- From: Michael Kifer <kifer@cs.sunysb.edu>
- Date: Fri, 07 Sep 2007 11:33:15 -0400
- To: Jos de Bruijn <debruijn@inf.unibz.it>
- Cc: RIF <public-rif-wg@w3.org>
> >> to define the semantics of a RIF-RDF combination. > >> Then, we come back to the question of how to define this semantics, > >> whether it is through an embedding or directly, using a model-theoretic > >> combination. > >> > >> In your e-mails I did not find any real argument why the embedding > >> approach would be better. Claiming it "is all we need" is not an argument. > >> In the e-mail starting this thread I outlined the advantages of a > >> model-theoretic approach, showing how it adds to the issue of > >> compatibility. > >> Finally, you claim that it "is not our business" to give a > >> model-theoretic account of the combination. However, you already agree > >> with me that an RDF compatibility is our business. From this I conclude > >> that if a model-theoretic approach is superior to the embedding > >> approach, then it is our business to define it. > > > > Your document already defines both approaches. > > The question is which one is the normative definition. > > > What I was saying was that > > embedding is more straightforward, > > That is debatable. I personally find the combined model approach more > straightforward. You think that it is a preference issue, so then the appropriate venue would be a group vote. But I do not even think it is a preference issue. Do you think that your document would be complete without the embedding part? If so, then why did you define the explicit embedding? If you think that the embedding is necessary, then this is precisely my point. I think that the embedding is necessary, while the combined semantics is not. > > > less imposing, > > I do not understand what you mean by that. The embedding is trivial. It is easy to explain to anybody. The combined semantics is not. > > and is equivalent to the > > first approach (the explicit combo) in your document. > > It is only equivalent with respect to checking entailment of RIF > condition or RDF graphs. One can certainly imagine query languages > (e.g. with counting), as well as extensions of RDF/RIF, such that the > equivalence no longer holds. If someone has an RDF-based query language with counting and other stuff then they would have to define a faithful embedding into RIF. Your combined semantics does not help here. That user would have to ensure that the embedding satisfies that commutative diagram, which was in my earlier msg to Axel. That diagram does not rely on the existence of a combined semantics. cheers --michael > best, Jos > > > So, the first > > approach is redundant. > > > > > > cheers > > --michael > > > > > >> Best, Jos. > >> > >> Michael Kifer wrote: > >>>> 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. > >>> Jos, > >>> this is an amazing and illogical inference. > >>> > >>> An embedding of RDF into RIF, as defined in your document, solves the RDF > >>> compatibility problem 100%, and is all what we need. But the first > >>> part of that document defined a combined RDF/RIF language and semantics. I > >>> claim that this is not adding much to the issue of compatibility and is not > >>> our business to do. > >>> > >>> > >>> --michael > >>> > >>>> 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 > >>>> -- > >>>> 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 > >> -- > >> Jos de Bruijn debruijn@inf.unibz.it > >> 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 > > --------------ms060607000707010601080606 > Content-Type: application/x-pkcs7-signature; name="smime.p7s" > Content-Transfer-Encoding: base64 > Content-Disposition: attachment; filename="smime.p7s" > Content-Description: S/MIME Cryptographic Signature > > MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIJEzCC > 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