问题
Say I have a predicate pred containing several facts.
pred(a, b, c).
pred(a, d, f).
pred(x, y, z).
Can I use findall/3 to get a list of all facts which can be pattern matched?
for example, if I have
pred(a, _, _)
I would like to obtain
[pred(a, b, c), pred(a, d, f)]
回答1:
Just summing up what @mbratch said in the comment section:
Yes, but you have to make sure that you either use named variables or construct a simple helper predicate that does that for you:
Named variables:
findall(pred(a,X,Y),pred(a,X,Y),List).
Helper predicate:
special_findall(X,List):-findall(X,X,List).
?-special_findall(pred(a,_,_),List).
List = [pred(a, b, c), pred(a, d, f)].
Note that this doesn't work:
findall(pred(a,_,_),pred(a,_,_),List).
Because it is equivalent to
findall(pred(a,A,B),pred(a,C,D),List).
And thus doesn't unify the Variables of Template with those of Goal.
来源：https://stackoverflow.com/questions/21082855/prolog-findall-3