How add finitely failed branches to a Prolog visualizer?

Question

Assume we want to visualize this Prolog execution. No goals from the fidschi islands, or something else exotic assumed, only good old SLDNF with the default selection rule:

p(a).
p(b).

?- \+ p(c).
Yes

But we have only a Prolog visualizer that can show derivations without negation as failure, like here. How can we boost the Prolog visualizer to also show negation as failure?

This is a good question but you doubt StackOverflow will deliver on this. (Also, what about frozen goals, when goals, general attributed variable goals, constraint goals, dif goals, setup_call_cleanup goals and catch goals?) — David Tonhofer, Mar 10 '21 at 14:56
Go on, ask such questions, demonstrate what you have done. By occams razor these questions are out of scope of my question, its only about default SLDNF, no delayed goals. — , Mar 10 '21 at 16:54

score 1 · Answer 1 · 2021-03-10T17:26:56.123

The good thing about negation as failure, writing a meta interpreter for negation as failure is much easier, than writing a meta interpreter for cut (!). So basically the vanilla interpreter for SLDNF can be derived from the vanilla interpreter for SLD by inserting one additional rule:

solve(true) :- !.
solve((A,B)) :- !, solve(A), solve(B).
solve((\+ A)) :- !, \+ solve(A). /* new */
solve(H) :- functor(H, F, A), sys_rule(F/A, H, B), solve(B).

We can now go on and extend solve/3 from here in the same vain. But we do something more, we also write out failure branches in the search tree, similar like Prolog visualizer does by strikethrough of a clause. So the amended solve/3 is as follows:

% solve(+Goal, +Assoc, +Integer, -Assoc)
solve(true, L, _, L) :- !.
solve((A, B), L, P, R) :- !, solve(A, L, P, H), solve(B, H, P, R).
solve((\+ A), L, P, L) :- !, \+ solve(A, L, P, _). /* new */
solve(H, L, P, R) :- functor(H, F, A), sys_rule(F/A, J, B),
   callable_property(J, sys_variable_names(N)),
   number_codes(P, U), atom_codes(V, [0'_|U]), shift(N, V, W),
   append(L, W, M), 
   (H = J -> true; offset(P), write(fail), nl, fail),  /* new */
   reverse(M, Z), triage(M, Z, I, K),
   offset(P), write_term(I, [variable_names(Z)]), nl,
   O is P+1, solve(B, K, O, R).

Here is an example run:

?- ?- \+ p(c).
fail
fail
Yes

How add finitely failed branches to a Prolog visualizer?

1 Answers1