- From: Axel Polleres <axel.polleres@deri.org>
- Date: Thu, 06 Sep 2007 15:52:34 +0200
- To: Michael Kifer <kifer@cs.sunysb.edu>
- CC: axel@polleres.net, Jos de Bruijn <debruijn@inf.unibz.it>, RIF <public-rif-wg@w3.org>
Michael Kifer wrote: >>I have to fully support Jos here, if we do not define a practically >>usable way to use RIF with RDF ( and probabley XML, OWL, SPARQL as the >>next steps). First, it is in the charter, second, otherwise, I don't see >>where we're going at all, to be honest and if we don't define the >>handles to RIF, I don't see who should pick it up in the end? > > > We DO define a practically usable way to use RIF with RDF. Did you follow > the discussion yes.... > or just took what Jos said in the last email for granted? well, actually what puzzled me was rather your mail before jos' and maybe thus I mis understood you the same way: >>>>>> But I see no reason why push that particular language >>>>>> onto others and exclude other possible combos of RDF and RIF. So, I'd kindly ask you to help me to understand better what you meant. I personally think it is good to provide some normative versions for semantic compatibility and embeddings for normative RDF (simple/RDF/RDFS) entailments. Whoever does something different should first define which subset of "RDF" entailment they mean. Can we agree on that? > What I am saying is that the first part of that RDF compatibility document > ADDS NOTHING to that "practical and usable way". All we need is the second > part of that document. I think that is is worthwhile to clarify the relation between RIF's and RDF's model theories, or do you want to leave that part out? I mean, it is maybe ok to put it in an appendix/separate document. Was this what you meant or to leave it out? If the latter, what is the reason? > ALL: please do not get swayed by rhetorical arguments. If you want to form > an opinion, go and read the RDF compatibility document. See if you can > understand the purpose of both of its parts and then think if you feel that > the first part of that document is actually needed for our purposes. As a foundation, I think part I is definitly needed, but probably not on the "user" level. best, Axel >>So what's the alternative? If we leave this open, why should people >>care? Who else should define the ways to interface with RIF if not we >>(at least for the core standards mentioned above)? >> >>best, >>Axel >> >>Jos de Bruijn wrote: >> >>>Michael, >>> >>>What you're saying is that we should not address RDF compatibility at all. >>>This means that it would not be possible to write rules about external >>>RDF data sets or with RDFS data models. Furthermore, if we should not >>>define compatibility with RDF because it is "not in our charter" (which >>>it actually is, see below), we should probably also not defined >>>compatibility with any other data language. This would mean that it >>>would not be possible to write rules about any kind of data sets work >>>with any kind of data model except for the one that you proposed. >>> >>>To come back to the charter [1]: it explicitly says (in section 1.2) >>>that RIF must address compatibility with XML (data), RDF, OWL, and >>>SPARQL. Your proposed (new) data model is not in the charter, but the >>>use of the XML, RDF, and OWL data models is in the charter. >>> >>>Best, Jos >>> >>>[1] http://www.w3.org/2005/rules/wg/charter#compatibility >>> >>>Michael Kifer wrote: >>> >>> >>>>>Michael, >>>>> >>>>> >>>>> >>>>>>these are all good arguments, but my point is that it is not our job to >>>>>>define one and the only natural combination of RDF and rules. >>>>>>A faithful embedding of RDF into RIF is all we need. Other languages will >>>>>>just need to translate themselves into RIF and they will add whatever >>>>>>necessary axioms they need in order to make themselves RIF-exchangeable. >>>>>> >>>>>>Incidentally, the most straightforward use of the embedding you proposed >>>>>>gives a language with the combined semantics that you proposed. But I see >>>>>>no reason why push that particular language onto others and exclude other >>>>>>possible combos of RDF and RIF. >>>>> >>>>>Whether we define the semantics of the combination of RDF and RIF based >>>>>on common models or based on an embedding of RDF in RIF, in both cases >>>>>we "push" a particular semantics onto the users of the language. We also >>>>>"push" the semantics of RIF onto users, and that is a good