(* stream.sml *)

(* implementation of streams *)

structure Stream :> STREAM =
struct

(* type of infinite streams and a supporting type *)

datatype 'a stream' = Cons of 'a * 'a stream

withtype 'a stream = 'a stream' Susp.susp;

(* val from : ('a -> 'a) -> 'a -> 'a stream *)

fun from f =
      let (* val frm : 'a -> 'a stream *)

          fun frm x = Susp.delay(fn () => frm' x)

          (* val frm' : 'a -> 'a stream' *)

          and frm' x = (print "<cons>"; Cons(x, frm(f x)))
      in frm end

(* val destruct : 'a stream -> 'a * 'a stream *)

fun destruct x =
      let val Cons(y, z) = Susp.force x
      in (y, z) end

end;