% simplest form
abs1(X,Y) :- X >= 0, Y is X.
abs1(X,Y) :- X < 0, Y is -X.

% some modification: We can have no condition checking in second rule.
abs2(X,Y) :- X >= 0, Y is X,!.
abs2(X,Y) :-  Y is -X.

% Y is not needed in the first rule.
abs3(X,X) :- X >= 0, !.
abs3(X,Y) :- Y is -X.

% We can do it with only one rule.
abs4(X,Y) :-  X <0 ,Y is -X,!.
abs4(X,X).