问题
Got this strange behaviour. I was running these test cases:
s1 :-
Q=[[lambda,symbol(_3026),[cons,[quote,_3434],
[quote,_3514]]],[quote,_3206]],
P=[_3434|_3514],
freeze(_3434, (write(foo), nl)),
unify_with_occurs_check(P, Q).
s2 :-
Q=[[lambda,symbol(_3026),[cons,[quote,_3434],
[quote,_3514]]],[quote,_3206]],
P=[_3434|_3514],
freeze(_3434, (write(foo), nl)),
freeze(_3514, (write(bar), nl)),
unify_with_occurs_check(P, Q).
Now I get these results, where the outcome of s2
is wrong. The outcome is wrong in two respects, first _3434
gets triggered and second unify_with_occurs_check
succeeds:
SWI-Prolog (threaded, 64 bits, version 8.3.16)
?- s1.
false.
?- s2.
foo
bar
true.
That _3434
shouldn't get triggered follows from 7.3.2 Herband Algorithm in ISO core standard. According to clause 7.3.2 f) 1) an instantiation of variable X to a term t is only propagated when it X does not occur in t.
That the unification should fail follows from clause 7.3.2 g). So it seems in SWI-Prolog, attributed variables in various incarnations such as freeze/2, dif/2, etc… seem to interfer with unify_with_occurs_check.
Any workaround?
回答1:
Here is another somewhat simpler workaround:
unify(X,X) :-
acyclic_term(X).
Certainly, this only works as expected if the two arguments are finite from the very start, but at least it does not loop in this case.
回答2:
One way out could be to roll your own unify_with_occurs_check/2. We can write it in Prolog itself, as was done in the past, for Prolog systems that did not have unify_with_occurs_check/2:
R.A.O'Keefe, 15 September 1984
http://www.picat-lang.org/bprolog/publib/metutl.html
Here is an alternative take that uses (=..)/2 and term_variables/2:
unify(X, Y) :- var(X), var(Y), !, X = Y.
unify(X, Y) :- var(X), !, notin(X, Y), X = Y.
unify(X, Y) :- var(Y), !, notin(Y, X), X = Y.
unify(X, Y) :- functor(X, F, A), functor(Y, G, B),
F/A = G/B,
X =.. [_|L],
Y =.. [_|R],
maplist(unify, L, R).
notin(X, Y) :-
term_variables(Y, L),
maplist(\==(X), L).
I now get the expected result:
?- s1.
false.
?- s2.
false.
来源:https://stackoverflow.com/questions/65532899/proper-unify-with-occurs-check-2-in-swi-prolog