Chapter 18 | The core library

This chapter describes the functions provided by the Objective Caml 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 8).
- 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.

18.1 | 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.

    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.

    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.

    exception Sys_blocked_io

> A special case of `Sys_error` raised when no I/O is possible on a non-blocking I/O channel.

    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 `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 integers, characters, strings 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.

    val (||) : bool -> bool -> bool
    val (or) : bool -> bool -> bool

> The boolean ``or''. Evaluation is sequential, left-to-right: in `e1 || 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 operations are taken modulo 231 (or 263). 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

> Integer division. Raise `Division_by_zero` if the second argument is 0.

    val (mod) : int -> int -> int

> Integer remainder. If `x >= 0` and `y > 0`, the result of `x mod y` satisfies the following properties: `0 <= x mod y < y` and `x = (x / y) * y + x mod y`. If `y = 0`, `x mod y` raises `Division_by_zero`. If `x < 0` or `y < 0`, the result of `x mod y` is not specified and depends on the platform.

    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.

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. The result is unspecified if `m < 0` or `m >= bitsize`, where `bitsize` is `32` on a 32-bit platform and `64` on a 64-bit platform.

    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`. The result is unspecified if `m < 0` or `m >= bitsize`.

    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. The result is unspecified if `m < 0` or `m >= bitsize`.

Floating-point arithmetic

> Caml's floating-point numbers follow the IEEE 754 standard, using double precision (64 bits) numbers. Floating-point operations never raise an exception on overflow, underflow, division by zero, etc. Instead, special IEEE numbers are returned as appropriate, such as `infinity` for `1.0 /. 0.0`, `neg_infinity` for `-1.0 /. 0.0`, and `nan` (``not a number'') for `0.0 /. 0.0`. These special numbers then propagate through floating-point computations as expected: for instance, `1.0 /. infinity` is `0.0`, and any operation with `nan` as argument returns `nan` as result.

    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

    val (/.) : float -> float -> float

> Floating-point division.

    val (**) : float -> float -> float

> Exponentiation

    val sqrt : float -> float

> Square root

    val exp : float -> float
    val log : float -> float
    val log10 : float -> float

> Exponential, natural logarithm, base 10 logarithm.

    val cos : float -> float
    val sin : float -> float
    val tan : float -> float
    val acos : float -> float
    val asin : float -> float
    val atan : float -> float
    val atan2 : float -> float -> float

> The usual trigonometric functions

    val cosh : float -> float
    val sinh : float -> float
    val tanh : float -> float

> The usual hyperbolic trigonometric functions

    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

> `mod_float a b` returns the remainder of `a` with respect to `b`. The returned value is `a -. n *. b`, where `n` is the quotient `a /. b` rounded towards zero to an integer.

    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` and `0.5 <= x < 1.0`.

    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
    val float_of_int : int -> float

> Convert an integer to floating-point.

    val truncate : float -> int
    val int_of_float : float -> int

> Truncate the given floating-point number to an integer. The result is unspecified if it falls outside the range of representable integers.

    val infinity: float

> Positive infinity.

    val neg_infinity: float

> Negative infinity.

    val nan: float

> A special floating-point value denoting the result of an undefined operation such as `0.0 /. 0.0`. Stands for ``not a number''.

    type fpclass =
        FP_normal           (* Normal number, none of the below *)
      | FP_subnormal        (* Number very close to 0.0, has reduced precision *)
      | FP_zero             (* Number is 0.0 or -0.0 *)
      | FP_infinite         (* Number is positive or negative infinity *)
      | FP_nan              (* Not a number: result of an undefined operation *)

> The five classes of floating-point numbers, as determined by the `classify_float` function.

    val classify_float: float -> fpclass

> Return the class of the given floating-point number: normal, subnormal, zero, infinite, or not a number.

String operations

> More string operations are provided in module `String`.

    val (^) : string -> string -> string

> String concatenation.

