Prolog programming - simple negative query

Question

My database is:

eat(magi,limo).
eat(nona,banana).

How do I ask: "Who's not eating limo?" This:

eat(X,not(limo)).

Doesn't work. :(

Answer 1

First of all limo is a symbol and you can't negate symbols. What you want to do is negate the predicate, i.e. not(eat(X, limo)).

However this still does not give you nona as a result. Why not? Well there are infinitely many values X for which eat(X, limo) will be false. The system needs more information than "X does not eat limo" to know which one you want. Instead we need to ask for an X such that "X eats something, but X does not eat limo". This leads us to the following query:

eat(X,Y), not(eat(X, limo)).

Which gives us nona as the solution for X.