src/HOL/Import/susp.ML

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/HOL/Import/susp.ML	Sat Oct 08 22:39:40 2005 +0200
@@ -0,0 +1,41 @@
+(*  Title:      HOL/Import/susp.ML
+    ID:         $Id$
+    Author:     Sebastian Skalberg, TU Muenchen
+
+Delayed evaluation.
+*)
+
+signature SUSP =
+sig
+
+type 'a susp
+
+val force : 'a susp -> 'a
+val delay : (unit -> 'a) -> 'a susp
+val value : 'a -> 'a susp
+
+end
+
+structure Susp :> SUSP =
+struct
+
+datatype 'a suspVal
+  = Value of 'a
+  | Delay of unit -> 'a
+
+type 'a susp = 'a suspVal ref
+
+fun force (ref (Value v)) = v
+  | force (r as ref (Delay f)) =
+    let
+	val v = f ()
+    in
+	r := Value v;
+	v
+    end
+
+fun delay f = ref (Delay f)
+
+fun value v = ref (Value v)
+
+end