What's an elegant way to unify X,Y with (1,2), (1,-2), (-1,2), (-1,-2), (2,1), (2,-1) , (-2,1), (-2,-1)?
Doing it this way seems error prone and tedious:
foo(1,2).
foo(1,-2).
foo(-1,-2).
...
...
...
And this way seems too expensive:
foo(X,Y) :-
L = [1,-1,2,-2],
member(X,L),
member(Y,L),
abs(X,X1), abs(Y,Y1),
X1 =\= Y1.
A further development on what was commented:
generate_pairs_foo(X,Y) :-
L = [1,-1,2,-2],
member(X,L),
member(Y,L),
abs(X,X1), abs(Y,Y1),
X1 =\= Y1.
assert_all_foo([]).
assert_all_foo([(X,Y)|T]) :-
assert(foo(X,Y)), assert_all_foo(T).
find_all((X,Y),generate_pairs_foo(X,Y),L), assert_all_foo(L).
Hmmmmmm... look, it's easier and shorter to just write all the cases xD
foo0(X,Y):-
member(X,[1,-1]),
member(Y,[2,-2]).
foo(X,Y):-
foo0(X,Y);
foo0(Y,X).
foo(1, 2).
foo(2, 1).
foo(X, Y) :-
foo(-X, Y).
foo(X, Y) :-
foo(X, -Y).