From: Hassan Aït-Kaci <hak@ilog.com>

Date: Wed, 08 Mar 2006 15:27:44 +0100

Message-ID: <440EE9E0.6060602@ilog.com>

To: "Peter F. Patel-Schneider" <pfps@inf.unibz.it>

CC: public-rif-wg@w3.org

Date: Wed, 08 Mar 2006 15:27:44 +0100

Message-ID: <440EE9E0.6060602@ilog.com>

To: "Peter F. Patel-Schneider" <pfps@inf.unibz.it>

CC: public-rif-wg@w3.org

Peter F. Patel-Schneider wrote: > In the absence of any demonstration that appending two lists can be performed > by recursive (Horn) rules, Here's the requested demonstration for pure Horn and no meta-rules: append([],L,L). append([H|T],L,[H|R]) :- append(T,L,R). > by recursive (Horn) rules, why should it be incumbent on me to show that it can > be done in pure production rules? I did my part. Now, it is your (or anyone's) turn in "Pure PRs" ... > I'm not claiming that pure production rules > can do everything, after all, just that pure production rules aren't any > different from recursive rules (as in Prolog). Nice wiggling... but won't do! ;-) I agree that any two Turing-complete computational systems "aren't any different". If this is your best argument to convince me, then I shall happily keep on living in my universe... :-) > peter > > > From: Hassan Aït-Kaci <hak@ilog.com> > Subject: Re: On production rules and phase I&II > Date: Wed, 08 Mar 2006 14:53:33 +0100 > > >>Peter F. Patel-Schneider wrote: >> >> >>>I don't understand which universe I've fallen into here. >>> >>>In my universe, production rule systems derived from OPS5. >>> >>>In my universe, OPS5 allowed rules like >>> (p (parent ^parent <x> ^child <y>) >>> (ancestor ^ancestor <y> ^descendant <z>) >>> --> >>> (make ancestor ^ancestor <x> ^descendant <z>)) >>> >>>In my universe, this was a recursive rule. >> >>Can you please explain what this rule does and where exactly is >>the recursion? Better: can you please write a PR program (in OPS5, >>CLIPS, JESS, Fair Isaac's Blaze, ILOG's JRules, or whatever) that >>appends two lists using only asserts, and without resorting to >>encoding a Horn Logic meta-interpreter as a PR? Thanks. >> >> >>>What is different in this universe? >> >>I think that the universe I live in is one where square pegs do >>not fit into round holes - at least, not naturally nor easily. At >>any rate, if you can do the very simple exercise that I am asking >>(append in OPS5 with only asserts), then maybe I'll join you in your >>shape-insensitive edenic universe... :-) > > > >>>peter >> >>-hak >> >> >>>From: Hassan Aït-Kaci <hak@ilog.com> >>>Subject: Re: On production rules and phase I&II >>>Date: Wed, 08 Mar 2006 08:57:16 +0100 >>> >>> >>> >>>>This is related to Frank's point and the replies he got from Peter and >>>>Michael. >>>> >>>>There is no recursion in PRs simply because PRs are not driven by names >>>>(as in Prolog, i.e., Horn Logic, or functional Rewrite Rules). For a >>>>computational system to be "recursive" there must be something that >>>>"recurses". In Prolog, e.g., the name of the head predicate is what >>>>"recurses" (in Rewrite Rules, the head's functional symbol is what >>>>recurses: >>>> >>>> Prolog: append([H|T],L,[H|R]) :- append(T,L,R). >>>> Rewrite Rule: append([H|T],L) -> [H|append(T,L)]. >>>> >>>>Clearly the symbol "append" is recursive (i.e., it reoccurs in its >>>>definition). >>>> >>>>Now PRs are NOT driven by names. Since a rule is not using a relational >>>>or functional name in its head to drive the rule's call, it is simply >>>>not possible to have recursive rules. >>>> >>>>Does it mean PR's cannot compute iteratively? Certainly not: there is >>>>an underlying loop that acytivates the rules based on the data present >>>>in the working memory (or Extensional DB, or Fact Base, etc...). Viz., >>>> >>>> WHILE [some rules match some objects in the WM] >>>> DO [choose a rule and all the objects it matches] >>>> [do the action of the rules on all these objects.] >>>> >>>>It is this "hidden loop" that ascribes PR's its Turing-equivalence. >>>>Furthermore, while Logical and Functional rules carry an environment >>>>along the computation, a "local" handle is passed from one rule to >>>>another as data is being constructed ot deconstructed (e.g., the >>>>"cons" cell [H|T] in the examples). PR rules communicate with each >>>>other only through a global store: the WM. This necessitates explicit >>>>modification of existing data (as opposed to inductive non-destructive >>>>building of structures passed in local environments). One realizes >>>>these subtle points when one tries to write a PR program to append >>>>two lists as specified by the recursive rules (as above). To say the >>>>least, it is not easy to do so with PRs and one would have to resort >>>>to contrived actions involving update and modify (not just assert). >>>> >>>>Therefore, what Frank meant is that what Harold presented as a Road >>>>Map defining a "Pure PR model" is not clear. If only asserts are >>>>allowed, one cannot write simple recursive schemed. That's all. >>>> >>>>Regarding the point of: >>>> >>>> C <- A & B. >>>>vs. >>>> isTrue(C) <- A & B. >>>>vs. >>>> mustBeTrue(C) <- A & B. >>>> >>>>I agree again with Frank. The last two are not logical - they are >>>>META-logical. Using them necessitates carrying an explicit structure >>>>(such as a list) to explicate the changed facts. (This is what monads >>>>do in Haskell to implement "purely" side-effects and procedural control.) >>>>Also, as Frank pointed out: What is true in Pure Horn must ALWAYS be >>>>true. So the question is, "When is C true?" in the example above? >>>>What Michael and Harold seem to propose is to identify a PR's model >>>>with the saturated construction derived from all possible asserts. >>>>These models are stable (as fix-points) only as the limit. Hence, >>>>this seems to be a valid approach only for finite models. Another >>>>snag of this approach is that such "Pure PRs" may very well compute >>>>inconsistent models (where "don't care" non-determinism is used). >>>>FOr example, a car-rental application will assign (via asserts) >>>>one car (any one) to any driver requesting one. However, a saturated >>>>model will generate rental contracts with the same car attributed >>>>to several drivers. >>>> >>>>In conclusion, while it is possible to simulate one system in the >>>>other (e.g., by mere Turing equivalence), it may be contended that >>>>the translations to and from each side (PRs and Pure Horn) are, IMHO, >>>>non-trivial and non-intuitive. >>>> >>>>So, Frank is basically right: what Harold et al.'s Road Map defines >>>>as "pure PRs" is computationally uninteresting and its rendition in >>>>Pure Horn is likely to be at odds with a rendition for Full PR. >>>> >>>>My 2 cents, >>>> >>>>-hak >>>>-- >>>>Hassan Aït-Kaci >>>>ILOG, Inc. - Product Division R&D >>>>tel/fax: +1 (604) 930-5603 - email: hak @ ilog . com >>> >>> >>> >> >>-- >>Hassan Aït-Kaci >>ILOG, Inc. - Product Division R&D >>tel/fax: +1 (604) 930-5603 - email: hak @ ilog . com > > -- Hassan Aït-Kaci ILOG, Inc. - Product Division R&D tel/fax: +1 (604) 930-5603 - email: hak @ ilog . comReceived on Wednesday, 8 March 2006 14:27:18 GMT

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