Re: [RIF-RDF] Embedding vs. model-theory [was: Re: [RIF-RDF] rationale for a model-theoretic semantics for combinations]

> 
> I guess I misunderstood your earlier comments. So, we agree that we need
> 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. What I was saying was that
embedding is more straightforward, less imposing, and is equivalent to the
first approach (the explicit combo) in your document. 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/

Received on Thursday, 6 September 2007 16:55:27 UTC