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Standard Library

Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!

If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.

Module Pervasives

module Pervasives: sig .. end
The initially opened module.

This module provides the basic operations over the built-in types (numbers, booleans, byte sequences, strings, exceptions, references, lists, arrays, input-output channels, ...).

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.



Exceptions

let raise: exn => 'a;
Raise the given exception value
let raise_notrace: exn => 'a;
A faster version raise which does not record the backtrace.
Since 4.02.0
let invalid_arg: string => 'a;
Raise exception Invalid_argument with the given string.
let failwith: string => 'a;
Raise exception Failure with the given string.
exception Exit;
The Exit exception is not raised by any library function. It is provided for use in your programs.

Comparisons

let (=): '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.
let (<>): 'a => 'a => bool;
Negation of Pervasives.(=).
let (<): 'a => 'a => bool;
let (>): 'a => 'a => bool;
let (<=): 'a => 'a => bool;
let (>=): 'a => 'a => bool;
Structural ordering functions. These functions coincide with the usual orderings over integers, characters, strings, byte sequences 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.
let compare: 'a => 'a => int;
compare x y returns 0 if x is equal to y, a negative integer if x is less than y, and a positive integer if x is greater than y. The ordering implemented by compare is compatible with the comparison predicates =, < and > defined above, with one difference on the treatment of the float value Pervasives.nan. Namely, the comparison predicates treat nan as different from any other float value, including itself; while compare treats nan as equal to itself and less than any other float value. This treatment of nan ensures that compare defines a total ordering relation.

compare applied to functional values may raise Invalid_argument. compare applied to cyclic structures may not terminate.

The compare function can be used as the comparison function required by the Set.Make and Map.Make functors, as well as the List.sort and Array.sort functions.

let min: 'a => 'a => 'a;
Return the smaller of the two arguments. The result is unspecified if one of the arguments contains the float value nan.
let max: 'a => 'a => 'a;
Return the greater of the two arguments. The result is unspecified if one of the arguments contains the float value nan.
let (==): 'a => 'a => bool;
e1 == e2 tests for physical equality of e1 and e2. On mutable types such as references, arrays, byte sequences, records with mutable fields and objects with mutable instance variables, e1 == e2 is true if and only if physical modification of e1 also affects e2. On non-mutable types, the behavior of ( == ) is implementation-dependent; however, it is guaranteed that e1 == e2 implies compare e1 e2 = 0.
let (!=): 'a => 'a => bool;
Negation of Pervasives.(==).

Boolean operations

let not: bool => bool;
The boolean negation.
let (&&): 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.
let (&): bool => bool => bool;
Deprecated.Pervasives.(&&) should be used instead.
let (||): 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.
let (or): bool => bool => bool;
Deprecated.Pervasives.(||) should be used instead.

Debugging

let __LOC__: string;
__LOC__ returns the location at which this expression appears in the file currently being parsed by the compiler, with the standard error format of OCaml: "File %S, line %d, characters %d-%d".
Since 4.02.0
let __FILE__: string;
__FILE__ returns the name of the file currently being parsed by the compiler.
Since 4.02.0
let __LINE__: int;
__LINE__ returns the line number at which this expression appears in the file currently being parsed by the compiler.
Since 4.02.0
let __MODULE__: string;
__MODULE__ returns the module name of the file being parsed by the compiler.
Since 4.02.0
let __POS__: (string, int, int, int);
__POS__ returns a tuple (file,lnum,cnum,enum), corresponding to the location at which this expression appears in the file currently being parsed by the compiler. file is the current filename, lnum the line number, cnum the character position in the line and enum the last character position in the line.
Since 4.02.0
let __LOC_OF__: 'a => (string, 'a);
__LOC_OF__ expr returns a pair (loc, expr) where loc is the location of expr in the file currently being parsed by the compiler, with the standard error format of OCaml: "File %S, line %d, characters %d-%d".
Since 4.02.0
let __LINE_OF__: 'a => (int, 'a);
__LINE__ expr returns a pair (line, expr), where line is the line number at which the expression expr appears in the file currently being parsed by the compiler.
Since 4.02.0
let __POS_OF__: 'a => ((string, int, int, int), 'a);
__POS_OF__ expr returns a pair (loc,expr), where loc is a tuple (file,lnum,cnum,enum) corresponding to the location at which the expression expr appears in the file currently being parsed by the compiler. file is the current filename, lnum the line number, cnum the character position in the line and enum the last character position in the line.
Since 4.02.0

