🚀 go-pugleaf

In article <MARK.90Dec7130758@adler.philosophie.uni-stuttgart.de> mark@adler.philosophie.uni-stuttgart.de (Mark Johnson) writes:

 * 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

What's wrong with "freeze"ing a goal which, in addition to calling the
main "Goal", will traverse the data structure the variable is
instantiated to, hooking up the same goal to all the variables in the
structure on its way?  For example:

    delay(Term, Goal) :-
	freeze(Term, (Goal, traverse(Term, Goal))).

    traverse(X, Goal) :- var(X), !, freeze(X, Goal).
    traverse(X, Goal) :- functor(X, _, N), traverse_args(N, X, Goal).

    traverse_args(0, _, _) :- !.
    traverse_args(N, X, Goal) :- 
	arg(N, X, Xn),
	traverse(Xn, Goal),
	N1 is N-1,
	traverse_args(N1, X, Goal).

which will execute as:

    | ?- delay(X, write(X)), X = f(Y,Z), Y = [a], Z = b.
    f(_92,_108)f([a],_343)f([a],b)

This calls "write" three times and I guess that's exactly what you
want.  Or do you want something completely different?

Takashi Chikayama
ICOT