=head1 NAME

AI::Prolog::Builtins - Builtin predicates that AI::Prolog supports

=head1 REVISION

 $Id: Builtins.pod,v 1.9 2005/08/06 23:28:40 ovid Exp $

=head2 Comments

Comments begin with a C<%> and terminate at the end of the line or begin with
C</*> and terminate with C<*/>. 

=head2 Variables

As in Prolog, all variables begin with an upper-case letter and are not quoted.
In the following example, C<STUFF> is a variable.

 steals(badguy, STUFF, "Some rich person").

=head2 Constants

Constants begin with lower-case letters.  If you need a constant that begins
with an upper-case letter or contains spaces or other non-alphanumeric
characters, enclose the constant in single or double quotes  The quotes will
not be included in the constant.

In the following example, C<badguy> and C<Some rich person> are both constants:

 steals(badguy, STUFF, "Some rich person").

=head2 Miscellaneous

This will not work:

 p(X) :- X. /* does not work */

Use this instead:

 p(X) :- call(X).

=head1 BUILTINS

=over 4

=item !/0

The "cut" operator.  This is used when you wish to tell Prolog that you only
need to satisfy a goal once.  For example, if you wish to deny someone the
right to rent videos if they have overdue videos, you might use the cut
operator as soon as you see they have any overdue video.  The fact that they
have more than one overdue video doesn't matter.

See the C<cut.pl> program in the C<examples/> directory that comes with this
distribution.

=item assert/1

Add new facts to the database.  Only facts can be added, not rules.  This may
change in the future.  See C<retract(X)>.

 assert(loves(ovid,perl)).

=item call/1

Invokes C<X> as a goal.

=item consult/1

Supplied the name of a file containing Prolog code, this will consult the
Prolog code in the file and add its contents to the current knowledgebase.

Will warn if the file cannot be opened.

=item div/2

Succeeds if both terms are bound.  The value of the term is X / Y.
Use with C<is(X,Y)>.

 is(X, div(N,3)).

This is the internal form of the infix operator:
 
 N / 3.

=item eq/2

Succeeds if C<X> and C<Y> are equal.

This is the internal form of the infix operator:

 X == Y.

=item fail/0

This goal always fails.  Useful when you've reached a condition you
know should not succeed.

  kill(Hero, Beast) :-
    not(has_weapon(Hero)), fail.

=item ge/2

Succeeds if both terms are bound and X >= Y.

This is the internal form of the infix operator:

 X >= Y.

=item gt/2

Succeeds if both terms are bound and X > Y.

This is the internal form of the infix operator:

 X > Y.

=item halt/1

In the C<aiprolog> shell, exist shell.  Currently has no other effect.

=item if/3

If C<X> succeeds as a goal, try C<Y> as a goal.  Otherwise, try C<Z>.

 thief(badguy).
 steals(PERP, X) :-
   if(thief(PERP), eq(X,rubies), eq(X,nothing)).

=item is/2

If X is unbound and Y is bound to a number, the goal succeeds and X becomes
bound to the value of Y.  Otherwise, succeeds if both terms are bound, numbers,
and equal.

All other conditions result in failure.

This is the internal form of the infix operator:

 X is Y.

=item le/2

Succeeds if both terms are bound and X <= Y.

This is the internal form of the infix operator:

 X <= Y.

=item listing/0

Dumps a listing of all user-defined predicates and how they are defined.

=item listing/1

Dumps a listing of the requested predicate.  C<X> must a variable or string
instantiated in I<functor/arity>) form.  Note that, unlike most Prolog's, this
means that the followig will not work:

 listing(foo/2).

Use this instead:

 listing('foo/2').

=item lt/2

Succeeds if both terms are bound and X < Y.

This is the internal form of the infix operator:

 X < Y.

=item minus/2

Succeeds if both terms are bound.  The value of the term is X - Y.
Use with C<is(X,Y)>.

 is(X, minus(N,1)).

This is the internal form of the infix operator:

 N - 1.

=item mod/2

Succeeds if both terms are bound.  The value of the term is X % Y. (modulus)
Use with C<is(X,Y)>.

 is(X, mod(N,3)).

This is the internal form of the infix operator:

 N % 3.

=item mult/2

Succeeds if both terms are bound.  The value of the term is X * Y.
Use with C<is(X,Y)>.

 is(X, mult(N,3)).

This is the internal form of the infix operator:

 N * 3.

=item ne/2

Succeeds if C<X> and C<Y> cannot be proven to be equal.

This is the internal form of the infix operator:

 X \= Y.

=item nl/0

Prints a newline.

=item not/1

Succeeds if C<X> cannot be proven.  This is not negation as we're used to
seeing it in procedural languages.

=item notrace/0

Turns off tracing of Prolog's attempt to satisfy goals.

=item once/1

Stop solving for C<X> if C<X> succeeds.  Defined as:

 once(X) :- X, !;

=item or/2

Succeeds as a goal if either C<X> or C<Y> succeeds.

=item plus/2

Succeeds if both terms are bound.  The value of the term is X + Y.
Use with C<is(X,Y)>.

 is(X, plus(N,1)).

=item print/1

Prints the current Term.  If the term is an unbound variable, it will print the
an underscore followed by the internal variable number (e.g., "_284").

 print(ovid).         % prints "ovid"
 print("Something").  % prints "Something"
 print(Something).    % prints whatever variable Something is bound to 

=item println/1

Same as C<print(Term)>, but automatically prints a newline at the end.

