- 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
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