samer@4: % lazy - Lazy evaluator, or memoiser
samer@4: %
samer@4: % lazy :: (unit -> A) -> (unit -> A).
samer@4: % lazy :: (unit -> action A) -> (unit -> action A). 
samer@4: %
samer@4: % If x=lazy(f), then f is not called until x is
samer@4: % called. Then x() returns x1=f(). Thereafter,
samer@4: % x() returns x1 and f is never called again.
samer@4: %
samer@4: % Eg, let x=lazy(@()rand).
samer@4: % Then x() returns the same random number each
samer@4: % time it is called.
samer@4: function f=lazy(thunk)
samer@4: 	value=thunk; 
samer@4: 	evalled=false;
samer@4: 	f=@get;
samer@4: 
samer@4: 	function v=get, 
samer@4: 		if evalled, 
samer@4: 			v=value;
samer@4: 		else 
samer@4: 			value=thunk(); 
samer@4: 			evalled=true; 
samer@4: 			v=value; 
samer@4: 		end
samer@4: 	end
samer@4: end