Prolog: Even and Odd

even_and_odd.pl

%% The `=` is Prolog unification operator, it does not perform arithmetic evaluation.
%% That means `8 = 4 + 4` is false in Prolog.
%% Instead you can use `8 is 4 + 4` or `8 =:= 4 + 4`.
%% The unification operator tries to unify intensional structure, and `8 = 4 + 4` is 
%% trying to unify `'+'(4, 4)` with `8`.
%% However `8 = 8` unifies!

even(0).
even(X) :- X > 0, X1 is X - 1, odd(X1).
even(X) :- X < 0, X1 is X + 1, odd(X1).

odd(1).
odd(X) :- X > 0, X1 is X - 1, even(X1).
odd(X) :- X < 0, X1 is X + 1, even(X1)

You could do much faster by using the "mod" function like this:
even(X) :- 0 is mod(X, 2).
odd(X) :- 1 is mod(X, 2).

Thanks @slevy97

thanks <3