Hi,
I used AI::Prolog once briefly, and that's the extent of my logic programming knowledge. There do seem to be a few Perl 6 features that may be useful for logic programming, although I'm not really qualified to judge.
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).
"let" variables and "hypothetical" assignments within rules may be a good starting point. Hypothetical variables are mentioned here: http://dev.perl.org/perl6/doc/design/syn/S04.html#Definition_of_Success While they usually occur within rules, hypothetical assignments can be to a target variable: http://dev.perl.org/perl6/doc/design/syn/S05.html#External_aliasing and "let" can now be used in any other closure. More details in A05: http://dev.perl.org/perl6/doc/design/apo/A05.html#Hypothetical_Variables,_er_Values
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?
Closures may have an "undo" trait which should be written to revert the side-effects of the closure when it receives an UNDO control exception (the block fails, see Definition of Success above). http://dev.perl.org/perl6/doc/design/syn/S04.html#Closure_traits For file reading the undo would seek back to the correct point. This does require the programmer to code the undo operation, which may not be possible (with non-seekable handles for examples). Are there other languages that blend logic and imperative styles that can be stolen from?
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.
There's some information about this in S05, with more info promised: http://dev.perl.org/perl6/doc/design/syn/S05.html#Matching_against_non-strings Brad -- Dohaku's wife suffered some severed fingers. Dohaku's wound was a severed neck bone, and since only his throat remained intact, his head hung down in front. Now boosting his head up with his own hands, Dohaku went off to the surgeon's. -- Hagakure http://bereft.net/hagakure/
