Prolog existence_error following Seven Languages in Seven Weeks

Question

I\'m just following the book Seven Languages in Seven Weeks.

I\'ve installed gprolog in my Mac machine using command port install gprolog-devel and run firs

Answer 1

The interactive gprolog interpreter runs queries against a loaded list of predicates, that is why you get the existence_error exception. You will have to load your predicates into it, either by using an IDE that does the loading for you or doing it manually.

Here's one approach:

| ?- [user].
compiling user for byte code...
likes(wallace, cheese).
likes(grommit, cheese).
likes(wendolene, sheep).

friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).

* Press Ctrl-D to end input. *
user compiled, 6 lines read - 909 bytes written, 15538 ms

yes
| ?- friend(wallace,grommit).

yes
| ?- friend(wallace,wendolene).

no

The gprolog manual writes about this in the chapter Consulting a Prolog program