Module Stdlib.Random

Pseudo-random number generators (PRNG).

With multiple domains, each domain has its own generator that evolves independently of the generators of other domains. When a domain is created, its generator is initialized by splitting the state of the generator associated with the parent domain.

In contrast, all threads within a domain share the same domain-local generator. Independent generators can be created with the Random.split function and used with the functions from the Random.State module.

before 5.0 Random value generation used a different algorithm. This affects all the functions in this module which return random values.

Basic functions

val init : int -> unit

let init: int => unit;

Initialize the domain-local generator, using the argument as a seed. The same seed will always yield the same sequence of numbers.

val full_init : int array -> unit

let full_init: array(int) => unit;

Same as Random.init but takes more data as seed.

val self_init : unit -> unit

let self_init: unit => unit;

Initialize the domain-local generator with a random seed chosen in a system-dependent way. If a cryptographically secure pseudorandom number generator is available on the host machine, it is used to provide a highly random initial seed. Otherwise, a less random seed is computed from system parameters (current time, process IDs, domain-local state).

val bits : unit -> int

let bits: unit => int;

Return 30 random bits in a nonnegative integer.

val int : int -> int

let int: int => int;

Random.int bound returns a random integer between 0 (inclusive) and bound (exclusive). bound must be greater than 0 and less than 230.

raises Invalid_argument if bound <= 0 or bound >= 230.

val full_int : int -> int

let full_int: int => int;

Random.full_int bound returns a random integer between 0 (inclusive) and bound (exclusive). bound may be any positive integer.

If bound is less than 231, then Random.full_int bound yields identical output across systems with varying int sizes.

If bound is less than 230, then Random.full_int bound is equal to Random.int bound.

If bound is at least 230 (on 64-bit systems, or non-standard environments such as JavaScript), then Random.full_int returns a value whereas Random.int raises Stdlib.Invalid_argument.

raises Invalid_argument if bound <= 0. since 4.13

val int32 : Int32.t -> Int32.t

let int32: Int32.t => Int32.t;

Random.int32 bound returns a random integer between 0 (inclusive) and bound (exclusive). bound must be greater than 0.

raises Invalid_argument if bound <= 0.

val int64 : Int64.t -> Int64.t

let int64: Int64.t => Int64.t;

Random.int64 bound returns a random integer between 0 (inclusive) and bound (exclusive). bound must be greater than 0.

raises Invalid_argument if bound <= 0.

val float : float -> float

let float: float => float;

Random.float bound returns a random floating-point number between 0 and bound (inclusive). If bound is negative, the result is negative or zero. If bound is 0, the result is 0.

val bool : unit -> bool

let bool: unit => bool;

Random.bool () returns true or false with probability 0.5 each.

val bits32 : unit -> Int32.t

let bits32: unit => Int32.t;

Random.bits32 () returns 32 random bits as an integer between Int32.min_int and Int32.max_int.

since 4.14

val bits64 : unit -> Int64.t

let bits64: unit => Int64.t;

Random.bits64 () returns 64 random bits as an integer between Int64.min_int and Int64.max_int.

since 4.14

Advanced functions

The functions from module State manipulate the current state of the random generator explicitly. This allows using one or several deterministic PRNGs, even in a multi-threaded program, without interference from other parts of the program.

module State : sig ... end

module State: { ... };

val get_state : unit -> State.t

let get_state: unit => State.t;

get_state() returns a fresh copy of the current state of the domain-local generator (which is used by the basic functions).

val set_state : State.t -> unit

let set_state: State.t => unit;

set_state s updates the current state of the domain-local generator (which is used by the basic functions) by copying the state s into it.