gistfile1.txt

type 'a stream = S of 'a * 'a stream Lazy.t

let rec map f s =
  match s with
    | lazy (S (h, t)) -> S (f h, lazy (map f t))

let rec nth s n =
  match s, n with
    | lazy (S (h, _)), 0 -> h
    | lazy (S (_, t)), n -> nth t (n - 1)

let rec merge t2 t3 t5 =
  match t2, t3, t5 with
    | lazy (S (h2, t2')), lazy (S (h3, t3')), lazy (S (h5, t5')) ->
        let h = min h2 (min h3 h5) in
        let t =
          lazy
            (merge
               (if h2 = h then t2' else t2)
               (if h3 = h then t3' else t3)
               (if h5 = h then t5' else t5)) in
        S (h, t)
    | _ -> assert false

let rec hamming = lazy (S (1, lazy (merge t2 t3 t5)))
and t2 = lazy (map (fun k -> k * 2) hamming)
and t3 = lazy (map (fun k -> k * 3) hamming)
and t5 = lazy (map (fun k -> k * 5) hamming)

;;

nth hamming 7