🚀 go-pugleaf

/* Here's a utility predicate freeze/3 for prologs like SICSTUS and BNR
 * which have freeze/2.  It uses a ``shared variable'' which is bound
 * when one of the frozen goals is run.  It's easy to generalize this
 * to arrange for Goal to be called once whenever *any* variable appearing
 * in a term (or list) is bound.
 *
 * But what I don't know how to do is the following: define a predicate
 * delay(Term, Goal) which calls Goal when Term is further instantiated,
 * possibly by unifying distinct variables occuring in Term.
 * Any ideas?
 *
 * Mark Johnson
 */

freeze(X, Y, Goal) :-               % Delay Goal until either X or Y is bound
	freeze(X, freeze_helper(SharedVar, Goal)),
	freeze(Y, freeze_helper(SharedVar, Goal)).

freeze_helper(SharedVar, _) :-      % If SharedVar is bound
	nonvar(SharedVar),          %  then the frozen goal
	!.                          %  has already been run.
freeze_helper(done, Goal) :-        % Bind SharedVar
	call(Goal).                 %  and call Goal.


test :-
	freeze(X, Y, write('Frozen goal activated')),
	X = a,
	Y = b.