Border case in Query Answer display

General
1

I guess this is a small glitch in the Query Answer display:

/* SWI-Prolog 9.3.22 */
?- length(L,2), f(L) = R, S = [_A].
L = [_A, _B],
R = f([_A, _B]),
S = [_A].

The newly generated _A and _B are not checked whether
they already appear in the answer elsewhere, causing a name

clash. The more correct answer would be:

/* Scryer Prolog 0.9.4-403 */
?- length(L,2), f(L) = R, S = [_A].
L = [_B, _C], 
R = f([_B, _C]), 
S = [_A].

Other Prolog systems that avoid the clash as well are
Trealla Prolog, and since today Dogelog Player.

2

Old SICStus 4.5.1 behaves correctly.

3

This is what Ciao does for reference:

Ciao 1.24
?- length(L,2), f(L) = R, S = [_A].

L = [_B,_C],
R = f([_B,_C]),
S = [_] ?
4

Different Ciao Prolog inspired test case, expectation:

/* Trealla Prolog 2.71.33, Scryer Prolog 0.9.4-403 */
?- length(L,2), f(L) = R, S = [_A].
L = [_B, _C], R = f([_B, _C]), S = [_A].

/* SWI-Prolog 9.3.22 */
?- length(L,2), f(L) = R, S = [_].
L = [_A, _B], R = f([_A, _B]), S = [_].

Reality:

/* Trealla Prolog 2.71.33, Scryer Prolog 0.9.4-403 */
?- length(L,2), f(L) = R, S = [_].
   L = [_A,_B], R = f([_A,_B]), S = [_C].

It seems that _ is not reconstructed.

5

In a sense it is correct as we first remove all _X variables from the answer :slight_smile: But, I agree it is highly confusing. Pushed 18147fa7cc7ed63f2ef2ebfa4b8f5149385b42f0 to both reserve variables from the original query and generated variables, e.g., for breaking cycles, etc.

1 Like
6

The clash detection works in listing/1, I didn’t find an issue so
far. Take this example, one needs to put it into a file case.pl:

test --> p, {q(A,A)}, r.

SWI-Prolog then preserves the variable A, but also
avoids a clash in the newly generated variables:

?- [case].

?- listing(test).
test(B, C) :-
    p(B, D),
    q(A, A),
    E=D,
    r(E, C).