class Reduced {
  constructor(value) {
    this.value = value
  }
}

const reduced = x => new Reduced(x)

const reduce = (reducer, init, arr) => {
  if (arr.length === 0) {
    return init
  }

  if (init instanceof Reduced) {
    return init.value
  }

  const [head, ...tail] = arr

  return reduce(reducer, reducer(init, head), tail)
}

const filter = pred => next =>
  (arr, x) =>
    pred(x)
      ? next(arr, x)
      : arr

const map = f => next =>
  (arr, x) =>
    next(arr, f(x))

const take = n => next =>
  (arr, x) =>
    arr.length < n
      ? next(arr, x)
      : reduced(arr)

const compose = (...fns) => a =>
  fns.reduceRight((x, f) => f(x), a)

const transduce = (xf, identity, init, arr) =>
	reduce(xf(identity), init, arr)

const append = (arr, x) =>
  arr.concat(x)

console.log(
  transduce(
    compose(
      filter(x => { console.log("filter", x); return x % 2 === 0 }),
      map(x => { console.log("map", x); return x * 10 }),
      take(2),
    ),
    append,
    [],
    Array.from({ length: 1000 }, (_, i) => i),
  )
)