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 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.
 AI::Prolog comes with ABSOLUTELY NO WARRANTY.  This library is free software;
 you can redistribute it and/or modify it under the same terms as Perl itself.

 Type '?' for help.

 ?- consult('gives.pro').

The second notation allows you to consult multiple files and add all of them to
the knowledge base.

After issuing the C<consult/1> command, the shell will appear to hang.  Hit
I<Enter> to continue.  We'll explain this behavior in a moment.

Now that you've loaded the program into the shell, issue the following query:

 ?- gives(X,Y,Z).

The shell should respond:

 gives(tom, book, bob) 

It will appear to hang.  It wants to know if you wish for more results or if
you are going to continue.  Typing a semicolon (;) tells Prolog that you want
it to I<resatisfy> the goal.  In other words, you're asking Prolog if there are
more solutions.  If you keep hitting the semicolon, you should see something
similar to the following:

 ?- gives(X,Y,Z).

 gives(tom, book, bob) ;

 gives(tom, book, alice) ;

 gives(alice, ring, bob) ;

 gives(tom, book, harry) ;

  No
 ?- 

That final "No" is Prolog telling you that there are no more results which
satisfy your goal (query).  (If you hit I<Enter> before Prolog prints "No", it
will print "Yes", letting you know that it found results for you.  This is
standard behavior in Prolog.)

One thing you might notice is that the last result, C<gives(tom, book, harry)>,
does not match the rule we set up for C<gives/3>.  However, we get this result
because we chose to hard-code this fact as the last line of the Prolog program.

=head2 How this works

At this point, it's worth having a bit of a digression to explain how this
works.

Many deductive systems in artificial intelligence are based on two algorithms:
backtracking and unification.  You're probably already familiar with backtracking
from regular expressions.  In fact, regular expressions are very similar to
Prolog in that you specify a pattern for your data and let the regex engine
worry about how to do the matching.

In a regular expression, if a partial match is made, the regex engine remembers
where the end of that match occurred and tries to match more of the string.  If
it fails, it backtracks to the last place a successful match was made and sees
if there are alternative matches it can try.  If that fails, it keeps
backtracking to the last successful match and repeats that process until it
either finds a match or fails completely.

Unification, described in a fit of wild hand-waving, attempts to take two
logical terms and "unify" them.  Imagine you have the following two lists:

 ( 1, 2, undef, undef,     5 )
 ( 1, 2,     3,     4, undef )

Imagine that undef means "unknown". We can unify those two lists because every
element that is known corresponds in the two lists. This leaves us with a list
of the integers one through five.

 ( 1, 2, 3, 4, 5 )

However, what happens if the last element of the first list is unknown?

 ( 1, 2, undef, undef, undef )
 ( 1, 2,     3,     4, undef )

We can still unify the two lists. In this case, we get the same five element
list, but the last item is unknown.

 ( 1, 2, 3, 4, undef )

If corresponding terms of the two lists are both bound (has a value) but not
equal, the lists will not unify:
 
 ( 1, 23, undef, undef, undef )
 ( 1,  2,     3,     4, undef )

Logic programming works by pushing these lists onto a stack and walking through
the stack and seeing if you can unify everything (sort of). But how to unify
from one item to the next? We assign names to the unknown values and see if
we can unify them. When we get to the next item in the stack, we check to see
if any named variables have been unified. If so, the engine will try to unify
them along with the other known variables.

That's a bad explanation, so here's how it works in Prolog. Imagine the
following knowledge base:

 parent(sally, tom)
 parent(bill, tom)
 parent(tom, sue)
 parent(alice, sue)
 parent(sarah, tim)
 
 male(bill)
 male(tom)
 male(tim)

Now let's assume we have a rule that states that someone is a father if they
are a parent and they are male.

 father(Person) :-
   parent(Person, _),
   male(Person).

In the above rule, the underscore is called an "anonymous vairable" and means
"I don't care what this value is."  Prolog may still bind the variable
internally (though this behavior is not guaranteed), but its value will not be
taken into account when trying to determine if terms unify.

Taking the first term in the rule, the logic engine might try to unify this
with the first fact in the knowledge base, C<parent(sally, tom)>. C<Person>
unifies with I<sally>.  The underscore, C<_>, unifies with I<tom> but since
we stated this unification is unimportant, we can ignore that.

We now have a fact which unifies with the first term in the rule, so we push
this information onto a stack.  Since there are still additional facts we can
try, we set a "choice point" in the stack telling us which fact we last tried.
If we have to backtrack to see a choice point, we move on to the next fact and
try again.

Moving on to the next term in the rule, C<male(Person)>, we know that "sally"
is unified to C<Person>, so we now try to unify C<male(sally)> with all of the
corresponding rules in the knowledge base. Since we can't, the logic engine
backs up to the last item where we could make a new choice and sees
C<parent(bill, tom)>. C<Person> gets unified with I<bill>. Then in moving to
the next rule we see that we unify with C<male(bill)>. Now, we check the first
item in the rule and see that it's C<father(Person)>. and the logic engine
reports that I<bill> is a father.

Note that we can then force the engine to backtrack and by continuously
following this procedure, we can determine who all the fathers are.

And that's how logic programming works. Simple, eh? (Well, individual items can
be lists or other rules, but you get the idea).

=head2 Executing Prolog in Perl

Getting back to Perl, how would we implement that in a Perl program?

