Prolog: How do I check whether item exists in the database before adding?

Question

I start out with a blank database. I need to add some database entries to it, for example:

person(John,male)
person(Veronica,female)
person(Jessica,female)

I am adding entries like this:

add_person(N,G):-asserta(person(N,G)).

Howerver, I do not want to add the same person twice, so I need to check before adding.

I am trying to do it this way:

add_person(N,G):- \+ person(N,G),asserta(person(N,G)).

The problem is that I cannot query the database with, say person(John,male) if my database is blank. So person(N,G) fails with existence error, and I cannot add at all.

Any thoughts?

Answer 1

I'm not sure which variant of Prolog you're using, but you can declare some predicates to be dynamic, by putting this line at the top of your code:

:- dynamic person/2.

This is described in the SWI-Prolog documentation.

Answer 2

The way you write John is a variable and not an atom. You should use only lowercase for atoms (john, veronica, etc.). I just tried your code and it works fine:

person(john, male).

add_person(N,G):-
    \+ person(N,G),
    asserta(person(N,G)).

Adding john again fails and returns false.