Composition operators

let (|>): 'a => ('a => 'b) => 'b;
Reverse-application operator: x |> f |> g is exactly equivalent to g (f (x)).
Since 4.01
let (@@): ('a => 'b) => 'a => 'b;
Application operator: g @@ f @@ x is exactly equivalent to g (f (x)).
Since 4.01

Integer arithmetic


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.
let (~-): int => int;
Unary negation. You can also write - e instead of ~- e.
let (~+): int => int;
Unary addition. You can also write + e instead of ~+ e.
Since 3.12.0
let succ: int => int;
succ x is x + 1.
let pred: int => int;
pred x is x - 1.
let (+): int => int => int;
Integer addition.
let (-): int => int => int;
Integer subtraction.
let ( * ): int => int => int;
Integer multiplication.
let (/): int => int => int;
Integer division. Raise Division_by_zero if the second argument is 0. Integer division rounds the real quotient of its arguments towards zero. More precisely, if x >= 0 and y > 0, x / y is the greatest integer less than or equal to the real quotient of x by y. Moreover, (- x) / y = x / (- y) = - (x / y).
let (mod): int => int => int;
Integer remainder. If y is not zero, the result of x mod y satisfies the following properties: x = (x / y) * y + x mod y and abs(x mod y) <= abs(y) - 1. If y = 0, x mod y raises Division_by_zero. Note that x mod y is negative only if x < 0. Raise Division_by_zero if y is zero.
let abs: int => int;
Return the absolute value of the argument. Note that this may be negative if the argument is min_int.
let max_int: int;
The greatest representable integer.
let min_int: int;
The smallest representable integer.

Bitwise operations

let (land): int => int => int;
Bitwise logical and.
let (lor): int => int => int;
Bitwise logical or.
let (lxor): int => int => int;
Bitwise logical exclusive or.
let lnot: int => int;
Bitwise logical negation.
let (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.
let (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.
let (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

OCaml'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 arithmetic operation with nan as argument returns nan as result.

let (~-.): float => float;
Unary negation. You can also write -. e instead of ~-. e.
let (~+.): float => float;
Unary addition. You can also write +. e instead of ~+. e.
Since 3.12.0
let (+.): float => float => float;
Floating-point addition
let (-.): float => float => float;
Floating-point subtraction
let ( *. ): float => float => float;
Floating-point multiplication
let (/.): float => float => float;
Floating-point division.
let ( ** ): float => float => float;
Exponentiation.
let sqrt: float => float;
Square root.
let exp: float => float;
Exponential.
let log: float => float;
Natural logarithm.
let log10: float => float;
Base 10 logarithm.
let expm1: float => float;
expm1 x computes exp x -. 1.0, giving numerically-accurate results even if x is close to 0.0.
Since 3.12.0
let log1p: float => float;
log1p x computes log(1.0 +. x) (natural logarithm), giving numerically-accurate results even if x is close to 0.0.
Since 3.12.0
let cos: float => float;
Cosine. Argument is in radians.
let sin: float => float;
Sine. Argument is in radians.
let tan: float => float;
Tangent. Argument is in radians.
let acos: float => float;
Arc cosine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and is between 0.0 and pi.
let asin: float => float;
Arc sine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and is between -pi/2 and pi/2.
let atan: float => float;
Arc tangent. Result is in radians and is between -pi/2 and pi/2.
let atan2: float => float => float;
atan2 y x returns the arc tangent of y /. x. The signs of x and y are used to determine the quadrant of the result. Result is in radians and is between -pi and pi.
let hypot: float => float => float;
hypot x y returns sqrt(x *. x + y *. y), that is, the length of the hypotenuse of a right-angled triangle with sides of length x and y, or, equivalently, the distance of the point (x,y) to origin.
Since 4.00.0
let cosh: float => float;
Hyperbolic cosine. Argument is in radians.
let sinh: float => float;
Hyperbolic sine. Argument is in radians.
let tanh: float => float;
Hyperbolic tangent. Argument is in radians.
let ceil: float => float;
Round above to an integer value. ceil f returns the least integer value greater than or equal to f. The result is returned as a float.
let floor: float => float;
Round below to an integer value. floor f returns the greatest integer value less than or equal to f. The result is returned as a float.
let abs_float: float => float;
abs_float f returns the absolute value of f.
let copysign: float => float => float;
copysign x y returns a float whose absolute value is that of x and whose sign is that of y. If x is nan, returns nan. If y is nan, returns either x or -. x, but it is not specified which.
Since 4.00.0
let 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.
let 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.
let ldexp: float => int => float;
ldexp x n returns x *. 2 ** n.
let modf: float => (float, float);
modf f returns the pair of the fractional and integral part of f.
let float: int => float;
let float_of_int: int => float;
Convert an integer to floating-point.
let truncate: float => int;
let int_of_float: float => int;
Truncate the given floating-point number to an integer. The result is unspecified if the argument is nan or falls outside the range of representable integers.
let infinity: float;
Positive infinity.
let neg_infinity: float;
Negative infinity.
let 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'. Any floating-point operation with nan as argument returns nan as result. As for floating-point comparisons, =, <, <=, > and >= return false and <> returns true if one or both of their arguments is nan.
let max_float: float;
The largest positive finite value of type float.
let min_float: float;
The smallest positive, non-zero, non-denormalized value of type float.
let epsilon_float: float;
The difference between 1.0 and the smallest exactly representable floating-point number greater than 1.0.
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 Pervasives.classify_float function.
let 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.

