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Re: [RIF-RDF] rationale for a model-theoretic semantics for combinations

From: Axel Polleres <axel.polleres@deri.org>
Date: Thu, 06 Sep 2007 15:52:34 +0200
Message-ID: <46E00622.6000101@deri.org>
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 


> 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 

> 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.


>>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)?
>>Jos de Bruijn 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.  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:
>>>>>>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
>>>>>>>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
>>>>>>>[4] Herman J. ter Horst: Combining RDF and Part of OWL with Rules:
>>>>>>>Semantics, Decidability, Complexity. International Semantic Web
>>>>>>>Conference 2005: 668-684.
>>>>>>>[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
>>>>> -- 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 GMT

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