Just Having Fun...

FunFolds.js

"use strict"

const identity = x => x

const of = x => [x]

const empty = () => []

const concat = (xs, ys) => [...xs, ...ys]

const foldRight = f => acc => ([x, ...xs]) => x
    ? f(x, foldRight(f)(acc)(xs))
    : acc

const foldLeft = f => acc => ([x, ...xs]) => x
    ? foldLeft(f)(f(acc, x))(xs)
    : acc

const map = f => foldLeft((acc, value) => concat(acc, of(f(value))))(empty())

const flatMap = f => foldLeft((acc, value) => concat(acc, f(value)))(empty())

const filter = f => foldLeft((acc, value) => f(value) ? concat(acc, of(value)) : acc)(empty())

const compose = (...args) => foldLeft((acc, value) => x => acc(value(x)))(identity)(args)

const process = compose(
    foldLeft((acc, value) => acc + value)(0),
    map(x => x * 5),
    filter(x => x % 2 === 0),
    flatMap(identity)
)

const input = [[1, 2], [3, 4], [5, 6], [7, 8]]

const output = process(input)

console.log(output)