A functional (pseudo) random number generator implementing Fantasy Land interfaces.

Usage no npm install needed!

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  import fantasyRng from '';



A functional (pseudo) random number generator implementing Fantasy Land interfaces.

Generator and factories

The library offers three factories to produce PRNG (pseuro random number generators).

RNG interface

Random number generators (RNG a) are objects with two method properties:

  • next :: RNG a ~> () -> [a, RNG a]: given a RNG, it returns a tuple with the next pseudo random number, and a new generator for the rest of the entries; this is the method mostly used in order to create a sequence of random numbers.
  • split :: RNG a ~> () => [RNG a, RNG a]: given a RNG, it returns two indipendent RNGs; this method is useful when multiple recursions are needed.


A function to produce N random numbers from a single one:

 * @param {number} n - the length of the resulting list
 * @param {RNG} rng - a random number generator
const randomlist = rng => n => {
  if (n === 0) {
    return [];
  const [value, newRng] =;
  return [value].concat(randomlist(newRng)(n - 1));

A function to produce a binary tree of depth N:

const createTree = rng => height => {
  if(height === 0) {
    return {
      type: 'leaf'
  } else {
    const [value, newRng] =;
    const [rng1, rng2] = newRng.split();
    return {
      type: 'node',
      left: createTree(rng1)(height - 1),
      right: createTree(rng2)(height - 1),

RNG Factories

There are some methods used to create standard RNGs.

  • defaultRandomGenerator :: RNG number - not a factory per se, but just the instance of the default, global random number generator, initialized at startup with a random seed.
  • createRandomGenerator :: number -> RNG number - a factory to create a pseudo random number generator starting from a seed; the implementation is based on tf-random.
  • createStaticGenerator :: number[] -> RNG number - a factory to create a generator that just repeats the values given in cycle; the split method creates two static generators, one with the even indices, the other with the odd indices.

Random monad

Sometimes it's useful to manipulate random numbers through monadic constructs. Random is the implementation of the Random monad based upon a RNG. It follows the specifications of Fantasy Land.

A Random instance is created passing it a random number generator. E.g.:

import { Random, defaultRandomGenerator } from 'fantasy-rng';
const myRandom = Random(defaultRandomGenerator);

The implementations of the monad methods have the following semantic:

  • fantasy-land/map: maps every value produced by the generator using the given function.
  • fantasy-land/of: creates a generator that produces the value provided over and over.
  • fantasy-land/ap: creates a generator that couples the values produces by the generator and the functions produced by the argument, and applies them one to the other to create the monad from resulting generator.
  • fantasy-land/chain: gets the first value of the main generator, applies the function and just returns a monad from the resulting generator.