different(Xs, Ys) :-
member(X, Xs),
non_member(X, Ys).
different(Xs, Ys) :-
member(Y, Ys),
non_member(Y, Xs).
While this definition using member/2
and non_member/2
is almost1 perfect from a declarative viewpoint, it produces redundant solutions for certain queries and leaves choice points all around.
What is a definition that improves upon this (in a pure manner probably using if_/3
and (=)/3
) such that exactly the same set of solutions is described by different/2
but is determinate at least for ground queries (thus does not leave any useless choice points open) and omits (if possible) any redundant answer?
1
Actually, different([a|nonlist],[]), different([],[b|nonlist])
succeeds. It could equally fail. So a solution that fails for both is fine (maybe even finer).
Let's take it to the limit---by the help of list_nonmember_t/3
, exists_in_t/3
, and
or_/2
!
some_absent_t(Xs,Ys,Truth) :-
exists_in_t(list_nonmember_t(Ys),Xs,Truth).
different(Xs,Ys) :-
or_(some_absent_t(Xs,Ys),
some_absent_t(Ys,Xs)).