Scott's loop/recur impl

loop-recur.js

const Loop = (fn, ... init) => { 
  let r = fn (... init)
  while (r instanceof Recur) r = fn (... r)
  return r
}

const Recur = ( ...v) => Object .create (
  Recur .prototype, 
  {[Symbol .iterator]: {value: _ => v .values ()}}
)

const count = (n) =>
  Loop ((i, n) => i < n ? Recur (i + 1, n) : i, 0, n)

console .log (count (1e6)) //=> 1000000, no recursion depth limits

const fib = (n = 0) =>
  Loop ((a, b, n) => n < 1 ? a : Recur (b, a + b, n - 1), 1, 1, n)

console .log (fib (15)) //=> 987


const nFibs = (n) => 
  Loop ((a, b, n, r) => n < 1 ? r : Recur (b, a + b, n - 1, r .concat (a)), 1, 1, n, [])

console .log (nFibs (10)) //=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]


// As in a mergesort
const _merge = (xs, ys) => Loop (
  (as, bs, i, j, rs) => i < as .length 
    ? j < bs .length
      ? as [i] < bs [j] 
        ? Recur (as, bs, i + 1, j, rs .concat (as [i]))
        : Recur (as, bs, i, j + 1, rs .concat (bs [j])) 
      : rs .concat (as) 
    : rs .concat (bs), 
  xs, ys, 0, 0, []
)