Debugging Simple Prolog Function

Question

I am trying to write my on mod function in Prolog, with a signature of modulo(A,B,C), where C is equivalent to A mod B. This is my code:

modulo(A,B,A) :- A<B. /* Not sure about this part.*/

modulo(A,B,C) :-
A==C -> 
    true ;
    A>C ->
        A1 is A-B,
        modulo(A1,B,C) ;
        false .

For some reason, when I run this code, it gives me an "Arguments are not sufficiently instantiated" error. I am very new to Prolog, and I was wondering if someone could please help me figure out what I am doing wrong/how to fix it?

Any help would be greatly appreciated! Thank you in advance!

Answer 1

You seem to be trying to get a Prolog predicate to return either true or false. That's not how Prolog works. A predicate either succeeds or it fails. They don't return anything.

Other than that, your code is close.

The first case you need to cover is when A < B.

modulo(A,B,A) :- A < B.

If A < B and the first and the third terms are the same then this predicate should succeed.

The next case is when A >= B.

modulo(A,B,C) :-
    A >= B,
    A1 is A - B,
    modulo(A1,B,C).

And that's it.

Notice I haven't returned anything. All we want to say is "did this succeed".

Trying on ?- modulo(7,7,1). I get No., but trying on ?- modulo(7,2,1). I get Yes..