I am trying to build a simple greatest common divisor (cmmdc) in prolog and, for the following code, I get a fail on newY = X mod Y. Can anyone help me out?
% cmmdc(X: intreg, Y: intreg, Ret: intreg)
% (i, i, o), (i, i, i)
cmmdc(X, 0, X):- !.
cmmdc(X, Y, Ret):-
newY = X mod Y,
cmmdc(Y, newY, Ret).
newY starts with a lowercase letter, so it is an atom instead of a varaible.
= is unification, not arithmetic.
The atom newY is not the same as the term mod(X,Y) so unification fails.
Use NewY is X mod Y instead.