let (^): string => string => string;
String concatenation.

Character operations

More character operations are provided in module Char.

let int_of_char: char => int;
Return the ASCII code of the argument.
let 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

let 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

let string_of_bool: bool => string;
Return the string representation of a boolean. As the returned values may be shared, the user should not modify them directly.
let 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".
let string_of_int: int => string;
Return the string representation of an integer, in decimal.
let int_of_string: string => int;
Convert the given string to an integer. The string is read in decimal (by default) or in hexadecimal (if it begins with 0x or 0X), octal (if it begins with 0o or 0O), or binary (if it begins with 0b or 0B). Raise Failure "int_of_string" if the given string is not a valid representation of an integer, or if the integer represented exceeds the range of integers representable in type int.
let string_of_float: float => string;
Return the string representation of a floating-point number.
let float_of_string: string => float;
Convert the given string to a float. Raise Failure "float_of_string" if the given string is not a valid representation of a float.

Pair operations

let fst: ('a, 'b) => 'a;
Return the first component of a pair.
let snd: ('a, 'b) => 'b;
Return the second component of a pair.

List operations

More list operations are provided in module List.

let (@): list 'a => list 'a => list 'a;
List concatenation.

Input/output
Note: all input/output functions can raise Sys_error when the system calls they invoke fail.
type in_channel;
The type of input channel.
type out_channel;
The type of output channel.
let stdin: in_channel;
The standard input for the process.
let stdout: out_channel;
The standard output for the process.
let stderr: out_channel;
The standard error output for the process.

Output functions on standard output

let print_char: char => unit;
Print a character on standard output.
let print_string: string => unit;
Print a string on standard output.
let print_bytes: bytes => unit;
Print a byte sequence on standard output.
Since 4.02.0
let print_int: int => unit;
Print an integer, in decimal, on standard output.
let print_float: float => unit;
Print a floating-point number, in decimal, on standard output.
let print_endline: string => unit;
Print a string, followed by a newline character, on standard output and flush standard output.
let 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

let prerr_char: char => unit;
Print a character on standard error.
let prerr_string: string => unit;
Print a string on standard error.
let prerr_bytes: bytes => unit;
Print a byte sequence on standard error.
Since 4.02.0
let prerr_int: int => unit;
Print an integer, in decimal, on standard error.
let prerr_float: float => unit;
Print a floating-point number, in decimal, on standard error.
let prerr_endline: string => unit;
Print a string, followed by a newline character on standard error and flush standard error.
let prerr_newline: unit => unit;
Print a newline character on standard error, and flush standard error.

