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 parent(bob, tim).           % parent/2
 parent(sue, alex, william). % parent/3
 male(sue).                  % male/1
 female(sue).                % female/1
 frobnitz.                   % frobnitz/0

You can name a predicate just about anything that makes sense, but note that
some predicates are built-in and their names are reserved.  The document
C<AI::Prolog::Builtins> has a list of the predicates that C<AI::Prolog>
directly supports.  If you're in the C<aiprolog> shell (explained later), you
can type C<help.> to get a list of built-in predicates and
C<help('functor/arity')> (e.g., C<help('consult/1')>) to get description of how
a particular built-in predicate works.

And that pretty much covers most of what you need to know about facts.

=head3 Rules

Rules, like facts, are stored in the program.  Rules describe how we can infer
new facts, even though we haven't explicitly stated them.

 gives(tom, book, SOMEONE) :-
     person(SOMEONE),
     likes(tom, SOMEONE).

This rule states that "Tom will give a book to anyone who Tom likes." Note that
we are not telling Prolog how to figure out to whom Tom will give books.
Instead, we have merely defined the conditions under which Tom is willing to
part with his material possessions.

To understand rules, read the neck operator, C<:->, as "if" and commas outside
of argument lists as "and."  Further, arguments beginning with upper-case
letters are I<variables>, such as C<SOMEONE> in the rule above.  Note that only
the first letter needs to be capitalized; C<Someone> would also be a variable,
as would C<SomeOne> or C<SOmeoNe>.

Of course, we could simply enumerate the relationships:

 gives(tom, book, alice).
 gives(tom, book, bob).
 gives(tom, book, martin).
 gives(tom, book, charlie).
 gives(tom, book, ovid).

However, this quickly become unweildy as the number of people in our program
grows.

=head3 Queries

Now that we have a rough understanding of facts and rules, we need to know how
to get answers from them.  Queries are typically entered in the "interactive
environment." This would be analogous to the query window in a database GUI.
With C<AI::Prolog>, a shell named C<aiprolog> is included for demonstration
purposes though we'll mainly focus on calling Prolog from within a Perl
program.

 ?- gives(tom, book, SOMEONE).

This looks just like a fact, but with some minor differences.  The C<?-> at the
beginning of the query is a query prompt seen in interactive environments.

You'll also note that C<SOMEONE> is capitalized, making it a variable (only the
first letter needs to capitalized.)  Thus, this query is asking who Tom will
gives books to.

 ?- gives(WHO, book, SOMEONE).

This query looks like the previous one, but since the first argument is
capitalized, we're asking "who will give books to whom?".

 ?- gives(WHO, WHAT, WHOM).

Finally, because all arguments are variables, we're asking for everyone who
will give anything to anybody.

Note that no code changes are necessary for any of this.  Because Prolog facts
and rules define relationships between things, Prolog can automatically infer
additional relationships if they are logically supported by the program.

Let's take a closer look at this, first focusing on the C<aiprolog> shell.
Assuming you've installed C<AI::Prolog> and said "yes" to installing the
C<aiprolog> shell, enter the following text in a file and save it as
I<gives.pro>.  Note that lines beginning with a percent sign (C<%>) are
single-line comments.

 % who are people?
 person(bob).
 person(sally).
 person(tom).
 person(alice).
 
 % who likes whom?
 likes(tom, bob).
 likes(tom, alice).
 likes(alice, bob).
 
 % who has what
 has(tom, book).
 has(alice, ring).
 has(bob, luck).
 
 % who will give what to whom?
 gives(WHO, WHAT, WHOM) :-
     has(WHO, WHAT),
     person(WHOM),
     likes(WHO, WHOM).
 
 gives(tom,book,harry).
 
When starting the shell, you can read in a file by supplying as a name on the
command line:

 $ aiprolog gives.pro

Alternately, you can I<consult> the file from the shell:
 
 $ aiprolog

 Welcome to AI::Prolog v 0.732
 Copyright (c) 2005, Curtis "Ovid" Poe.