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Re: New editor's draft of RDF Datatset Semantics

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Fri, 13 Dec 2013 16:49:20 +0100
Message-ID: <52AB2C80.10708@emse.fr>
To: public-rdf-wg@w3.org
Le 13/12/2013 13:35, Andy Seaborne a écrit :
> On 12/12/13 21:08, Antoine Zimmermann wrote:
>> The new draft addresses most of Pat's comments. I added 2 issues to it.
>> I also added a section that reflects Sandro's box dataset semantics,
>> following Guus' suggestion, and also made some changes in response to
>> Peter initial comments.
>> https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-dataset/index.html
>> If it looks ok, we can vote by email to make it a FPWD. Sorry for not
>> having done this before yesterday teleconf.
> [[
> 3.2 Default graph as union or as merge
> It is sometimes assumed that named graphs are simply a convenient way of
> sorting the triples but all the triples participate in a united
> knowledge base that takes the place of the default graph. More
> precisely, a dataset is considered to be true if all the triples in all
> the graphs, named or default, are true together.
> ]]
> This starts talking of dftGraph = union of named graphs, not a super
> set, but then says it is all the triples in graphs, named or default.
> Shouldn't the default graph be exactly the union (version 2: merge) of
> the named graphs?

The semantics defines the truth of all datasets. Not only the datasets 
with a default graph equal to the union of the named graphs.

> Or there needs to be some rewording because "default graph" is being
> used is different ways.

In different ways? What way is different from what other way?

> [[
> This description allows two formalizations of dataset semantics,
> depending on how blank nodes spanning several named graphs are treated.
> ]]
> It is worth nothing that TriG and N-quads use document-scoped blank node
> labels.

worth nothing? or worth noting? In any case, the formal semantics is 
agnostic wrt the serialisation syntax used. The definition of RDF 
datasets allows for shared blank nodes across several named graphs, and 
the semantics has to apply to all RDF datasets.

> I don't know of any system that does NGs+dftgraph -> dftGraph of
> dataset.  Are there any?

Sesame and OWLIM apply reasoning on the union of all named graphs to 
generate triples that are then inserted into the default graph. This 
materialisation of inferences creates a dataset where the default graph 
is a strict superset of the union of named graphs.

> 3.6 Quad semantics
> Worth mentioning about the empty graph.

Ok, empty named graphs cannot be represented as quads, but this is 
essentially a syntactic issue. Still, I'd say it could be noted nonetheless.


>      Andy

Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
42023 Saint-Étienne Cedex 2
Tél:+33(0)4 77 42 66 03
Fax:+33(0)4 77 42 66 66
Received on Friday, 13 December 2013 15:49:47 UTC

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