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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:

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.

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. 
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 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. 
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 operations are taken modulo 2^{31} (or 2^{63}). 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.

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 
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. 
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. 
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 : 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. 
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. 
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
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 order when the program executes exit. They will not be called if the program terminates because of an uncaught exception.
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