Re: To embed or combine

Dave,

I'm afraid I don't really see the difference between "accessing RDF
data" and "entailment regimes", so I don't really understand why they
should be treated differently.
Wouldn't one simply use a combination with the simple entailment regime
in this case?

Best, Jos


Dave Reynolds wrote:
> 
> On reflection I think we have to slice this slightly differently.
> 
> There's the question of how RIF rule sets can simply access RDF data. I
> believe that requires parts of the "Embedding triples and Graphs"
> section to be normative (see below).
> 
> Then there's the question of the various RDF(S) entailment regimes and
> how those interact with RIF. I think those are specified better by means
> of the model theoretic semantics (see below).
> 
> I also think we need a separate informative document (or document part)
> which offers a "Guide to using RIF with the semantic web" that is more
> accessible to implementers and users and which opens up the options of
> non-normative subset semantics.
> 
> 
> ** Accessing RDF data
> 
> We have a UCR requirement that RIF rules should be able to "cover RDF
> triples as data where compatible with phase 1 semantics". To me this
> means it should be possible write RIF rules which can access RDF
> statements.
> 
> This requires the syntax section (how literals and URIs are mapped) and
> the tr and tr\sub(s) mappings from "Embedding triples and Graphs" to be
> normative.
> 
> Further, as I've argued several times, in practice RDF rule languages
> include builtins like "isBlankNode". Supporting those requires a
> modification to the tr mapping to distinguish the skolem constants as
> referenced by the blue text in Jos' document.
> 
> ** Entailment regimes
> 
> Since RDF is not simply data but also comes with a set of entailment
> regimes we also have to specify how RIF rule sets that access RDF data
> interact with those regimes.
> 
> [The RIF processor will also need to know what the required RDF
> entailment regime is - hence the Data Sets section in Arch.]
> 
> I think the interaction of the entailment regimes is best specified
> using a model theoretic approach as Jos has done. I say that for the
> following reasons:
> 
> o In the RDF specification the rule sets for RDF and RDFS entailment are
> purely informative, it is preferable for the RIF specification to
> reference the normative parts of the RDF specification.
> 
> o We agreed in RIF that dialect semantics should be specified model
> theoretically where possible, in preference to proof-theoretic or
> operational semantics. That seems consistent with preferring a model
> theoretic formulation of how the RDF and RIF entailment regimes interact.
> 
> o If we made the RIF rule sets for RDF and RDFS entailment normative we
> would be directing implementers to use this approach. Whereas in
> practice RDF implementations will already have solutions to especially
> the RDF entailment and should be free to use that and connect to a RIF
> processor in "black box" style if desired.
> 
> o The model theoretic approach seems more extensible towards defining
> the interaction with OWL (which we also have to do) than one based on
> translation rules.
> 
> At first I had been concerned that Jos' proposal forces the full
> RDF/RDFS semantics on implementations whereas in practice many people
> implement subsets of the entailments. However, when people want to
> support subset semantics they can do this by specifying just simple
> entailment and conveying the subset semantics by imported RIF rulesets,
> such as that for rho-df. That practice is sufficiently important that I
> think we should enable it by providing a rule import mechanism and
> document it in a non-normative "Guide to using RIF with the semantic web".
> 
> We could simply duck the question of how RDF and RIF semantics interact
> altogether. We could just specify the data access embedding and give
> neither the model theoretic nor the rule-based-embedding normative
> status. I don't think that would be ideal but would probably be
> acceptable to me.
> 
> So my conclusion is to restructure the document slightly:
>   o RDF graph embedding (normative) combining current syntax section and
> definition of tr and tr\sub(s)
>   o Semantics of RIF rules combined with RDF entailment, model
> theoretic, normative
>   o Embedding RDF semantics, informative
> 
> Note that putting the tr definition in the first section also clarifies
> for implementers what is actually going on and may help to alleviate
> some of Michael's concerns.
> 
> Dave

-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
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