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Re: Adaptation of the semantics

From: Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu>
Date: Wed, 10 Mar 2021 09:50:26 +0100
To: "public-rdf-star@w3.org" <public-rdf-star@w3.org>
Message-ID: <7fe51d48-1925-747c-99b1-b9ca74581474@ercim.eu>
Update on this new semantics proposal:

I re-introduced one hidden IRI, for the same reason as before (RDF-star 
systems should not be required to "compile" descriptions of embedded 
triples into proper embeded triples).

Unlike its predecessors, however, that IRI /can/ hide from SPARQL (see 
issue #101 <https://github.com/w3c/rdf-star/issues/101>) because it is 
not entailed by the embedded triple.


On 05/03/2021 19:02, Pierre-Antoine Champin wrote:
> I forgot to put the link to the preview, for those not quite familiar 
> with our github:
> https://pr-preview.s3.amazonaws.com/w3c/rdf-star/pull/127.html#rdf-star-semantics
> On 05/03/2021 19:00, Pierre-Antoine Champin wrote:
>> Hi all,
>> I just pushed a pull-request adapting the semantics:
>> https://github.com/w3c/rdf-star/pull/127
>> I believe it has some advantages over the current version:
>>   * it does not rely anymore on "hidden" predicates (see issue #101
>>     <https://github.com/w3c/rdf-star/issues/101>)
>>   * it does not have the "merging" issue warned about in §6.3.1
>>     <https://w3c.github.io/rdf-star/cg-spec/2021-02-18.html#combining-rdf-star-graphs>
>>   * I think that it allows us to align SPARQL query semantics with
>>     simple entailment (as newly defined)
>>   * I think that it allows the Interpolation Lemma
>>     <https://www.w3.org/TR/rdf11-mt/#dfn-interpolation> to extend to
>>     RDF-star
>> (I didn't formally prove the last two items, hence "I think"...)
>> The trick is that we do not map anymore RDF-star graphs to a single, 
>> semantically equivalent RDF graph.
>> Instead, we map it to a pair of RDF graphs, which can be thought of 
>> as a "lower and upper bound" of the RDF-star graph, in terms of 
>> entailment. The semantics of the RDF-star graph is defined through 
>> the semantics of its "bounds", reusing RDF semantics as is (as we 
>> currently do).
>> In this new semantics, a strict RDF-star graph (i.e. one that 
>> contains embedded triples) has no exactly equivalent RDF graph, so it 
>> still can not be conveyed exactly using RDF syntaxes (but we do not 
>> rely anymore on hidden predicates for that). However, either of the 
>> two "bounds" can be used to approximate the RDF-star graph in legacy 
>> RDF. The "lower bound" will produce correct but incomplete 
>> inferences. The "upper bound" will produce complete inferences, with 
>> a few spurious (but generally harmless) ones.
>> I am curious to get some feedback on this.

Received on Wednesday, 10 March 2021 08:50:31 UTC

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