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Re: Adaptation of the semantics

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 11 Mar 2021 12:28:14 -0500
To: public-rdf-star@w3.org
Message-ID: <65ac3e55-8431-f160-45d0-e95c214afa79@gmail.com>
I don't see this as much of an advance.

The semantics now uses two graphs and also uses a hidden predicate.

As far as I can see, the reason to use a hidden predicate is to prevent RDF* 
graphs with embedded triples from beng equivalent to an RDF graph.  I'm not in 
favour of this attribute of RDF*, as it prevents the construction of 
variations on embedded triples.

As far as I can see, the reason to use two graphs is to hide the triples 
generated by embedded triples from SPARQL.

So there are two hiding operations.  Are two really needed?


On 3/5/21 1:00 PM, 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 Thursday, 11 March 2021 17:28:29 UTC

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