(* Title: Pure/General/susp.ML
ID: $Id$
Author: Sebastian Skalberg and Florian Haftmann, TU Muenchen
Delayed evaluation.
*)
signature SUSP =
sig
type 'a T
val value: 'a -> 'a T
val delay: (unit -> 'a) -> 'a T
val force: 'a T -> 'a
val peek: 'a T -> 'a option
val same: 'a T * 'a T -> bool
end
structure Susp : SUSP =
struct
datatype 'a susp =
Value of 'a
| Delay of unit -> 'a;
type 'a T = 'a susp ref;
fun value v = ref (Value v);
fun delay f = ref (Delay f);
fun force susp = NAMED_CRITICAL "susp" (fn () =>
(case ! susp of
Value v => v
| Delay f =>
let
val v = f ();
val _ = susp := Value v;
in v end));
fun peek susp =
(case ! susp of
Value v => SOME v
| Delay _ => NONE);
fun same (r1 : 'a T, r2) = (r1 = r2); (*equality on references*)
end