utilities.js

/* --------------------------------------------------------------------------- *
        Utilities for managing lists, implemented using only recursion
* --------------------------------------------------------------------------- */

const head = ([x]) => x

const tail = ([x, ...rest]) => rest

const last = xs => xs[xs.length - 1]

const length = xs => xs.length

const drop = (n, xs) => n ? drop(n - 1, tail(xs)) : xs

const take = (n, [x, ...rest]) => n ? [x, ...take(n - 1, rest)] : []

const reverse = ([x, ...rest]) => length(rest) ? [...reverse(rest), x] : [x]

const map = (f, [x, ...rest]) => length(rest) ? [f(x), ...map(f, rest)] : [f(x)]

const filter = (f, [x, ...rest]) => length(rest) ? f(x) ? [x, ...filter(f, rest)] : filter(f, rest) : f(x) ? [x] : []

const reduce = (f, acc, [x, ...rest]) => length(rest) ? reduce(f, f(acc, x), rest) : f(acc, x)

const reduceRight = (f, acc, xs) => reduce(f, acc, reverse(xs))

const fact = x => x ? x * fact(x - 1) : 1

const compose = (...funcs) => x => reduceRight((acc, f) => f(acc), x, funcs)