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sub do {
    my ( $self, $query ) = @_;
    $self->query($query);
    1 while $self->results;
    $self;
}

sub query {
    my ( $self, $query ) = @_;

    # make that final period optional
    $query .= '.' unless $query =~ /\.$/;
    $self->{_query} = Term->new($query);
    unless ( defined $self->{_engine} ) {

        # prime the pump
        $self->{_engine} = Engine->new( @{$self}{qw/_query _prog/} );
    }
    $self->{_engine}->query( $self->{_query} );
    return $self;
}

sub results {
    my $self = shift;
    unless ( defined $self->{_query} ) {
        croak "You can't fetch results because you have not set a query";
    }
    $self->{_engine}->results;
}

sub trace {
    my $self = shift;
    if (@_) {
        $self->{_engine}->trace(shift);
        return $self;
    }
    return $self->{_engine}->trace;
}

sub raw_results {
    my $class = shift;
    if (@_) {
        Engine->raw_results(shift);
        return $class;
    }
    return Engine->raw_results;
}

my $QUOTER;

sub quote {
    my ( $proto, $string ) = @_;
    $QUOTER = Text::Quote->new unless $QUOTER;
    return $QUOTER->quote_simple($string);
}

sub list {
    my $proto = shift;
    return
        join ", " => map { /^$RE{num}{real}$/ ? $_ : $proto->quote($_) } @_;
}

sub continue {
    my $self = shift;
    return 1 unless $self->{_engine};    # we haven't started yet!
    !$self->{_engine}->halt;
}

1;

__END__

=head1 NAME

AI::Prolog - Perl extension for logic programming.

=head1 SYNOPSIS

 use AI::Prolog;
 use Data::Dumper;

 my $database = <<'END_PROLOG';
   append([], X, X).
   append([W|X],Y,[W|Z]) :- append(X,Y,Z).
 END_PROLOG

 my $prolog = AI::Prolog->new($database);
 
 my $list   = $prolog->list(qw/a b c d/);
 $prolog->query("append(X,Y,[$list]).");
 while (my $result = $prolog->results) {
     print Dumper $result;
 }

=head1 ABSTRACT

 AI::Prolog is merely a convenient wrapper for a pure Perl Prolog compiler.
 Regrettably, at the current time, this requires you to know Prolog.  That will
 change in the future.

=head1 EXECUTIVE SUMMARY

In Perl, we traditionally tell the language how to find a solution.  In logic
programming, we describe what a solution would look like and let the language
find it for us.

=head1 QUICKSTART

For those who like to just dive right in, this distribution contains a Prolog
shell called C<aiprolog> and two short adventure games, C<spider.pro> and
C<sleepy.pro>.  If you have installed the C<aiprolog> shell, you can run
either game with the command:

 aiprolog data/spider.pro
 aiprolog data/sleepy.pro

When the C<aiprolog> shell starts, you can type C<start.> to see how to play
the game.  Typing C<halt.> and hitting return twice will allow you to exit.

See the C<bin/> and C<data/> directories in the distribution.