Prolog not unique elements in predicate call

Question

In my Prolog script, I have defined:

mother(X,Y) :-
    parent_of(X,Y),
    female(X).

I then want to know if there are any mothers with more than two children, so I run:

mother(X,Y), mother(X,Z)

With the result:

X = pam,
Y = M, M = bob

Which has left me quite baffled.... I figured that if I add

not(Y = Z)

This will fix it, but am unsure as to why...

Answer 1

It looks like that you assumed that variables with different names can't have the same value. That's not true. You have to specify this explicitly (like in mathematics, for example, - variable X can have the same value as a different variable Y, unless you explicitly specify the opposite).

Answer 2

If you execute a query like

mother(X,Y).

The result would bring back mothers that have two children as well.

So if your database was something like

female(maria).
female(irini).
parent_of(maria,nick).
parent_of(maria,dario).
parent_of(irini,dewey).

and you executed the mother(X,Y). query, the result would bring back

1 ?- mother(X,Y).
X = maria,
Y = nick ;
X = maria,
Y = dario ;
X = irini,
Y = dewey.

So your result would have the mother (maria) that has two children.

If you only want a mother with two children, you should modify your mother query as:

mother(X,Y) :-
    parent_of(X,Y),
    parent_of(X,M),
    Y \= M,
    female(X).

The result of this query would be:

3 ?- mother(X,Y).
X = maria,
Y = nick ;
X = maria,
Y = dario ;
false.

( false means that Prolog did't find any more results).