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Re: a blank node issue

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 2 Mar 2011 11:24:25 -0600
Cc: nathan@webr3.org, Ivan Shmakov <oneingray@gmail.com>, semantic-web@w3.org
Message-Id: <EBE30BD5-C1B2-42A8-B6A0-BE7BDCAC6B7E@ihmc.us>
To: William Waites <ww@styx.org>
In the RDF documents we used a convention which could be described as treating the underline character as the base URI of Qnames for blanknodes, so that one gets things like _:23  or _:myfavoriteblanknode. That seems to work. Of course, one has to be careful about the scope of these, um, identifiers: unlike IRIs, they only extend to the edge of the document or graph in which they occur. 

BTW, part of our motivation for having blank nodes in the RDF conceptual model was precisely to avoid having to get into questions of scope and the resulting syntactic messes. In this we did not fully succeed, but in any case, as you point out, actual implementations have to get involved with this stuff. 

Pat

On Mar 2, 2011, at 8:18 AM, William Waites wrote:

> * [2011-03-01 17:26:44 -0600] Pat Hayes <phayes@ihmc.us> écrit:
> 
> ] No. Indeed, a blank node has no identifier, in the RDF abstract 
> ] graph syntax. However, this does not mean it has no semantic value
> 
> Fine as far as it goes. However if you descend into actual
> real implementations this quickly becomes a problem. This
> is why triplestores have some kind of skolemisation that
> they do. 4store does bnode:abc123, Virtuoso does
> nodeID://b1234.
> 
> Every time I have written a wrapper so that a library like
> rdflib can talk to these stores, typically using SPARQL
> over some sort of channel I have to write these special
> cases in for the vendor extensions. Otherwise operations
> like SELECT and DELETE cannot work predictably when
> operating on a single statement containing a blank node.
> 
> So I know that several of us have seen this in practice
> and had to deal with it one way or another, and obviously
> the major triplestore authors have thought about it and
> implemented vendor extensions, maybe some convention or
> standard for skolemising blank nodes so they can be 
> referred to might be a good thing?
> 
> Cheers,
> -w
> -- 
> William Waites                <mailto:ww@styx.org>
> http://river.styx.org/ww/        <sip:ww@styx.org>
> F4B3 39BF E775 CF42 0BAB  3DF0 BE40 A6DF B06F FD45
> 
> 

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Received on Wednesday, 2 March 2011 17:26:43 UTC

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