[RIF-RDF] Embedding vs. model-theory [was: Re: [RIF-RDF] rationale for a model-theoretic semantics for combinations] from Jos de Bruijn on 2007-09-06 (public-rif-wg@w3.org from September 2007)

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Thu, 06 Sep 2007 15:12:32 +0200
To: Michael Kifer <kifer@cs.sunysb.edu>
CC: RIF <public-rif-wg@w3.org>
Message-ID: <46DFFCC0.80200@inf.unibz.it>
Michael,

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.


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/
----------------------------------------------
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
Received on Thursday, 6 September 2007 13:13:00 UTC