Prolog difference lists: code works on SWI and GNU Prolog, but not Yap

3
votes

This code correctly calculates the number of elements in a difference list when I run it on GNU Prolog and SWI Prolog. However, Yap enters an infinite loop.

count(X-X1,0) :- unify_with_occurs_check(X,X1), !.
count([H|T]-T1,N) :- count(T-T1,M), N is M+1.

?- count([1,2|A]-A,N).

Why would a Prolog interpreter (like Yap) not terminate when asked this query?

1
prologyap
I tried your code in Yap 6.2.2 and it worked fine. ?- count([1,2|A]-A,N). N = 2 - gusbro
@gusbro falls in loop in yap 5.1.3: trace: ?- count([1,2|A]-A,N). (1) call:count([1,2|_1059]-_1059,_1063) ? (1) redo:count([1,2,1,2,1,2,1,2,1,2,...]-_1059,_1063) ? (2) call:count([2,1,2,1,2,1,2,1,2,1,...]-_1059,_1147) ? (2) redo:count([2,1,2,1,2,1,2,1,2,1,...]-_1059,_1147) ? - Thanos Tintinidis
Hm, I'm using 5.1.3, so it could be a fixed bug... - Jay
I have just installed Yap 6.2.3 and it works! Thanks! - Jay
@gusbro: you have actually answered the question, so it would be nice if you posted it as an answer. - Jay

1 Answers

3
votes

There seems to be a bug in Yap 5.1.3

Newer versions (tested with Yap 6.2.2) work fine:

?- count([1,2|A]-A,N).
   N = 2