Re: To embed or combine

Jos de Bruijn wrote:
> 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.

I'm not suggesting they be treated differently.

To write rules that work with the data, or to implement a translator for 
those rules, the data mapping does have to normatively defined. To me 
this means that tr and the treatment of bNodes in data (not necessarily 
queries), literals etc have to all be normative. In the current document 
almost all of that is covered - literals are in the normative part, tr 
is implicit in the model theory but the handling of bNodes is not 
spelled out. Spelling tr out and defining the bNode handling in the 
normative part would resolve this, and would further increase the 
clarity of the document, at the trivial cost of moving a very small 
number of lines of text around.

> Wouldn't one simply use a combination with the simple entailment regime
> in this case?

Yes, I pointed this out in the "at first I was concerned" paragraph 
(since subset semantics includes the trivial subset).

Dave
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> 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
>

Received on Tuesday, 11 September 2007 07:54:40 UTC