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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?


peter



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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