Larry pointed out that this topic is better suited for perl6-language instead of perl6-users, so I'm forwarding this along. Feel free to exercise your "delete" key.
Cheers, Ovid -- If this message is a response to a question on a mailing list, please send follow up questions to the list. Web Programming with Perl -- http://users.easystreet.com/ovid/cgi_course/ ----- Forwarded Message ---- From: Ovid <[EMAIL PROTECTED]> To: perl6-users@perl.org Sent: Thursday, May 25, 2006 11:40:54 AM Subject: Re: Logic Programming for Perl6 (Was Re: 3 Good Reasons... (typo alert!)) ----- Original Message ---- From: David Romano <[EMAIL PROTECTED]> > > duplicate results and this is almost always wrong. (See > > http://use.perl.org/~Ovid/journal/28378 > > for an SQL example of this problem). > I re-read your journal entry and comments (I had read it back when you > first had posted it), and I'm curious about what obstacles that you > think need to be overcome for Perl6 to support logic programming. If anyone wants to know what the heck this is about, you can read http://search.cpan.org/dist/AI-Prolog/lib/AI/Prolog/Article.pod That's an article I originally had published in The Perl Review (http://www.theperlreview.com/) and it explains the basics of logic programming and how to do it in Perl5. It also explains the append/3 predicate, something I mention below. The first hurdle would be the syntax. The programmer just looking at the code would need to know when one section of code represents a snippet of logic programming. Is the following a function call or a Prolog fact? loves( 'foo', 'bar' ); How would one assert facts and rules in Perl6? How would one know that a variable is a logic variable and not a normal one? Assignment to a logic variable which is still subject to rebinding could break code. On the other hand, using normal variables for logic variables could let us use objects for them and I think this might get us contraint programming (long story). There's also the question of non-logical behavior in logic programming. What happens if you try to use math in Perl6 logic programming? Generally speaking, math is "non-logical" in the sense that it's used in Prolog (see the aforementioned article). Opening and reading from a file is also non-logical (you can't backtrack over it). How are those issues to be handled? My biggest concern, though, is a rule like the following: customer_cities(City) :- 700 <= credit_rating(Customer), customer(Customer, City). That's logically equivalent to the SQL I posted in a previous email and, like that SQL, will very likely return duplicates. Prolog students are constantly faced wtih the "how do I get rid of duplicates?" problem. This results in inefficient queries and a lot of time spent culling the duplicate answers when the person just wants to know the list of cities which meets their requirements. Perl6 grammars may be able to assist with this, but grammars (as far as I can tell) are still based around strings and not data structures. Admittedly, Prolog is essentially a fancy string rewriting system, but it's not clear how the grammars would be applied to this problem. For anyone familiar with logic programming and the append/3 predicate, here's one way of representing it in Perl5: use re 'eval'; my $string = "abc"; my $length = length $string; my $regex = qr/(\G[$string]{0,$length}(?{print "# [$&][$'][$string]\n"}))/ x 2; $string =~ $regex; Can you say "yuck"? However, that only represents a limited subset of append/3's functionality and trying to generalize it would be a very difficult task. One approach might be to write a wrapper around http://www.perlmonks.org/?node_id=318350. That's an approach I found to leverage Perl5's regex engine for logic programming, but I discovered that Perl5's regexen aren't re-entrant, killing that project. Perl6 rules might be perfect for this. In short, there are lots of questions which would need to be answered to get logic programming done correctly in Perl6. I'm quite happy that @Larry decided not to answer those questions since things would have been delayed. We can hack in logic programming later. Cheers, Ovid -- If this message is a response to a question on a mailing list, please send follow up questions to the list. Web Programming with Perl -- http://users.easystreet.com/ovid/cgi_course/
