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Re: [Graph] Comparison of two semantics (first step)

From: Ivan Herman <ivan@w3.org>
Date: Thu, 23 Feb 2012 10:11:05 +0100
Cc: RDF WG <public-rdf-wg@w3.org>
Message-Id: <5F04AA7E-A322-4D83-BA1B-5A12CCEA4F0E@w3.org>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Thanks Antoine,

On Feb 22, 2012, at 18:43 , Antoine Zimmermann wrote:

> As I said at today's telecon, I'd like us to compare Pat's proposed semantics to the already existing proposal that is on the wiki [1].
> 
> As Andy mentioned in the telecon, the best to do would be that we take a look at the use cases and show in what way the semantics (Pat's and [1]) help addressing them.
> 
> But first, just a high level comparison, I'll send description of how to address use cases in a different email:
> 
> - the proposal of [1] is based on Datasets which makes it a bit more inline with SPARQL specs, while Pat's is based on quads and quad-graphs. In practice, it does not make a big different as you can go from one to the other and back without any loss, but it makes the connection with SPARQL more explicit
> - the proposal of [1] does not change the RDF semantics or syntax, it only refers to it. So a RDF Graph is still a set of triples with the same semantics. A dataset is a distinct data structure that is used only by people who *want* to separate and manage different graphs, for whatever reasons (time variations, provenance issues, trust, endorsement, etc). If I understand well Pat's proposal, his goal is to make RDF interpretation evolve to the point where they can satisfy quadruples as well as triples.
> - since [1] is based on the current RDF semantics, it externalises most of the definitions to the RDF Semantics document and the whole proposal is very concise. What is in [1] is a complete formal logic and can be implement right now.
> - [1] currently assumes that the underlying logic of the individual graphs is RDF-entailment, but adapting the definitions to take account of RDFS or D-entailment, or OWL, or SWRL would simply need to replace the phrase "RDF-model" by "X-model" where X is an entailment regime. It's not yet clear what Pat's proposal does wrt RDFS and extensions, see espcially Q6 in Pat's FAQ from his email [2].
> 

I do find these arguments compelling, and I agree we should indeed look at [1] as an alternative. It does look like a smaller 'delta' from where we are... But yes, use cases count.


> The proposal in [1] is not really well addressing use cases that are important for me, especially UC 6.2 [3] but it could with adaptation. Pat's proposal, however, in its main approach (ternary IEXT), would do the job very well, provided that some adaptation and clarification are made.
> 

Yes, I can see that Pat's approach *may* lead towards those, but it is not immediate. You give the example:

ex:chadhurley  rdf:type  ex:YoutubeEmployee . [2005,2010]
ex:YoutubeEmployee  rdf:type  ex:GoogleEmployee . [2006,2011]

=>

ex:chadhurley  rdf:type  ex:GoogleEmployee . [2006,2010]

This *could* be described in Pat's formulation if, at the fourth position, we'd define some sort of a datatype for date intervals (maybe it already exist in xsd, I have not checked) but it would also require to reformulate the semantics in some more complex manner. Indeed, the inference, in this case, would have to take into account some sort of an interval calculus, too. I do not believe this is a road we want to engage into.

Ivan



> So if I was trying to only address the use cases that are important to me, I would have probably gone towards the road of a quad-based semantics à la Pat's, but I feel that the Dataset-based semantics is better addressing important use cases and would be much easier to implement and deploy.
> 
> I also think that an educated comparison can only be made if Pat's proposal is fully described in terms of formal details.
> 
> 
> [1] http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/RDF-Datasets-Proposal#Semantics
> [2] http://lists.w3.org/Archives/Public/public-rdf-wg/2012Feb/0094.html
> [3] http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs-UC#.28B_priority.29_Reasoning_over_annotations
> -- 
> Antoine Zimmermann
> ISCOD / LSTI - Institut Henri Fayol
> École Nationale Supérieure des Mines de Saint-Étienne
> 158 cours Fauriel
> 42023 Saint-Étienne Cedex 2
> France
> Tél:+33(0)4 77 42 83 36
> Fax:+33(0)4 77 42 66 66
> http://zimmer.aprilfoolsreview.com/
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
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Received on Thursday, 23 February 2012 09:11:29 UTC

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