Why doesn't < evaluate the arguments?

Help!
2

The evaluation for (<)/2 was being done, but unification doesn’t evaluate:

gcd(X, X, X). % does not evaluate X

Instead, you need to do something like this for the first clause:

gcd(X, Y, D) :-
    X =:= Y  % evaluate
    D is X.  % also evaluate

So, as a general rule, it’s best to use is/2 to evaluate an expression rather than passing unevaluated expressions around.

BTW, this is how I’d write it (using if-then-else) without using is/2:

gcd(X, Y, Gcd) :-
    (   X =:= Y
    ->  Gcd is X
    ;   X < Y
    ->  gcd(X, Y-X, Gcd)
    ;   % Y < X
        gcd(Y, X-Y, Gcd)
    ).

and using if-then-else and is/2:

gcd(X, Y, D) :-
    (   X = Y
    ->  D = X
    ;   X < Y
    ->  Z is Y-X,
        gcd(X, Z, D)
    ;   % Y < X,
        Z is X-Y,
        gcd(Y, Z, D)
    ).

The if-then-else avoids creating choicepoints.

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