Input functions on standard input

let 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.
let 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.
let 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 for reading.
*)
| Open_wronly (*
open for writing.
*)
| Open_append (*
open for appending: always write at end of file.
*)
| Open_creat (*
create the file if it does not exist.
*)
| Open_trunc (*
empty the file if it already exists.
*)
| Open_excl (*
fail if Open_creat and 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.
*)
let 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.
let open_out_bin: string => out_channel;
Same as Pervasives.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 Pervasives.open_out.
let open_out_gen: list open_flag => int => string => out_channel;
open_out_gen mode perm filename opens the named file for writing, as described above. The extra argument mode specify the opening mode. The extra argument perm specifies the file permissions, in case the file must be created. Pervasives.open_out and Pervasives.open_out_bin are special cases of this function.
let 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.
let flush_all: unit => unit;
Flush all open output channels; ignore errors.
let output_char: out_channel => char => unit;
Write the character on the given output channel.
let output_string: out_channel => string => unit;
Write the string on the given output channel.
let output_bytes: out_channel => bytes => unit;
Write the byte sequence on the given output channel.
Since 4.02.0
let output: out_channel => bytes => int => int => unit;
output oc buf pos len writes len characters from byte sequence buf, starting at offset pos, to the given output channel oc. Raise Invalid_argument "output" if pos and len do not designate a valid range of buf.
let output_substring: out_channel => string => int => int => unit;
Same as output but take a string as argument instead of a byte sequence.
Since 4.02.0
let 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.
let output_binary_int: out_channel => int => unit;
Write one integer in binary format (4 bytes, big-endian) on the given output channel. The given integer is taken modulo 232. The only reliable way to read it back is through the Pervasives.input_binary_int function. The format is compatible across all machines for a given version of OCaml.
let 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 Pervasives.input_value. See the description of module Marshal for more information. Pervasives.output_value is equivalent to Marshal.to_channel with an empty list of flags.
let 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.
let pos_out: out_channel => int;
Return the current writing position for the given channel. Does not work on channels opened with the Open_append flag (returns unspecified results).
let out_channel_length: out_channel => int;
Return the size (number of characters) of the regular file on which the given channel is opened. If the channel is opened on a file that is not a regular file, the result is meaningless.
let close_out: out_channel => unit;
Close the given channel, flushing all buffered write operations. Output functions raise a Sys_error exception when they are applied to a closed output channel, except close_out and flush, which do nothing when applied to an already closed channel. Note that close_out may raise Sys_error if the operating system signals an error when flushing or closing.
let close_out_noerr: out_channel => unit;
Same as close_out, but ignore all errors.
let 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

let 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.
let open_in_bin: string => in_channel;
Same as Pervasives.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 Pervasives.open_in.
let open_in_gen: list open_flag => int => string => in_channel;
open_in_gen mode perm filename opens the named file for reading, as described above. The extra arguments mode and perm specify the opening mode and file permissions. Pervasives.open_in and Pervasives.open_in_bin are special cases of this function.
let 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.
let 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.
let input: in_channel => bytes => int => int => int;
input ic buf pos len reads up to len characters from the given channel ic, storing them in byte sequence 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 range of buf.
let really_input: in_channel => bytes => int => int => unit;
really_input ic buf pos len reads len characters from channel ic, storing them in byte sequence 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 range of buf.
let really_input_string: in_channel => int => string;
really_input_string ic len reads len characters from channel ic and returns them in a new string. Raise End_of_file if the end of file is reached before len characters have been read.
Since 4.02.0
let input_byte: in_channel => int;
Same as Pervasives.input_char, but return the 8-bit integer representing the character. Raise End_of_file if an end of file was reached.
let input_binary_int: in_channel => int;
Read an integer encoded in binary format (4 bytes, big-endian) from the given input channel. See Pervasives.output_binary_int. Raise End_of_file if an end of file was reached while reading the integer.
let input_value: in_channel => 'a;
Read the representation of a structured value, as produced by Pervasives.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.
let 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.
let pos_in: in_channel => int;
Return the current reading position for the given channel.
let in_channel_length: in_channel => int;
Return the size (number of characters) of the regular file on which the given channel is opened. If the channel is opened on a file that is not a regular file, the result is meaningless. The returned size does not take into account the end-of-line translations that can be performed when reading from a channel opened in text mode.
let close_in: in_channel => unit;
Close the given channel. Input functions raise a Sys_error exception when they are applied to a closed input channel, except close_in, which does nothing when applied to an already closed channel.
let close_in_noerr: in_channel => unit;
Same as close_in, but ignore all errors.
let 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.

Operations on large files

module LargeFile: sig .. end
Operations on large files.

References

type 'a ref = {
   mutable contents : 'a;
}
The type of references (mutable indirection cells) containing a value of type 'a.
let ref: 'a => ref 'a;
Return a fresh reference containing the given value.
let (!): ref 'a => 'a;
!r returns the current contents of reference r. Equivalent to fun r -> r.contents.
let (:=): ref 'a => 'a => unit;
r := a stores the value of a in reference r. Equivalent to fun r v -> r.contents <- v.
let incr: ref int => unit;
Increment the integer contained in the given reference. Equivalent to fun r -> r := succ !r.
let decr: ref int => unit;
Decrement the integer contained in the given reference. Equivalent to fun r -> r := pred !r.