thing, >>>>>because it enables interoperability. >>>> >>>>There are many good things that we could do, but we are not chartered to do >>>>them. For instance, defining the means for exchanging data models along the >>>>lines of my earlier email would be a good thing, because it enables >>>>interoperability. But some members of the WG think that this is not in our >>>>charter. Even better would be to define ONE AND ONLY rule language, because >>>>it will eliminate the issue of interoperability altogether. But this is not >>>>in our charter. >>>> >>>> >>>> >>>>>This does not preclude other possible combinations of RDF and RIF; if >>>>>someone would want to use another kind of combination (say, abc) which >>>>>is embeddable in RIF, the person can use RIF for exchanging this >>>>>combination; however, he/she cannot expect interoperability with anyone >>>>>using a combination other than (abc). >>>> >>>>This does not preclude other languages, but sends a message that they >>>>better shut up, because we have defined the TRUE combo of rules and RDF. >>>>Is it in our charter to do so? >>>> >>>> >>>> >>>>>So, it *is* our job to define the semantics of the combination of RDF >>>>>and RIF. My arguments for defining the semantics based on common models >>>>>rather than based on an embedding can be found in my earlier e-mail. >>>> >>>>As I said, I do not think it is our job to do so. I propose that this be an >>>>item in one of the telecons or even at the F2F. >>>> >>>> >>>> cheers >>>> --michael >>>> >>>> >>>> >>>> >>>>>Best, Jos >>>>> >>>>> >>>>> >>>>>> --michael >>>>>> >>>>>> >>>>>> >>>>>> >>>>>>>Dear all, >>>>>>> >>>>>>>In the original e-mail [1] I sent introducing my proposal for RIF-RDF >>>>>>>combinations based on combined models [6], I included a short rationale >>>>>>>for choosing such a combination as the normative way for combining RIF >>>>>>>and RDF, rather than an embedding of RDF in RIF. >>>>>>> >>>>>>>Some doubt has been raised [2] about the value of this approach. >>>>>>>In this e-mail I try to formulate my rationale for this model-theoretic >>>>>>>semantics of RIF-RDF combinations a bit more clearly. >>>>>>>Before I present my main argument, I will first try to put the issue in >>>>>>>its context by considering the relationship between RIF and other >>>>>>>semantic Web languages, and recount the main use cases for RDF in RIF. >>>>>>> >>>>>>> >>>>>>>Some background >>>>>>>==== >>>>>>>RIF is a W3C working group in the semantic Web activity. Like the Basic >>>>>>>Logic Dialect (BLD) of RIF, RDF is a logical language with a declarative >>>>>>>model-theoretic semantics. >>>>>>>Since RDF is the primary semantic Web language, RIF needs to account for >>>>>>>this language; RIF needs to be compatible with RDF. >>>>>>>Now, "compatible" can be interpreted in various ways. Extending the >>>>>>>model theory of RDF is a possible way to ensure a high level of >>>>>>>compatibility; the language OWL Full extends the RDF semantics in such a >>>>>>>way. Despite it being mentioned as a possibility in the charter, the >>>>>>>RIF working group decided not to base its model theory on that of RDF, >>>>>>>but rather, like OWL DL, develop a new one. >>>>>>>There could never be any real compatibility between RDF and OWL DL, >>>>>>>because the description logic paradigm does not work well with the >>>>>>>syntactic freedom of RDF. >>>>>>>There is, however, a potential to achieve a high degree of compatibility >>>>>>>between RDF and RIF, because the RDF semantics has been carefully >>>>>>>crafted so that it works well with the rule-based reasoning paradigm. >>>>>>>In fact, the RDF semantics can be embedded in a simple Datalog-like >>>>>>>language [3], so that rule reasoners can be used for reasoning with RDF. >>>>>>> >>>>>>> >>>>>>>The use cases for RDF in RIF >>>>>>>=== >>>>>>>Now, the RIF working group has identified two primary use cases related >>>>>>>to RDF compatibility: (a) using an RDF graph as an external data >>>>>>>model/data set and (b) exchanging RDF rules, i.e. rules which extend an >>>>>>>RDF graph (e.g. N3, [4]). >>>>>>> >>>>>>>In both cases, we are essentially dealing with the combination of an RDF >>>>>>>graph (which may include an RDFS ontology) and a set of (RIF BLD) rules >>>>>>>[*]. >>>>>>>The question now is: what is an appropriate semantics for this combination. >>>>>>> >>>>>>> >>>>>>>The semantics of combinations >>>>>>>=== >>>>>>>Earlier [5], I proposed to