streams.ml

let print_list f l =
  print_char '[';
  begin
    match l with
      [] -> ()
    | hd :: tl ->
      print_string (f hd);
      List.iter (fun x -> print_string (", " ^ (f x))) tl
  end;
  print_char ']';;

let print_int_list l = print_list (string_of_int) l;;

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

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

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

let rec filter f (Cons (h, tf)) =
  if (f h) then Cons(h, fun () -> filter f (tf ()))
  else filter (f) (tf ())
;;

let rec take n s =
  if n=0 then []
  else hd s :: take (n-1) (tl s);;

let rec sum (Cons(h1, tf1)) (Cons(h2, tf2)) =
  Cons(h1 + h2, fun () -> sum (tf1()) (tf2()));;

let rec square (Cons(h, tf)) = Cons(h*h, fun () -> square (tf ()));;


let rec primes sum xs =
  let x = hd xs in
  if sum > 4000000 then []
  else x ::
       (
         primes (sum + x) (filter (fun p -> (p mod x) <> 0) (tl xs))
       )

let rec fibs =
  Cons(1, fun () ->
      Cons(1, fun () ->
          sum fibs (tl fibs)))

let () = print_int_list (primes 0 (from 2));   print_newline ();;

let () = print_int_list (take 10 fibs); print_newline ();;

  let () = print_int_list
      (take 10
         (
           filter (fun x -> (x mod 3) = 0) (from 1)
         )
      );;
let () = print_newline ();