# Module Pervasives

    module Pervasives: sig .. end

The initially opened module.

This module provides the basic operations over the built-in types (numbers, booleans, 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

    val raise : exn -> 'a

Raise the given exception value

    val invalid_arg : string -> 'a

Raise exception `Invalid_argument` with the given string.

    val 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

    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

See `(>=)`.

    val (>) : 'a -> 'a -> bool

See `(>=)`.

    val (<=) : 'a -> 'a -> bool

See `(>=)`.

    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` 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 `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.

    val 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`.

    val 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`.

    val (==) : 'a -> 'a -> bool

`e1 == e2` tests for physical equality of `e1` and `e2`. On mutable types such as references, arrays, strings, 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`.

    val (!=) : 'a -> 'a -> bool

Negation of `(==)`.

###### Boolean operations

    val not : bool -> bool

The boolean negation.

    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

Deprecated.`(&&)` should be used instead.

    val (||) : 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.

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

Deprecated.`(||)` should be used instead.

###### Composition operators

    val (|>) : 'a -> ('a -> 'b) -> 'b

Reverse-application operator: `x |> f |> g` is exactly equivalent to `g (f (x))`.
Since 4.01

    val (@@) : ('a -> 'b) -> 'a -> 'b

Application operator: `g @@ f @@ x` is exactly equivalent to `g (f (x))`.
Since 4.01

###### 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 (~+) : int -> int

Unary addition. You can also write `+ e` instead of `~+ e`.
Since 3.12.0

    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. 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)`.

    val (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.

    val abs : int -> int

Return the absolute value of the argument. Note that this may be negative if the argument is `min_int`.

    val max_int : int

The greatest representable integer.

    val min_int : int

The smallest representable integer.
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

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.

    val (~-.) : float -> float

Unary negation. You can also write `-. e` instead of `~-. e`.

    val (~+.) : float -> float

Unary addition. You can also write `+. e` instead of `~+. e`.
Since 3.12.0

    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

Exponential.

    val log : float -> float

Natural logarithm.

    val log10 : float -> float

Base 10 logarithm.

    val 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

    val 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

    val cos : float -> float

Cosine. Argument is in radians.

    val sin : float -> float

Sine. Argument is in radians.

    val tan : float -> float

Tangent. Argument is in radians.

    val 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`.

    val 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`.

    val atan : float -> float

Arc tangent. Result is in radians and is between `-pi/2` and `pi/2`.

    val 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`.

    val 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

    val cosh : float -> float

Hyperbolic cosine. Argument is in radians.

    val sinh : float -> float

Hyperbolic sine. Argument is in radians.

    val tanh : float -> float

Hyperbolic tangent. Argument is in radians.

    val 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.

    val 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.

    val abs_float : float -> float

`abs_float f` returns the absolute value of `f`.

    val 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

    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

Same as `float_of_int`.

    val float_of_int : int -> float

Convert an integer to floating-point.

    val truncate : float -> int

Same as `int_of_float`.

    val 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.

    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'. 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`.

    val max_float : float

The largest positive finite value of type `float`.

    val min_float : float

The smallest positive, non-zero, non-denormalized value of type `float`.

    val 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 `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. As the returned values may be shared, the user should not modify them directly.

    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 (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`.

    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. Raise `Failure "float_of_string"` 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

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.

    val stdin : in_channel

The standard input for the process.

    val stdout : out_channel

The standard output for the process.

    val stderr : out_channel

The 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 and flush 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 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. | `*)`

Opening modes for `open_out_gen` and `open_in_gen`.

    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.

    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 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. `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 flush_all : unit -> unit

Flush all open output channels; ignore errors.

    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 oc buf pos len` writes `len` characters from string `buf`, starting at offset `pos`, to the given output channel `oc`. 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 (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 `input_binary_int` function. The format is compatible across all machines for a given version of OCaml.

    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. Does not work on channels opened with the `Open_append` flag (returns unspecified results).

    val 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.

    val 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.

    val close_out_noerr : out_channel -> unit

Same as `close_out`, but ignore all errors.

    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.

    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 perm filename` opens the named file for reading, as described 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 -> string -> int -> int -> int

`input ic buf pos len` reads up to `len` characters from the given channel `ic`, 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 `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 -> string -> int -> int -> unit

`really_input ic buf pos len` reads `len` characters from channel `ic`, 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 (4 bytes, big-endian) 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 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.

    val 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.

    val close_in_noerr : in_channel -> unit

Same as `close_in`, but ignore all errors.

    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.
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`.

    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`.

###### 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`.

    type ('a, 'b, 'c, 'd) format4 = ('a, 'b, 'c, 'c, 'c, 'd) format6

Format strings have a general and highly polymorphic type `('a, 'b, 'c, 'd, 'e, 'f) format6`. Type `format6` is built in. 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 ('a, 'b, 'c) format = ('a, 'b, 'c, 'c) format4

    val string_of_format : ('a, 'b, 'c, 'd, 'e, 'f) format6 -> string

Converts a format string into a string.

    val format_of_string : ('a, 'b, 'c, 'd, 'e, 'f) format6 -> ('a, 'b, 'c, 'd, 'e, 'f) format6

`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.

    val (^^) : ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
           ('f, 'b, 'c, 'e, 'g, 'h) format6 -> ('a, 'b, 'c, 'd, 'g, 'h) format6

`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

    val 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.

    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`, 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.