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2011-10-21 10:32:22

Randomness in the style of Common Lisp

This proposal for random numbers is not a literal transcription of the Common Lisp (CL) interface, but it uses the same concepts and provides the same facilities. The text is a heavily edited version of the CL Hyperspec.

A random-source object is an encapsulation of the state information used by an implementation-dependent pseudo-random number generator. The state can be printed out and successfully read back in by the same implementation, but might not function correctly as a state in another implementation. Random-source objects are a disjoint type. This facility should not be used where strong randomness is required.


(make-random-source . args)

Constructs and returns a random-source object that has been randomly initialized by some implementation-defined means. The CL equivalent is (make-random-state t). If args are provided, they are used in an implementation-defined way to affect what result is returned.

(random-source? obj)

Returns #t if obj is a random-source object, and #f otherwise. The CL equivalent is random-state-p.

(random-source-state random-source)

Returns an implementation-specific object representing a copy of the state encapsulated by random-source. This object MUST be printable and rereadable using standard Scheme lexical syntax. It MUST also be suitable for passing to make-random-source-from-state. Providing this mechanism makes it possible to save and reconstitute a random-source in a file or database, or to pass it across a network to an equivalent implementation. Mutating the result of this procedure does not affect random-source. There is no CL equivalent of this procedure, because CL random-state objects are themselves required to be printable and rereadable (they are typically CL structs).

(make-random-source-from-state state)

Constructs and returns a random-source object whose state is a copy of state, so that mutating state does not affect the random-source object. The result will generate the same sequence of pseudo-random numbers that the original random-source object would have generated as of the time random-source-state was invoked on it. It is an error to pass a state object that has been mutated. There is no CL equivalent of this procedure.

(copy-random-source random-source)

Constructs and returns a random-source object whose state is an independent copy of the state of random-source. Calling this procedure is equivalent to calling (make-random-source-from-state (random-source-state random-source)), but potentially more efficient because it can avoid copying the state twice. The result and random-source will henceforth return the same sequence of values, allowing the same series of pseudo-random numbers to be generated many times within a single program. The CL equivalent is make-random-state with a random-state argument.


A parameter that returns the default random-source object used by random. Its initial value MUST be a random-source object, but is implementation-dependent. The CL equivalent is *random-state*.

Note: The equivalent of CL (make-random-state) and (make-random-state nil) is (copy-random-source (current-random-source)).

(random limit [random-source])

Returns the next pseudo-random number from random-source. The result is a non-negative number less than limit. Limit MUST be either an exact integer (in which case random returns an exact integer), or an inexact real number (in which case random returns an inexact real number). If random-source is not specified, the value of (current-random-source) is used. The CL equivalent of this procedure is random.

An approximately uniform choice distribution is used. If limit is an integer, each of the possible results occurs with (approximate) probability 1/limit.


(<= 0 (random 1000) 1000) => true (let ((state1 (copy-random-source (current-random-source))) (state2 (copy-random-source (current-random-source)))) (= (random 1000 state1) (random 1000 state2))) => true