🌱 Permutations in JS

'use strict'

// GENERIC FUNCTIONS

// replicateM n act performs the action n times, gathering the results.
// replicateM :: (Applicative m) => Int -> m a -> m [a]
const replicateM = (n, f) => {
  const loop = (x) => (x <= 0 ? [[]] : liftA2(cons, f, loop(x - 1)))
  return loop(n)
}

// Lift a binary function to actions.
// liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
const liftA2 = (f, a, b) => listApply(a.map(curry(f)), b)

// <*>
// listApply :: [(a -> b)] -> [a] -> [b]
const listApply = (fs, xs) =>
  [].concat.apply(
    [],
    fs.map((f) =>
      [].concat.apply(
        [],
        xs.map((x) => [f(x)])
      )
    )
  )

// curry :: ((a, b) -> c) -> a -> b -> c
const curry = (f) => (a) => (b) => f(a, b)

// cons :: a -> [a] -> [a]
const cons = (x, xs) => [x].concat(xs)

// show :: a -> String;
const show = JSON.stringify

// TEST
console.log(show(replicateM(8, [0, 1])))
javascript math