Character operations

> More character operations are provided in module `Char`.

    val int_of_char : char -> int

> Return the ASCII code of the argument.

    val char_of_int : int -> char

> Return the character with the given ASCII code. Raise `Invalid_argument "char_of_int"` if the argument is outside the range 0--255.

Unit operations

    val ignore : 'a -> unit

> Discard the value of its argument and return `()`. For instance, `ignore(f x)` discards the result of the side-effecting function `f`. It is equivalent to `f x; ()`, except that the latter may generate a compiler warning; writing `ignore(f x)` instead avoids the warning.

String conversion functions

    val string_of_bool : bool -> string

> Return the string representation of a boolean.

    val bool_of_string : string -> bool

> Convert the given string to a boolean. Raise `Invalid_argument "bool_of_string"` if the string is not `"true"` or `"false"`.

    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`.

    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.

    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.

    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 : mode:open_flag list -> perm:int -> string -> out_channel

> Open the named file for writing, as above. The extra argument `mode` specify the opening mode. The extra argument `perm` 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 -> buf:string -> pos:int -> len:int -> unit

> Write `len` characters from string `buf`, starting at offset `pos`, to the given output channel. Raise `Invalid_argument "output"` if `pos` and `len` do not designate a valid substring of `buf`.

    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.

    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. A `Sys_error` exception is raised if any of the functions above is called on a closed channel.

    val set_binary_mode_out : out_channel -> bool -> unit

> `set_binary_mode_out oc true` sets the channel `oc` to binary mode: no translations take place during output. `set_binary_mode_out oc false` sets the channel `oc` to text mode: depending on the operating system, some translations may take place during output. For instance, under Windows, end-of-lines will be translated from `\n` to `\r\n`. This function has no effect under operating systems that do not distinguish between text mode and binary mode.

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 : mode:open_flag list -> perm:int -> string -> in_channel

> Open the named file for reading, as above. The extra arguments `mode` and `perm` 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.

    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 -> buf:string -> pos:int -> len:int -> int

> Read up to `len` characters from the given channel, storing them in string `buf`, starting at character number `pos`. 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 not all requested `len` characters were read, either because no more characters were available at that time, or because the implementation found it convenient to do a partial read; `input` must be called again to read the remaining characters, if desired. (See also `Pervasives.really_input` for reading exactly `len` characters.) Exception `Invalid_argument "input"` is raised if `pos` and `len` do not designate a valid substring of `buf`.

    val really_input : in_channel -> buf:string -> pos:int -> len:int -> unit

> Read `len` characters from the given channel, storing them in string `buf`, starting at character number `pos`. Raise `End_of_file` if the end of file is reached before `len` characters have been read. Raise `Invalid_argument "really_input"` if `pos` and `len` do not designate a valid substring of `buf`.

    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

> 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. A `Sys_error` exception is raised if any of the functions above is called on a closed channel.

    val set_binary_mode_in : in_channel -> bool -> unit

> `set_binary_mode_in ic true` sets the channel `ic` to binary mode: no translations take place during input. `set_binary_mode_out ic false` sets the channel `ic` to text mode: depending on the operating system, some translations may take place during input. For instance, under Windows, end-of-lines will be translated from `\r\n` to `\n`. This function has no effect under operating systems that do not distinguish between text mode and binary mode.

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`. Equivalent to `fun r -> r.contents`.

    val (:=) : 'a ref -> 'a -> unit

> `r := a` stores the value of `a` in reference `r`. Equivalent to `fun r v -> r.contents <- v`.

    val incr : int ref -> unit

> Increment the integer contained in the given reference. Equivalent to `fun r -> r := succ !r`.

    val decr : int ref -> unit

> Decrement the integer contained in the given reference. Equivalent to `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 when the program executes `exit`. They will not be called if the program terminates because of an uncaught exception. The functions are called in ``last in, first out'' order: the function most recently added with `at_exit` is called first.