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Re: [BlankNodeRefs] Questions on blank node refs

From: Axel Polleres <axel.polleres@deri.org>
Date: Wed, 18 Mar 2009 22:52:29 +0000
Message-ID: <49C17B2D.7070603@deri.org>
To: Bijan Parsia <bparsia@cs.manchester.ac.uk>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>
Hi Bijan, all,

Bijan Parsia wrote:
 > Blank nodes have been a bear for the OWL WG as well.
 >
 > One thing to consider is what the real future of their current semantics
 > (and operational behavior) really is. We might also look at how RIF is
 > handling them.

* concerning RIF, the status is as follows:
As far as RIF is concerned, blank nodes are treated as existentially 
quantified variables.

In more detail: In RIF itself, there is no such thing as blank nodes.
RDF triples are mapped to slots in RIF. When mapping RDF(S) entailment 
to RIF, blanknodes of the entailing graph are skolemized whereas the 
entailed graph is mapped to a RIF condition formula such that blank 
nodes are mapped to variables.
As for more details, cf. [1]

* As for OWL:
Can you elaborate a bit more why are blank nodes a problem in OWL?
Per se, some RDF triples with blank nodes can be mapped to a OWL1 
statements already, i.e.

s p _:b.

would become:

SubclassOf(oneOf(s)  restriction( p minCardinality(1) )

but that is more problematic with bnodes in subject positions, right?
Is that the kind of problems you are referring to or the problem to 
define the semantics of a SPARQL query on top of OWL entailment, on 
datasets with blank nodes such as co-reference in solution sets, etc., 
cf. your part of our ESWC Tutorial [2]?

If you could summarize/review the issues briefly, it might be nice for 
the group.

Thanks for further explanations,
Axel

1. RIF RDF and OWL Compatibility. As the latest public snapshot
http://www.w3.org/TR/rif-rdf-owl/
is a bit dated, better check the wiki version:
http://www.w3.org/2005/rules/wiki/SWC


2. ESWC 2007 Tutorial: "SPARQL - Where are we? Current state, theory and 
practice", Unit 5b, 
http://www.polleres.net/sparqltutorial/ESWC2007_SPARQL_Tutorial_unit5b.pdf

> 
> People do use blank nodes in fairly specific ways and we might 
> considering "fixing" them if we can show clear requirements from SPARQL 
> on this. SPARQL exposes some aspect of the semantics of bnodes in a very 
> user salient way, so this might be the right time.
> 
> (Obviously "fixing" blank nodes is one of my hobby horses...so, grain of 
> salt and all.)
> 
> Cheers,
> Bijan.
> 
> 


-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/
Received on Wednesday, 18 March 2009 22:53:11 GMT

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