base the semantics on an embedding of the RDF >>>>>>>graph and the RDF semantics in RIF. It can be shown that this embedding >>>>>>>behaves the same as the RDF semantics, with respect to entailment, if >>>>>>>the rule set is empty. >>>>>>>There is, however, no way (apart from anecdotal evidence such as test >>>>>>>cases) to determine whether the semantics of the combination behaves in >>>>>>>a natural way. >>>>>>>In fact, there is no way to see whether the combination is in any way >>>>>>>faithful to the RDF semantics, and it is very hard to reconstruct from >>>>>>>such an embedding how the RDF and RIF semantics interact. >>>>>>> >>>>>>>These problems prompted me to reconsider the semantics of RDF and RIF >>>>>>>combinations. Since both RDF and RIF have a model-theoretic semantics, >>>>>>>it is possible to give a declarative, model-theoretic account of their >>>>>>>combination, thereby providing a clear understanding of the semantics of >>>>>>>the combination. It is thereby possible to verify whether the >>>>>>>combination is "natural", and whether it has the properties you want off >>>>>>>a combination. >>>>>>>I am sure that an important property of an RIF-RDF combination is that >>>>>>>it is faithful to both the RDF and RIF semantics. So, the combinations >>>>>>>is defined such that the models are based on the combination of RIF and >>>>>>>RDF models, thereby ensuring that the semantics is faithful to RDF and >>>>>>>RIF, on the model level. >>>>>>>The interaction between the semantics is defined through a total of 8 >>>>>>>conditions, so that it is very easy to verify whether the interaction is >>>>>>>natural. >>>>>>>It turns out that satisfiability checking and entailment of combinations >>>>>>>can be reduced to satisfiability checking and entailment, respectively, >>>>>>>of RIF rules, through an embedding. This embedding gives implementers an >>>>>>>idea of these combinations could be processed, as did the RDF entailment >>>>>>>rules for the model-theoretic semantics of RDF. >>>>>>> >>>>>>>Another concern which has been raised about the model-theoretic >>>>>>>semantics for combinations is that it is supposedly complex, and bothers >>>>>>>the reader too much. >>>>>>>Actually, I find this a non-issue: if the reader is not interested in >>>>>>>model-theoretic semantics, the reader would probably have already >>>>>>>skipped the definition of the RIF semantics, and will probably also skip >>>>>>>the definition of the semantics of combinations, and perhaps instead >>>>>>>read the embedding, but, most likely, he/she will just read the language >>>>>>>reference or the language guide. >>>>>>>In fact, the definition of the model-theoretic semantics is much clearer >>>>>>>and much more concise than the proposed embedding (also in [6]), so I >>>>>>>would argue that an embedding actually bothers the reader more than the >>>>>>>proposed model-theoretic semantics. >>>>>>> >>>>>>> >>>>>>>Best, Jos >>>>>>> >>>>>>> >>>>>>>[1] http://lists.w3.org/Archives/Public/public-rif-wg/2007Aug/0012.html >>>>>>>[2] http://lists.w3.org/Archives/Public/public-rif-wg/2007Aug/0039.html >>>>>>>[3] >>>>>>>http://www.inf.unibz.it/~jdebruijn/publications-type/Bruijn-Heymans-LogiFoun-07.html >>>>>>>[4] Herman J. ter Horst: Combining RDF and Part of OWL with Rules: >>>>>>>Semantics, Decidability, Complexity. International Semantic Web >>>>>>>Conference 2005: 668-684. >>>>>>>http://www.springerlink.com/content/366474250nl35412/ >>>>>>>[5] http://lists.w3.org/Archives/Public/public-rif-wg/2007May/0077.html >>>>>>>[6] http://www.w3.org/2005/rules/wg/wiki/Core/RIF-RDF_Compatibility >>>>>>> >>>>>>>[*] Not considering, for the moment, that RDF rules may require >>>>>>>additional constructs or symbols, such as (rigid) blank nodes and >>>>>>>certain built-ins. >>>>>>>-- >>>>>>> debruijn@inf.unibz.it >>>>>>> >>>>>>>Jos de Bruijn, http://www.debruijn.net/ >>>>> >>>>>-- >>>>>Jos de Bruijn debruijn@inf.unibz.it >>>>> http://www.debruijn.net/ >>>>>---------------------------------------------- >>>>>As far as the laws of mathematics refer to >>>>>reality, they are not certain; and as far as >>>>>they are certain, they do not refer to >>>>>reality. >>>>> -- Albert Einstein >>>> >>>> >> >>-- >>Dr. Axel Polleres >>email: axel@polleres.net url: http://www.polleres.net/ >> >> >> >> > > > -- Dr. Axel Polleres email: axel@polleres.net url: http://www.polleres.net/
Received on Thursday, 6 September 2007 13:53:06 UTC