=item pow/2

Succeeds if both terms are bound.  The value of the term is X ** Y 
(X raised to the Y power).
Use with C<is(X,Y)>.

=item retract/1

Remove facts from the database.  You cannot remove rules.  This may change in
the future.  See C<assert(X)>.

 retract(loves(ovid,java)).

=item trace/0

Turns on tracing of Prolog's attempt to satisfy goals.

=item true/0

True goal.  Automatically succeeds.

=item var/1

Succeeds if X is an unbound variable.  Otherwise, this goal fails.

=item write/1

Prints the current Term.  If the term is an unbound variable, it will print the
an underscore followed by the internal variable number (e.g., "_284").

 write(ovid).         % prints "ovid"
 write("Something").  % prints "Something"
 write(Something).    % prints whatever variable Something is bound to 

=item writeln/1

Same as C<write(Term)>, but automatically prints a newline at the end.

=back

=head1 LIMITATIONS

These are known limitations that I am not terribly inclined to fix.  See the
TODO list for those I am inclined to fix.

 IF -> THEN; ELSE not allowed.

Use C<if(IF, THEN, ELSE)> instead.

Chaining terms with a semicolon for "or" does not work.  Use C<or/2> instead.

=head1 TODO

There are many things on this list.  The core functionality is there, but I do
want you to be aware of what's coming.

=over 4

=item Improve printing.

There are some bugs with printing and escaping characters.  Maybe I'll look
into them :)

=item More builtins.

Currently, we only have a tiny subset of builtins available.  More are coming.

=back

=head1 MATH

Since version .70, math is fully available in C<AI::Prolog>.  Note that math is
implemented via the
L<AI::Prolog::Parser::PreProcessor::Math|AI::Prolog::Parser::PreProcessor::Math>
module.  This module rewrites Prolog math to an internal, predicate-based form
with the L<AI::Prolog::Parser|AI::Prolog::Parser> can parse.  This may cause
confusion when debugging.

 X is 5 + 7.
 % internally converted to 
 % is(X, plus(5, 7)).

The math predicates are officially deprecated and I<cannot> be used in the same
expression with regular Prolog math.  

Number may be integers, floats, doubles, etc.  A number that starts with a
minus sign (-) is considered negative.  No number may end in a decimal point as
the period is interpreted as the end of a clause.  The following is therefore a
syntax error:

 X is 5. + 7.

Unfortunately, the parser doesn't yet yell about that.  We'll try and figure
out why later.

Omit the period after the number or put a zero after it:

 X is 5.0 + 7.
 X is 5 + 7.

Because numbers use Perl scalars, you may mix types (ints and floats) and they
will behave as you expect in Perl.

Precedence is C<*> and C</>, left to right, followed by C<+> and C<->, left to
right followed by C<%>, left to right.  (I probably should change that.)
Naturally, parentheses may be used for grouping:

 X is 3 * 5 + 2.   % is(X, plus(mult(3, 5), 2)).
 X is 3 * (5 + 2). % is(X, mult(3, plus(5, 2))).

When using math, note that C<is> is similar to Perl's assignment operator, C<=>.
This can be confusing.

 X is 3 + 2.

Sets C<X> to the value of C<5>.  

If C<X> is already instantiated, this goal succeeds if the value of C<X> is the
value of the result of the right-hand side of the equation.  Internally, if X is not
instantiated, it looks like this:

 is(5, plus(3,2)).

The C<=> operator tries to unify the left-hand side with the right-hand side:

 X = 3 + 2.

If C<X> is already instantiated, this goal succeeds if the value of C<X> is the
same goal as the right-hand side of the equation.  Internally, if X is not
instantiated, it looks like this:

 eq(plus(3,2), plus(3,2)).

When you first start using Prolog, you probably was C<is> instead of C<=>.

Logical comparisons are straightforward:

 3 >= X.
 Y > (4 + 3) * X.
 X == Y. % a test for equality
 X \= Y. % Not equal.  See caveats for ne/2
 % etc.

=head1 BUGS

Currently we cannot directly access bound variables in the tail of a list used
in a query when using unformatted results:

 $prolog->query('foo([Head|Tail], bar).');
 while (my $result = $prolog->results) {
    print Dumper $result->Head;
    print Dumper $result->Tail; # doesn't work
 }

You'll have to switch to C<raw_results> and parse the data manually:

 $prolog->raw_results(1);
 $prolog->query('foo([Head|Tail], bar).');
 while (my $result = $prolog->results) {
     print Dumper $result;
 }    

=head1 SEE ALSO

L<AI::Prolog::Introduction>

L<AI::Prolog>

W-Prolog:  L<http://goanna.cs.rmit.edu.au/~winikoff/wp/>

X-Prolog:  L<http://www.iro.umontreal.ca/~vaucher/XProlog/>

Roman BartE<225>k's online guide to programming Prolog:
L<http://kti.ms.mff.cuni.cz/~bartak/prolog/index.html>

=head1 AUTHOR

Curtis "Ovid" Poe, E<lt>moc tod oohay ta eop_divo_sitrucE<gt>

Reverse the name to email me.

This work is based on W-Prolog, http://goanna.cs.rmit.edu.au/~winikoff/wp/,
by Dr. Michael Winikoff.  Many thanks to Dr. Winikoff for granting me
permission to port this.

=head1 COPYRIGHT AND LICENSE

Copyright 2005 by Curtis "Ovid" Poe

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself. 

=cut