I'm observing a rather strange behaviour (GnuProlog 1.4.4 on Windows and Linux,
idem for older versions) and I'm not able to completely isolate it; it seems to
be related to singleton variables and backtracking. Here is a stupid code
demonstrating the issue:
%--------------
z(A,B, A,B).
z(A,B, B,A).
p5(Ls):-
_ = [B, C],
% write(B), % write(C),
Ls = [A, E],
z(A, _, 1, 2),
E=3,
z(A, _, B, C).
%--------------
Asking for every solution leads to an infinite loop...
?- p5(X).
X = [1,3] ? a
...... [infinine loop].....
Writing B and C removes the problem; and writing only one of them leads to
another strange behavior.
Running the call with the tracer also changes the behavior, and reveals
strange d(...) sub-terms:
{trace}
| ?- p5(X).
1 1 Call: p5(_23) ?
2 2 Call: z(_53,_57,1,2) ?
2 2 Exit: z(1,2,1,2) ?
3 2 Call: z(1,_85,_126,_127) ?
3 2 Exit: z(1,_85,1,_85) ?
1 1 Exit: p5([1,3]) ?
X = [1,3] ? a
1 1 Redo: p5([1,3]) ?
3 2 Redo: z(1,_85,1,_85) ?
3 2 Exit: z(1,_85,_85,1) ?
1 1 Exit: p5([1,3]) ?
X = [1,3]
1 1 Redo: p5([1,3]) ?
2 2 Redo: z(1,2,1,2) ?
2 2 Exit: z(2,1,1,2) ?
3 2 Call:
z(2,_85,d(3,[a(z(2,_85,d(3,[a(z(2,_85,...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),2)
?
3 2 Exit:
z(2,d(3,[a(z(2,d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),d(3,[a(z(2,d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),2)
?
1 1 Exit: p5([2,3]) ?
X = [2,3]
(16 ms) yes
{trace}
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