Operations on format strings


Operations on format strings


Format strings are character strings with special lexical conventions that defines the functionality of formatted input/output functions. Format strings are used to read data with formatted input functions from module Scanf and to print data with formatted output functions from modules Printf and Format.

Format strings are made of three kinds of entities:

  • conversions specifications, introduced by the special character '%' followed by one or more characters specifying what kind of argument to read or print,
  • formatting indications, introduced by the special character '@' followed by one or more characters specifying how to read or print the argument,
  • plain characters that are regular characters with usual lexical conventions. Plain characters specify string literals to be read in the input or printed in the output.
There is an additional lexical rule to escape the special characters '%' and '@' in format strings: if a special character follows a '%' character, it is treated as a plain character. In other words, "%%" is considered as a plain '%' and "%@" as a plain '@'.

For more information about conversion specifications and formatting indications available, read the documentation of modules Scanf, Printf and Format.

Operations on format strings


Format strings are character strings with special lexical conventions that defines the functionality of formatted input/output functions. Format strings are used to read data with formatted input functions from module Scanf and to print data with formatted output functions from modules Printf and Format.

Format strings are made of three kinds of entities:

  • conversions specifications, introduced by the special character '%' followed by one or more characters specifying what kind of argument to read or print,
  • formatting indications, introduced by the special character '@' followed by one or more characters specifying how to read or print the argument,
  • plain characters that are regular characters with usual lexical conventions. Plain characters specify string literals to be read in the input or printed in the output.
There is an additional lexical rule to escape the special characters '%' and '@' in format strings: if a special character follows a '%' character, it is treated as a plain character. In other words, "%%" is considered as a plain '%' and "%@" as a plain '@'.

For more information about conversion specifications and formatting indications available, read the documentation of modules Scanf, Printf and Format.

Format strings have a general and highly polymorphic type ('a, 'b, 'c, 'd, 'e, 'f) format6. The two simplified types, format and format4 below are included for backward compatibility with earlier releases of OCaml.

The meaning of format string type parameters is as follows:

  • 'a is the type of the parameters of the format for formatted output functions (printf-style functions); 'a is the type of the values read by the format for formatted input functions (scanf-style functions).
  • 'b is the type of input source for formatted input functions and the type of output target for formatted output functions. For printf-style functions from module Printf, 'b is typically out_channel; for printf-style functions from module Format, 'b is typically Format.formatter; for scanf-style functions from module Scanf, 'b is typically Scanf.Scanning.in_channel.
Type argument 'b is also the type of the first argument given to user's defined printing functions for %a and %t conversions, and user's defined reading functions for %r conversion.

  • 'c is the type of the result of the %a and %t printing functions, and also the type of the argument transmitted to the first argument of kprintf-style functions or to the kscanf-style functions.
  • 'd is the type of parameters for the scanf-style functions.
  • 'e is the type of the receiver function for the scanf-style functions.
  • 'f is the final result type of a formatted input/output function invocation: for the printf-style functions, it is typically unit; for the scanf-style functions, it is typically the result type of the receiver function.

type format6 'a 'b 'c 'd 'e 'f = CamlinternalFormatBasics.format6 'a 'b 'c 'd 'e 'f;
type format4 'a 'b 'c 'd = format6 'a 'b 'c 'c 'c 'd;
type format 'a 'b 'c = format4 'a 'b 'c 'c;
let string_of_format: format6 'a 'b 'c 'd 'e 'f => string;
Converts a format string into a string.
let format_of_string: format6 'a 'b 'c 'd 'e 'f => format6 'a 'b 'c 'd 'e 'f;
format_of_string s returns a format string read from the string literal s. Note: format_of_string can not convert a string argument that is not a literal. If you need this functionality, use the more general Scanf.format_from_string function.
let (^^): format6 'a 'b 'c 'd 'e 'f => format6 'f 'b 'c 'e 'g 'h => format6 'a 'b 'c 'd 'g 'h;
f1 ^^ f2 catenates format strings f1 and f2. The result is a format string that behaves as the concatenation of format strings f1 and f2: in case of formatted output, it accepts arguments from f1, then arguments from f2; in case of formatted input, it returns results from f1, then results from f2.

Program termination

let exit: int => 'a;
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. All open output channels are flushed with flush_all. An implicit exit 0 is performed each time a program terminates normally. An implicit exit 2 is performed if the program terminates early because of an uncaught exception.
let 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 Pervasives.exit, or terminates, either normally or 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.