# Module Stdlib

    module Stdlib: sig .. end

The OCaml Standard library.

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 `Stdlib`.

It particular, it provides the basic operations over the built-in types (numbers, booleans, byte sequences, strings, exceptions, references, lists, arrays, input-output channels, ...) and the standard library modules.

## Exceptions

    val raise : exn -> 'a

Raise the given exception value

    val raise_notrace : exn -> 'a

A faster version `raise` which does not record the backtrace.

- Since 4.02.0

    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.

    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 match keyword in the source code (file name, line number, column number).

    exception Assert_failure of (string * int * int)

Exception raised when an assertion fails. The arguments are the location of the assert keyword in the source code (file name, line number, column number).

    exception Invalid_argument of string

Exception raised by library functions to signal that the given arguments do not make sense. The string gives some information to the programmer. As a general rule, this exception should not be caught, it denotes a programming error and the code should be modified not to trigger it.

    exception Failure of string

Exception raised by library functions to signal that they are undefined on the given arguments. The string is meant to give some information to the programmer; you must not pattern match on the string literal because it may change in future versions (use Failure _ instead).

    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. (Not reliable for allocations on the minor heap.)

    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.

Before 4.10, it was not fully implemented by the native-code compiler.

    exception Sys_error of string

Exception raised by the input/output functions to report an operating system error. The string is meant to give some information to the programmer; you must not pattern match on the string literal because it may change in future versions (use Sys_error _ instead).

    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 integer division and remainder operations when their second argument is zero.

    exception Sys_blocked_io

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

    exception Undefined_recursive_module of (string * int * int)

Exception raised when an ill-founded recursive module definition is evaluated. The arguments are the location of the definition in the source code (file name, line number, column number).

## 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. Left-associative operator, see `Ocaml_operators` for more information.

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

Negation of `(=)`. Left-associative operator, see `Ocaml_operators` for more information.

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

See `(>=)`. Left-associative operator, see `Ocaml_operators` for more information.

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

See `(>=)`. Left-associative operator, see `Ocaml_operators` for more information.

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

See `(>=)`. Left-associative operator, see `Ocaml_operators` for more information.

    val (>=) : '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. Left-associative operator, see `Ocaml_operators` for more information.

    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, 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`. Left-associative operator, see `Ocaml_operators` for more information.

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

Negation of `(==)`. Left-associative operator, see `Ocaml_operators` for more information.

## 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. Right-associative operator, see `Ocaml_operators` for more information.

    val (&) : bool -> bool -> bool

Deprecated. `(&&)` should be used instead. Right-associative operator, see `Ocaml_operators` for more information.

    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. Right-associative operator, see `Ocaml_operators` for more information.

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

Deprecated. `(||)` should be used instead. Right-associative operator, see `Ocaml_operators` for more information.

## Debugging

    val __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

    val __FILE__ : string

`__FILE__` returns the name of the file currently being parsed by the compiler.

- Since 4.02.0

    val __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

    val __MODULE__ : string

`__MODULE__` returns the module name of the file being parsed by the compiler.

- Since 4.02.0

    val __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

    val __FUNCTION__ : string

`__FUNCTION__` returns the name of the current function or method, including any enclosing modules or classes.

- Since 4.12.0

    val __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

    val __LINE_OF__ : 'a -> int * 'a

`__LINE_OF__ 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

    val __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

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

Reverse-application operator: `x |> f |> g` is exactly equivalent to `g (f (x))`. Left-associative operator, see `Ocaml_operators` for more information.

- Since 4.01

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

Application operator: `g @@ f @@ x` is exactly equivalent to `g (f (x))`. Right-associative operator, see `Ocaml_operators` for more information.

- Since 4.01

## Integer arithmetic

Integers are `Sys.int_size` bits wide. All operations are taken modulo 2`Sys.int_size`. They do not fail on overflow.

    val (~-) : int -> int

Unary negation. You can also write `- e` instead of `~- e`. Unary operator, see `Ocaml_operators` for more information.

    val (~+) : int -> int

Unary addition. You can also write `+ e` instead of `~+ e`. Unary operator, see `Ocaml_operators` for more information.

- 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. Left-associative operator, see `Ocaml_operators` for more information.

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

Integer subtraction. Left-associative operator, , see `Ocaml_operators` for more information.

    val ( * ) : int -> int -> int

Integer multiplication. Left-associative operator, see `Ocaml_operators` for more information.

    val (/) : int -> int -> int

Integer division. 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)`. Left-associative operator, see `Ocaml_operators` for more information.

- Raises `Division_by_zero` if the second argument is 0.

