 211 unlink, 198
tcflush, 211 unlock, 225
tcgetattr, 210 uppercase, 158, 188
tcsendbreak, 211 usage, 155
tcsetattr, 210 utimes, 204
temp_file, 160
text_size, 232 wait, 195, 225, 227
Thread (module), 223 wait_next_event, 234
ThreadUnix (module), 227 wait_pid, 224
time, 203 wait_read, 224
timed_read, 227 wait_timed_read, 224
timed_write, 227 wait_timed_write, 224
times, 203 wait_write, 224
tl, 173 waitpid, 195, 227
to_buffer, 178 Weak (module), 191
to_channel, 178 white, 231
to_list, 157 word_size, 189
to_string, 178, 180 wrap, 226
total_size, 179 write, 197, 227
transp, 233
truncate, 145, 197 yellow, 231
try_lock, 225
Index of keywords
and, see let, type, 65 method, 63--65
as, 49--52, 63, 65 module, 65, 67--70
assert, 73 mutable, 61--65
begin, 53, 54 new, 53, 60
class, 63, 65, 67, 70 not, 53
closed, 63, 65 of, see type, exception
constraint, 61--63, 65 open, 65, 68--70
do, see while, for or, 53, 57
done, see while, for parser, 72
downto, see for private, 63--65
else, see if protected, 63, 64
end, 53, 54, 63, 65, 66, 69 rec, see let
exception, 62, 65, 67, 69, 70
external, 65, 66, 69, 70 sig, 65, 66
false, 48 struct, 69
for, 53, 57 then, see if, 65
fun, 53, 55 to, see for
function, 53, 55 true, 48
functor, 65, 68, 69, 71 try, 53, 58
if, 53, 56 type, 61, 65, 66, 68--70
in, see let val, 63--66
inherit, 63--65 virtual, 63, 65
lazy, 74 when, 53, 55
let, 53, 55, 69, 70 while, 57
match, 53, 57, 72 with, see match, try, 65, 68
246