Ovid has asked for the wisdom of the Perl Monks concerning the following question:

A few days ago I started to again work on implementing predicate logic in Perl. I know it can be done. I just need that epiphany that makes me slap my forehead. I stopped working on the latest implementation because it was awful, but I kept thinking about the problem. Basically, to get anything similar to Prolog in Perl, I need backtracking. The only thing in Perl which implements backtracking is a regular expression. I'm pretty sure the key lies there.

This morning, I thought I had the epiphany. What if, given a set of facts, I would like to know who owns gold?

my $facts = <<END_FACTS; owns merlyn gold owns Ovid books gives Ovid books kudra owns kudra gold END_FACTS + + my $query = qr/owns (\S+) gold/; my @who = $facts =~ /$query/mg;

That assigns "merlyn" and "kudra" to @who, but it's not really useful. What if I want the query to be something like "who owns something valuable?" I can do this:

use Data::Dumper; use constant SUCCEED => qr{(?=)}; use constant FAIL => qr{(?!)}; + + my $owns = <<END_FACTS; owns merlyn gold owns Ovid books owns kudra gold END_FACTS + + my $valuable = <<END_VALUABLE; valuable Perl valuable gold valuable Corinna END_VALUABLE + + my $is_valuable = qr[(\S+)(??{match($valuable, "valuable $2")})]; my $owns_query = qr/owns (\S+) $is_valuable/; # here's the magic! my @who = $owns =~ /$owns_query/mg; print Dumper \@who; + + sub match { my ($facts, $query) = @_; my $sleep_count = 0; my $pid; do { $pid = open KID_TO_READ, "-|"; unless (defined $pid) { die "Couldn't fork" if $sleep_count++ > 6; sleep 1; } } until defined $pid; + + unless ($pid) { # child if ($facts =~ $query) { print SUCCEED; } else { print FAIL; } } else { # parent my $result = <KID_TO_READ>; close KID_TO_READ; return $result; } }

The prints out:

$VAR1 = [ 'merlyn', 'gold', 'kudra', 'gold' ];

That's great. Once I can autogenerate those regexes and clean up the output I can wrap this up in a pretty framework and do all of the predicate logic programming I want.

Uh, no I can't. Regular expressions are not reentrant, thus forcing me to fork off a new process for every regex "nested" inside of the main query (such as the "is_valuable" condition). This winds up being so ridiculously slow that it's probably not worth much more than a toy. I also suspect that a complicated query will suck memory like a Hoover. Any suggestions to get around this, or am I stuck?


New address of my CGI Course.