(* this is stolen from Harper's notes (p. 36), but I have generalized it.
   In my version you give makestream an initial value and a successor function
   and it returns the corresponding stream
*)

abstype 'a stream = Stream of unit -> ('a * 'a stream)
  with
   fun makestream (first,succ) =
     let fun step n = Stream (fn () => (n, step (succ n)))
     in step first end
  fun next(Stream f) = f()
 end;


(* Example use #1 (natural numbers):

  val nats = makestream (0, fn n => n+1) ;
  val (a,nats) = next nats ;
  val (b,nats) = next nats ;

*)

(* Example use #2 (powers of 2):

  val pow2 = makestream (1, fn n => 2*n) ;
  val (a,pow2) = next pow2 ;
  val (b,pow2) = next pow2 ;
  val (c,pow2) = next pow2 ;

*)