Re: [TED] Action-188, ISSUE: production rule systems have "difficulty" with recursive rules in RIF Core

Christian de Sainte Marie <csma@ilog.fr> wrote:
> 
> I wonder if we are not missing the point, here... See comment below.
> 
> Gary Hallmark wrote:
> > 
> > A naive translation from RIF Core to a "generic" production rule 
> > language might produce the following:
> > 
> > assert(factorial(0, 1))
> > IF factorial(?x, ?y)
> > THEN assert(factorial(?x + 1, (?x + 1) * ?y))
> > 
> > The problem with the naive translation is it will generate *all* 
> > factorial facts:
> > factorial(1 1)
> > [...]
> > ...etc....
> > until memory is exhausted.  In other words, the naive translation using 
> > forward chaining is not "goal directed".  In contrast, a backward 
> > chaining implementation would start with a query such as:
> > 
> > :- factorial(4 ?out)
> > 
> > and may terminate after generating subgoals factorial(3 ?), factorial(2 
> > ?), and factorial(1 ?).
> 
> So, the rules are correctly mapped into the production rule language 
> (their semantics is preserved). Only, they may have undesirable effect 
> if applied carelessly. But is that relevant to RIF?
> 
> You may also try to compute all the factorials until the memory is full 
> by asking :- factorial(?in ?out) to your LP system, no? Similarly, you 
> might compute the factorial of 4 with your PR engine by executing step 
> by step, checking after each cycle if you have your answer and stopping 
> when you have it.
> 
> The point, as many stressed, is that PR and LP are designed for 
> different usage: PR engines are mostly not used to answer queries (which 
> implies somehow backward chaining), and they do not usually have a query 
> answering control strategy.
> 
> But, again, is that really relelvant to RIF?
> 
> More generally, to what degree should RIF be concerned with what systems 
>   do with the rules they retrieve from a RIF document? The answer may 
> range from: "not at all, provided that the semantics is preserved" - and 
> recursion is not a problem in Core - to: "all the way to, and including 
> making sure that retrieved rules can be run efficiently" - which might 
> make sense for specific dialects; but probably not for Core.

I agree. In fact, it is hard to find a reasonably complete yet expressive
rule system. For instance, even XSB is incomplete if one uses function symbols.


	--michael

Received on Tuesday, 19 December 2006 16:16:52 UTC