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

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.
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).

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.


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/
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As far as the laws of mathematics refer to
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they are certain, they do not refer to
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