From: Michael Kifer <kifer@cs.sunysb.edu>

Date: Tue, 10 Nov 2009 14:48:27 -0500

To: Chris Welty <cawelty@gmail.com>

Cc: Jos de Bruijn <debruijn@inf.unibz.it>, RIF WG Public list <public-rif-wg@w3.org>

Message-ID: <20091110144827.4b8335df@kiferdesk>

Date: Tue, 10 Nov 2009 14:48:27 -0500

To: Chris Welty <cawelty@gmail.com>

Cc: Jos de Bruijn <debruijn@inf.unibz.it>, RIF WG Public list <public-rif-wg@w3.org>

Message-ID: <20091110144827.4b8335df@kiferdesk>

Yes, I agree that 3a is a reasonable fix. 3b is too big of a change, and I was not suggesting it for this round. michael On Tue, 10 Nov 2009 10:53:32 -0500 Chris Welty <cawelty@gmail.com> wrote: > > I read this a little more carefully. > > Basically, the issue is whether to add some correspondence between rif:subclass > and rdf:subclass and between rif:type and rdf:type *in the OWL compatibility > section* of SWC. Such a correspondence is already there for RDF compatibility, > but Michael noted that it is not "inherited" by the "OWL-DL" (now know as OWL > Direct Semantics) section. So, currently in SWC, the OWL-DL compatibility has > no correspondence between the rather obvious type/subclass relations in the two > languages. > > I agree this is a problem and should be fixed, and option #1 in Michael's > analysis, copied below (to leave it as is) is unacceptable. > > Option #2 is to just add a sentence to the text saying there is no > correspondence between owl and rif type/subclass. This is less than satisfactory. > > Option #3 is to "fix" it somehow, and there are two variations there, I'll call > them 3a (just repeat the correspondences from RDFS in OWL-DL) and 3b (do the > best possible job mapping between owl and rif subclass). > > </chair>I prefer option 3a. I agree with Jos' analysis of option 3b and think > it is too big a change.<chair> > > As chair, I am also willing to accept 2 or 3a as an oversight and bug fix (I > personally thought the correspondence between type and subclass were "inherited" > from the RDF correspondence, so 3a would just make it the way I thought it was), > however 3b seems to me, procedurally, to be much more significant and requires a > new last call for SWC. > > -Chris > > Jos de Bruijn wrote: > >> In today's telecon I was asked to reanimate the issue of OWL compatibility, > >> which was discussed 1 month ago. > >> > >> Here is the relevant message: > >> > >> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0017.html > >> > >> The current situation is a bug, IMO. If it isn't a bug then at least that part > >> of the document is very unsatisfactory and obscure. Jos proposed 3 solutions: > >> > >> 1- leave things as they are, assuming that # and ## are not of interest > >> to users of RIF-OWL DL combinations > >> 2- explain the use of # and ## in the document (this would certainly not > >> be a substantive change, so we should not run into procedural problems) > >> 3- define the semantics of # and ## in RIF-OWL DL combinations in a > >> similar fashion as in RIF-RDF combinations: a one-to-one correspondence > >> between # and OWL class membership statements and implication between ## > >> and OWL subclassing. > >> > >> The easiest for him would be to do nothing (1), thus leaving things > >> unsatisfactory and obscure. His next choice is (3), which is also my choice and > >> the "right thing to do." (3) stretches things a little, but it can be argued > >> that it is a simple fix. > > > > In my earlier e-mail to Michael referred to I did not say what my > > preference is among the mentioned options. I guess arguments can be > > made for all three options, so in fact I do not have a strong > > preference, but I do have a concern about option (3): implementation > > might be harder. If, for example, implementation is done through > > embedding in other rules system, like the embedding of RIF-OWL2RL > > combination in the appendix of the document, quite a few rules need to > > be added for the ## construct. > > In particular, for every pair of distinct class names (A,B), we need to > > add the rule: > > > > Forall ?x (?x[rdf:type -> B] :- And(?x[rdf:type -> A] A##B)) > > > > This means adding a quadratic number of rules. > > > > Dealing with # is easy: in the mapping of RIF DL-document formulas to > > RIF documents [1] we simply map a#b to tr'(b)(a). Clearly, we would > > restrict b in formulas a#b to constant symbols. > > > > > > [1] > > http://www.w3.org/TR/rif-rdf-owl/#Embedding_RIF_DL-document_formulas_into_RIF_BLD > > > > > >> Solution (2) is more work. It fixes the obscurity aspect, not the > >> unsatisfactory aspect of the definitions. So, (3) seems like the best way to > >> proceed. > >> > >> Solution (3) still leaves some problems, which are unrelated to the above > >> issues. In the current semantics, subclassing in RIF implies subclassing in > >> OWL/RDF, but not vice versa. > >> > >> In this regard, I would like to point to my follow-up message > >> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0019.html > >> Here I proposed a stronger semantics, which fixes this non-entailment problem. > > > > Michael proposed the following semantics: > > > > {(A,B) | A rdfs:subclassOf B and A != B on the RDF side} > > = {(A,B) | A##B on the RIF side} > > > > I feel that this would take us out of Horn, even when considering Simple > > entailment, because implementation would require (classical) negation. > > At least, that is the only way I current see how this could be > > implemented. As we know, classical negation in the body amounts to > > disjunction in the head, so we would end up adding the following rule to > > the embedding of RDF-RDF combinations: > > > > Forall ?x, ?y (Or(?x##?y ?x=?y) :- ?x[rdfs:subClassOf -> ?y]) > > > > For RIF-OWL DL combinations such a semantics is even more problematic, > > because subclass in OWL DL means subset relation between class > > extensions, so the condition would look something like (X^C is the class > > extension of X): > > > > {(A,B) | A^C subset B^C and A^C != B^C on the RDF side} > > = {(A,B) | A##B on the RIF side} > > > > (Actually, we will need to apply some tricks here, since A and B are not > > constants on the OWL side, but I guess we can come up with a definition > > that kind-of achieves this semantics) > > > > A formula implementing the => direction of the condition for a pair of > > class names A,B would look something like (again, negation in the body > > becomes disjunction in the head): > > > > Forall ?x ( > > Or(A##B > > And(Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A]) > > Forall ?x(?x[rdf:type -> A] :- ?x[rdf:type -> B]))) > > :- > > Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A])) > > > > > > So, I would not be in favor of extending either the semantics of RDF or > > the semantics of OWL DL combinations with such a condition. > > > > > > Best, Jos > >> This would certainly be a substantive change semantically (although not > >> significant textually). If we don't have the energy to do it this time, > >> maybe for RIF 1.1. > >> > >> michael > >> > > >Received on Tuesday, 10 November 2009 19:49:02 GMT

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