type 'a infstream = Cons of 'a * (unit -> 'a infstream)

let hd (Cons (h, _)) = h
let tl (Cons (_, t)) = t ()

let rec from n = Cons (n, fun () -> from (n + 1))

let rec take n (Cons (h, t)) =
  if n <= 0 then []
  else h :: take (n - 1) (t ())

let rec drop n (Cons (h, t) as s) =
  if n <= 0 then s
  else drop (n - 1) (t ())

let rec map f (Cons (h, t)) =
  Cons (f h, fun () -> map f (t ()))

let rec map2 f (Cons (h1, t1)) (Cons (h2, t2)) =
  Cons (f h1 h2, fun () -> map2 f (t1 ()) (t2 ()))

let rec fibs =
  Cons (0, fun () -> Cons (1, fun () -> map2 (+) fibs (tl fibs)))

let rec unfold f x =
  let (v, x') = f x in
  Cons (v, fun () -> unfold f x')

let fibs' = unfold (fun (a, b) -> (a, (b, a + b))) (0, 1)