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Re: B-scopes

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 14 Nov 2012 09:16:28 -0800
Cc: public-rdf-wg@w3.org
Message-Id: <FF6366C5-9BF5-403E-B19F-E1B5C36E3113@ihmc.us>
To: Andy Seaborne <andy.seaborne@epimorphics.com>

On Nov 14, 2012, at 3:32 AM, Andy Seaborne wrote:

> 
> 
> On 14/11/12 11:02, Richard Cyganiak wrote:
>> Following recent discussions, I've written up a proposal to change the design of blank nodes in RDF by explicitly introducing scoped blank node identifiers into the abstract syntax.
>> 
>> http://www.w3.org/2011/rdf-wg/wiki/User:Rcygania2/B-Scopes
>> 
>> Requirements:
>> 
>>  Consistency with all resolutions the WG has made so far
>>  No changes to other specs beyond Concepts and Semantics required
>>  No changes to conforming implementations required
>> 
>> All further details are in the wiki.
>> 
>> Comments welcome.
>> 
>> Best,
>> Richard
>> 
> 
> The idea may be OK although adding a new piece of terminology to explain other unclear concepts may not achieve what you hope for.

I dont think bnodes are *unclear* right now, they are just hard to describe in a way that non-mathematicians find easy to grasp. 

> 
> Comments:
> 
> I have no idea what a "blank node scope" is.
> 
> [[
> There is a widely held misconception in the RDF community that graphs cannot share blank nodes. This stems from the fact in practice they rarely do and almost never need to,
> ]]
> 
> Any systems that has the default query graph as the union of named graphs is sharing bnodes across graphs.
> 
> Such systems are not 'rare'.

But if you take two graphs from random places on the Web, do they share bnodes? Most people think they obviously don't, but the specs implicitly say they might (by conspicuously not saying they don't.) 

> 
> ... sharing ...
> [[
> but (for the time being) not between graph stores.
> ]]
> 
> The skolemization note makes this confusing.  While not strictly at odds with it, the appearance of skolemization suggests global bnodes.

Bnodes are still global in this model. Remember, a bnode is a pair (identifier, b-scope). That is still globally unique. 

> Alternative for consideration:
> 
> 1/ Make clear that parsing produces a new blank nodes every time for the same label.

What does "new" mean? (Do I have to check whether someone in Afghanistan has already used it before going ahead?) 

> 2/ Talk about "fresh blank node" and not the universal not-quite-arbitrary set of blank nodes.

The trouble for me with this way of talking is that it seems to suggest some kind of global source for new, fresh bnodes, like the garden of Eden being the single source of the four rivers. But where is this magic bnode spring? I guess maybe TImBL is the only person who knows?

Pat

> 3/ Bnodes are global.
> 
> Then the "b-scope" and "bNode = (b-scope, label)" is an implementation approach.
> 
> 	Andy
> 
> 
> 

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Received on Wednesday, 14 November 2012 17:17:04 GMT

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