diff general/funutils/lazy.m @ 4:e44f49929e56

Adding reorganised general toolbox, now in several subdirectories.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/general/funutils/lazy.m	Sat Jan 12 19:21:22 2013 +0000
@@ -0,0 +1,27 @@
+% lazy - Lazy evaluator, or memoiser
+%
+% lazy :: (unit -> A) -> (unit -> A).
+% lazy :: (unit -> action A) -> (unit -> action A). 
+%
+% If x=lazy(f), then f is not called until x is
+% called. Then x() returns x1=f(). Thereafter,
+% x() returns x1 and f is never called again.
+%
+% Eg, let x=lazy(@()rand).
+% Then x() returns the same random number each
+% time it is called.
+function f=lazy(thunk)
+	value=thunk; 
+	evalled=false;
+	f=@get;
+
+	function v=get, 
+		if evalled, 
+			v=value;
+		else 
+			value=thunk(); 
+			evalled=true; 
+			v=value; 
+		end
+	end
+end