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

 %who_loves = reverse %loves;
 $who       = $who_loves{perl};

(C<AI::Prolog>, being written in Perl, is slow.  So the C<O(n)> versus
C<O(1)> argument doesn't hold.  Versions written in C are far more practical in
this regard.)

But this fails, too.  The first problem is that we have duplicated data.  If we
have to add additional data to the C<%loves> hash, we'll have to remember to
synchronize the hashes.  The second problem is that Perl is just a little too
popular:

 loves(ovid,    perl).
 loves(alice,   perl).
 loves(bob,     perl).
 loves(charlie, perl).

If we simply reverse the hash for those entries, we lose three of those names.
So we have to play with this some more.

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

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