stream.re

type lazylist('a) =
  | Cons('a, unit => lazylist('a));

let rec lseq = n => Cons(n, () => lseq(n + 1));

let lhd = (Cons(n, _)) => n;
let ltl = (Cons(_, tf)) => tf();

let rec ltake = (Cons(h, tf), n) =>
  switch (n) {
  | 0 => []
  | _ => [h, ...ltake(tf(), n - 1)]
  };

let rec ldrop = (Cons(h, tf) as ll, n) =>
  switch (n) {
  | 0 => ll
  | _ => ldrop(tf(), n - 1)
  };

let rec lmap = (f, Cons(h, tf)) =>
  Cons(f(h), () => lmap(f, tf()));

let rec lfilter = (f, Cons(h, tf)) =>
  if (f(h)) {
    Cons(h, () => lfilter(f, tf()));
  } else {
    lfilter(f, tf());
  };

let squares = lmap(a => a * a, lseq(2));
Js.log(ltake(lfilter(x => x > 50, squares), 10));