    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`. Left-associative operator, see `Ocaml_operators` for more information.

- Raises `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. Left-associative operator, see `Ocaml_operators` for more information.

    val (lor) : int -> int -> int

Bitwise logical or. Left-associative operator, see `Ocaml_operators` for more information.

    val (lxor) : int -> int -> int

Bitwise logical exclusive or. Left-associative operator, see `Ocaml_operators` for more information.

    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 > Sys.int_size`. Right-associative operator, see `Ocaml_operators` for more information.

    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 > Sys.int_size`. Right-associative operator, see `Ocaml_operators` for more information.

    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 > Sys.int_size`. Right-associative operator, see `Ocaml_operators` for more information.

## 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`, basic arithmetic operations (`+.`, `-.`, `*.`, `/.`) with `nan` as an argument return `nan`, ...

    val (~-.) : float -> float

Unary negation. You can also write `-. e` instead of `~-. e`. Unary operator, see `Ocaml_operators` for more information.

    val (~+.) : float -> float

Unary addition. You can also write `+. e` instead of `~+. e`. Unary operator, see `Ocaml_operators` for more information.

- Since 3.12.0

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

Floating-point addition. Left-associative operator, see `Ocaml_operators` for more information.

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

Floating-point subtraction. Left-associative operator, see `Ocaml_operators` for more information.

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

Floating-point multiplication. Left-associative operator, see `Ocaml_operators` for more information.

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

Floating-point division. Left-associative operator, see `Ocaml_operators` for more information.

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

Exponentiation. Right-associative operator, see `Ocaml_operators` for more information.

    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. If one of `x` or `y` is infinite, returns `infinity` even if the other is `nan`.

- 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 acosh : float -> float

Hyperbolic arc cosine. The argument must fall within the range `[1.0, inf]`. Result is in radians and is between `0.0` and `inf`.

- Since 4.13.0

    val asinh : float -> float

Hyperbolic arc sine. The argument and result range over the entire real line. Result is in radians.

- Since 4.13.0

    val atanh : float -> float

Hyperbolic arc tangent. The argument must fall within the range `[-1.0, 1.0]`. Result is in radians and ranges over the entire real line.

- Since 4.13.0

    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. Right-associative operator, see `Ocaml_operators` for more information.

- Raises `Invalid_argument` if the result is longer then than `Sys.max_string_length` bytes.

## 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.

- Raises `Invalid_argument` 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_opt : string -> bool option

Convert the given string to a boolean.

Return `None` if the string is not `"true"` or `"false"`.

- Since 4.05

    val bool_of_string : string -> bool

Same as `bool_of_string_opt`, but raise `Invalid_argument "bool_of_string"` instead of returning `None`.

    val string_of_int : int -> string

Return the string representation of an integer, in decimal.

    val int_of_string_opt : string -> int option

Convert the given string to an integer. The string is read in decimal (by default, or if the string begins with `0u`), in hexadecimal (if it begins with `0x` or `0X`), in octal (if it begins with `0o` or `0O`), or in binary (if it begins with `0b` or `0B`).

The `0u` prefix reads the input as an unsigned integer in the range `[0, 2*max_int+1]`. If the input exceeds `max_int` it is converted to the signed integer `min_int + input - max_int - 1`.

The `_` (underscore) character can appear anywhere in the string and is ignored.

Return `None` 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`.

- Since 4.05

    val int_of_string : string -> int

Same as `int_of_string_opt`, but raise `Failure "int_of_string"` instead of returning `None`.

    val string_of_float : float -> string

Return the string representation of a floating-point number.

    val float_of_string_opt : string -> float option

Convert the given string to a float. The string is read in decimal (by default) or in hexadecimal (marked by `0x` or `0X`).

The format of decimal floating-point numbers is `[-] dd.ddd (e|E) [+|-] dd` , where `d` stands for a decimal digit.

The format of hexadecimal floating-point numbers is `[-] 0(x|X) hh.hhh (p|P) [+|-] dd` , where `h` stands for an hexadecimal digit and `d` for a decimal digit.

In both cases, at least one of the integer and fractional parts must be given; the exponent part is optional.

The `_` (underscore) character can appear anywhere in the string and is ignored.

Depending on the execution platforms, other representations of floating-point numbers can be accepted, but should not be relied upon.

Return `None` if the given string is not a valid representation of a float.

- Since 4.05

    val float_of_string : string -> float

Same as `float_of_string_opt`, but raise `Failure "float_of_string"` instead of returning `None`.

## 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. Not tail-recursive (length of the first argument). Right-associative operator, see `Ocaml_operators` for more information.

## 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_bytes : bytes -> unit

Print a byte sequence on standard output.

