(* sieve-skeleton.sml *)

(* the type of suspensions and a supporing type *)

datatype 'a delay = Value of 'a
                  | Delay of unit -> 'a;

type 'a susp = 'a delay ref;

(* val delay : (unit -> 'a) -> 'a susp *)

fun delay f = ref(Delay f);

(* val force : 'a susp -> 'a *)

fun force(ref(Value x))      = x
  | force(r as ref(Delay f)) =
      let val x = f()
      in r := Value x; x end;

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

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

withtype 'a stream = 'a stream' susp;

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

fun from' n = Cons(n, from(n + 1))

and from n = delay(fn () => from' n);