The basic process for using C<AI::Prolog> looks something like this:

 use AI::Prolog;
 my $prolog = AI::Prolog->new($prolog_code);

Create a new C<AI::Prolog> object, passing Prolog code as the argument.  If you
prefer, you can wrap the constructor in a C<BEGIN> block:

 my $prolog;
 BEGIN {
   $prolog = AI::Prolog->new(<<'  END_PROLOG');
     % some Prolog code goes here
   END_PROLOG
 }

This is not strictly necessary, but if your Prolog code has a syntax error, it
will be a compile-time error, not a run-time error, and you'll get an error
message similar to:

 Unexpected character: (Expecting: ')'.  Got (.)) at line number 12.
 BEGIN failed--compilation aborted at test.pl line 7.

Note that the line number for "Unexpected character" is relative to the Prolog
code, not the Perl code.

After the contructor, issue your query:

 $prolog->query($some_query);

And do something with the results:

 while ( my $results = $prolog->results ) {
     print "@$results\n";
 }

Results are usually each returned as an array reference with the first argument
being the functor and subsequent arguments being the values.  If any value is a
list, it will be represented as an array reference.  We'll see more on that

lib/AI/Prolog/Article.pod  view on Meta::CPAN

 while ( my ($person, $thing) = each %loves ) {
     push @{ $who_loves{$thing} }, $person;
 }

Oh, wait.  This fails too.  You see, I'm fickle.

 loves(ovid, perl).
 loves(ovid, prolog).
 loves(ovid, 'Perl 6').

(The quotes around "Perl 6" tell Prolog that this is a single value and that
it's a constant, not a variable.)

How do I find out everything Ovid loves?  In Prolog, the query doesn't change:

 loves(ovid, WHAT).

In Perl, our original hash wasn't enough.

 my %loves = (
     ovid    => [ qw/perl prolog Perl6/ ],
     alice   => [ 'perl' ],
     bob     => [ 'perl' ],
     charlie => [ 'perl' ],
 );

 my %who_loves;
 while ( my ($person, $things) = each %loves ) {
     foreach my $thing ( @$things ) {
         push @{ $who_loves{$thing} }, $person;
     }
 }

Now that's starting to get really ugly.  To represent and search that data in
Prolog is trivial.  Now do you really want to have fun?

 gives(tom, book, sally).

How would you represent that in Perl?  There could be multiple gift-givers,
each gift-giver could give multiple things.  There can be multiple recipients.
Representing this cleanly in Perl would not be easy.  In fact, when handling
relational data, Perl has several weaknesses in relation to Prolog.

=over 4

=item * Data must often be duplicated to handle bi-directional relations.

=item * You must remember to synchronize data structures if the data changes.

=item * Often you need to change your code if your relations change.

=item * The code can get complex, leading to more bugs.

=back

=head2 Prolog versus SQL

At this point, there's a good chance that you're thinking that you would just
stuff this into a relational database. Of course, this assumes you need
relational data and want to go to the trouble of setting up a database and
querying it from Perl.  This is a good solution if you only need simple
relations.  In fact, Prolog is often used with relational databases as the two
are closely related.  SQL is a I<special purpose> declarative language whereas
Prolog is a I<general purpose> declarative language.

Firing up SQLite, let's create two tables and insert data into them.

 sqlite> CREATE TABLE parent_2 (parent VARCHAR(32), child VARCHAR(32));
 sqlite> CREATE TABLE male_1   (person VARCHAR(32));

 sqlite> INSERT INTO parent_2 VALUES ('sally', 'tom');
 sqlite> INSERT INTO parent_2 VALUES ('bill', 'tom');
 sqlite> INSERT INTO parent_2 VALUES ('tom', 'sue');
 sqlite> INSERT INTO parent_2 VALUES ('alice', 'sue');
 sqlite> INSERT INTO parent_2 VALUES ('sarah', 'tim');

 sqlite> INSERT INTO male_1   VALUES ('bill');
 sqlite> INSERT INTO male_1   VALUES ('tom');
 sqlite> INSERT INTO male_1   VALUES ('tim');

We can then find out who the fathers are with the following query.

 SELECT parent 
 FROM   parent_2, male_1 
 WHERE  parent_2.parent = male_1.person;

This is very similar to Prolog but we are forced to explicitly state the
relationship.  Many Prolog queries are conceptually similar to SQL queries but
the relationships are listed directly in the program rather than in the query.
When working with a relational database, we can get around this limitation with
a view:

 CREATE VIEW father AS
     SELECT parent 
     FROM   parent_2, male_1 
     WHERE  parent_2.parent = male_1.person;

And finding out who the fathers are is trivial:

 SELECT * FROM father;

Further, databases, unlike many Prolog implementations, support indexing,
hashing, and reordering of goals to reduce backtracking.  Given all of that,
why on earth would someone choose Prolog over SQL?

As stated earlier, Prolog is a general purpose programming language.  Imagine
if you had to write all of your programs in SQL.  Could you do it?  You could
with Prolog.  Further, there's one huge reason why one might choose Prolog:
recursion.  Some SQL implementations have non-standard support for recursion,
but usually recursive procedures are simulated by multiple calls from the
programming language using SQL.

Let's take a look at recursion and see why this is a win.

=head2 Recursion and Lists

In Prolog, lists are not data structures.  They're actually implemented in
terms of something referred to as the "dot functor" (./2).  For our purposes,
we'll ignore this and pretend they're really data types.  In fact, there's
going to be a lot of handwaving here so forgive me if you already know Prolog.