- Since 4.02.0

    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_bytes : bytes -> unit

Print a byte sequence on standard error.

- Since 4.02.0

    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.

- Raises `End_of_file` if the end of the file is reached at the beginning of line.

    val read_int_opt : unit -> int option

Flush standard output, then read one line from standard input and convert it to an integer.

Return `None` if the line read is not a valid representation of an integer.

- Since 4.05

    val read_int : unit -> int

Same as `read_int_opt`, but raise `Failure "int_of_string"` instead of returning `None`.

    val read_float_opt : unit -> float option

Flush standard output, then read one line from standard input and convert it to a floating-point number.

Return `None` if the line read is not a valid representation of a floating-point number.

- Since 4.05.0

    val read_float : unit -> float

Same as `read_float_opt`, but raise `Failure "float_of_string"` instead of returning `None`.

### 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, positioned 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` specifies 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_bytes : out_channel -> bytes -> unit

Write the byte sequence on the given output channel.

- Since 4.02.0

    val 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`.

- Raises `Invalid_argument` if `pos` and `len` do not designate a valid range of `buf`.

    val 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

    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). For files opened in text mode under Windows, the returned position is approximate (owing to end-of-line conversion); in particular, saving the current position with `pos_out`, then going back to this position using `seek_out` will not work. For this programming idiom to work reliably and portably, the file must be opened in binary mode.

    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, positioned 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.

- Raises `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.

- Raises `End_of_file` if the end of the file is reached at the beginning of line.

    val 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 `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`.

    val 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`.

- Raises
  - `End_of_file` if the end of file is reached before `len` characters have been read.
  - `Invalid_argument` if `pos` and `len` do not designate a valid range of `buf`.

    val 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.

- Since 4.02.0
- Raises `End_of_file` if the end of file is reached before `len` characters have been read.

    val input_byte : in_channel -> int

Same as `input_char`, but return the 8-bit integer representing the character.

- Raises `End_of_file` if the 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`.

- Raises `End_of_file` if the 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. For files opened in text mode under Windows, the returned position is approximate (owing to end-of-line conversion); in particular, saving the current position with `pos_in`, then going back to this position using `seek_in` will not work. For this programming idiom to work reliably and portably, the file must be opened in binary mode.

    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`. Unary operator, see `Ocaml_operators` for more information.

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

`r := a` stores the value of `a` in reference `r`. Equivalent to `fun r v -> r.contents <- v`. Right-associative operator, see `Ocaml_operators` for more information.

    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`.

## Result type

    type ('a, 'b) result =

`|` | `Ok of `'a``
`|` | `Error of `'b``

- Since 4.03.0

## 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 ('a, 'b, 'c, 'd, 'e, 'f) format6 = ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.format6

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

    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`. Right-associative operator, see `Ocaml_operators` for more information.

## 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 does any of the following:

- executes `exit`
- terminates, either normally or because of an uncaught exception
- executes the C function `caml_shutdown`. The functions are called in 'last in, first out' order: the function most recently added with `at_exit` is called first.

## Standard library modules

    module Arg: Arg

    module Array: Array

    module ArrayLabels: ArrayLabels

    module Atomic: Atomic

    module Bigarray: Bigarray

    module Bool: Bool

    module Buffer: Buffer

    module Bytes: Bytes

    module BytesLabels: BytesLabels

    module Callback: Callback

    module Char: Char

    module Complex: Complex

    module Digest: Digest

    module Either: Either

    module Ephemeron: Ephemeron

    module Filename: Filename

    module Float: Float

    module Format: Format

    module Fun: Fun

    module Gc: Gc

    module Genlex: Genlex

    module Hashtbl: Hashtbl

    module In_channel: In_channel

    module Int: Int

    module Int32: Int32

    module Int64: Int64

    module Lazy: Lazy

    module Lexing: Lexing

    module List: List

    module ListLabels: ListLabels

    module Map: Map

    module Marshal: Marshal

    module MoreLabels: MoreLabels

    module Nativeint: Nativeint

    module Obj: Obj

    module Oo: Oo

    module Option: Option

    module Out_channel: Out_channel

    module Parsing: Parsing

    module Pervasives: Pervasives

    module Printexc: Printexc

    module Printf: Printf

    module Queue: Queue

    module Random: Random

    module Result: Result

    module Scanf: Scanf

    module Seq: Seq

    module Set: Set

    module Stack: Stack

    module StdLabels: StdLabels

    module Stream: Stream

    module String: String

    module StringLabels: StringLabels

    module Sys: Sys

    module Uchar: Uchar

    module Unit: Unit

    module Weak: Weak