src/Pure/General/susp.ML

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Pure/General/susp.ML	Tue Sep 19 15:21:55 2006 +0200
@@ -0,0 +1,45 @@
+(*  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 (ref (Value v)) = v
+  | force (r as ref (Delay f)) =
+      let
+        val v = f ()
+      in
+        r := Value v;
+        v
+      end;
+
+fun peek (ref (Value v)) = SOME v
+  | peek (ref (Delay _)) = NONE;
+
+fun same (r1 : 'a T, r2) = (r1 = r2); (*equality on references*)
+
+end