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if ($file) {
    open FH, "< $file" or die "Could not open ($file) for reading: $!";
    $program = do { local $/; <FH> };
}
my $prolog  = Prolog->new($program);
my $version = Prolog->VERSION;

print $OUT <<"END_WELCOME";

Welcome to AI::Prolog v $version
Copyright (c) 2005-2006, 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 'help.' for for a list of built-ins or 'help("\$builtin").' for help on a
specific built-in.

END_WELCOME

my $COMMAND = qr/^%\s*/;
my $RESULTS = 0;
my $MORE    = 1;

while ($prolog->continue) {
    my $query = $term->readline("?- ");
    chomp $query;
    next unless $query;
    $term->addhistory($query);
    print $OUT "\n";
    
    if ( $query =~ /^\s*\?/ ) {
        help();
        next;
    }
    elsif ( $query =~ $COMMAND ) {
        last if $query =~ /$COMMAND?(?:halt|quit|exit|stop)/i;
        if ($query =~ /$COMMAND(?:help)/i) {
            help();
        }
        elsif ($query =~ /${COMMAND}more/i) {
            $MORE = 1;
        }
        elsif ($query =~ /${COMMAND}no\s*more/i) {
            $MORE = 0;
        }
        next;
    }
    
    eval {$prolog->query($query)};
    if ($@) {
        warn $@;
        next;
    }
    $RESULTS = 1;
    show_results($prolog);
    while ($MORE && user_wants_more()) {
        show_results($prolog);
    }
}

sub show_results {
    return unless $RESULTS;
    my ($prolog) = @_;
    my $results = $prolog->results;
    $results ||= ''; # otherwise it's an arrayref
    print $OUT $results, " ";
    unless ($results) {
        print $OUT "No\n";
        $RESULTS = 0;
    }
}

sub user_wants_more {
    return unless $RESULTS;
    ReadMode 'cbreak';
    my $key = ReadKey(0);
    ReadMode 'normal';
    if (';' eq $key) {
        print $OUT ";\n\n";
        return 1;
    }
    print $OUT "\n\nYes\n" if $RESULTS;
    return;
}

my $offset;
sub help {
    $offset ||= tell DATA;
    seek DATA, $offset, 0;
    pod2usage({
        -verbose => 2, 
        -input   => \*DATA,
        -exitval => 'NOEXIT',
    });
}

__DATA__

=head1 NAME 

aiprolog --  A simple Prolog shell using AI::Prolog.

=head1 SYNOPSIS

 usage: aiprolog <optional prolog program name>

=head1 DESCRIPTION

C<aiprolog> is a simple prolog shell using L<AI::Prolog> as the backend.

See the documentation for more detail on the Prolog features that L<AI::Prolog>
currently accepts.

=head2 Commands

Commands specific to aiprolog shell:

 "% more"     -- enables prompting for more results (default)
 "% no more"  -- disables prompting for more results
 "% nomore"   -- same as "no more"
 "% halt"     -- stops the shell
 "% help"     -- display this message

Note that the percent sign must preceed the command.  The percent sign
indicates a Prolog comment.  Without that, aiprolog will think you're trying to
execute a prolog command.

aiprolog-specific commands are case-insensitive.

=head2 Typical session

Save the following to a file named "append.pro":

 append([],X,X).
 append([W|X], Y, [W|Z]) :- append(X,Y,Z).

Then load it into the C<aiprolog> shell by typing this at a shell:

 aiprolog path/to/append.pro

Alternatively, once in the shell, you can load the program with:

 consult('path/to/append.prog').

In the shell, you should be greeted by a query prompt "?-".  At this prompt,
you can issue queries against the program.  Try entering the following query: