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Re: Labelled graphs

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 30 Apr 2012 02:36:45 -0500
Cc: Ivan Herman <ivan@w3.org>, Steve Harris <steve.harris@garlik.com>, public-rdf-wg WG <public-rdf-wg@w3.org>
Message-Id: <D3683875-D9F6-4E33-8FB2-C1C97E1FE8F2@ihmc.us>
To: Sandro Hawke <sandro@w3.org>

On Apr 25, 2012, at 6:59 AM, Sandro Hawke wrote:

> On Wed, 2012-04-25 at 13:34 +0200, Ivan Herman wrote:
>> On Apr 25, 2012, at 12:51 , Sandro Hawke wrote:
>>> On Wed, 2012-04-25 at 11:08 +0100, Steve Harris wrote:
>>>> On 24 Apr 2012, at 13:04, Sandro Hawke wrote:
>>>>>>> * When the same label is used multiple times in the same dataset, the
>>>>>>> graph is
>>>>>>> assumed to be the union of the graphs labeled with it
>>>>> This is the "partial-graph semantics" view, which I can live with, but
>>>>> some people have expressed opposition.  We should probably try some
>>>>> straw polling on it.
>>>> The choice here needs to be made carefully, to avoid unintended consequences on implementations, and data generation processes.
>>>> The corner cases are around bNodes (aren't they always), e.g.
>>> I believe you're talking about a different issue here.  The question of
>>> the scope of bNode labels comes up whether we have partial- or
>>> complete-graph semantics.
>>> 6.1 says the scope of bNode labels is the document (or dataset, I
>>> suppose).  I know that gives you a memory cost, but it's important for
>>> several use cases, such as Keeping Inferred Triples Separate.
>>> There are some SPARQL test cases for this here:
>>>       http://www.w3.org/2011/rdf-wg/wiki/Graphs_Design_6.1#Blank_Nodes
>>> I don't think you can test for it with trig entailment unless you have a
>>> way to get at the triples inside the named graphs and exposing them to
>>> RDF semantics.  Folks have been proposing doing that by flagging the
>>> dataset as a default-is-union dataset; if you can do that, then you
>>> could ask:
>>>       Does
>>>               @default-is-union
>>>               <u1> { _:x <b> <c> }
>>>               <u2> { _:x <b> <d> }
>>>       entail
>>>               { _:y <b> <c>,<d> }
>>> I claim the answer should be "yes".
>> I must admit I am not sure what http://www.w3.org/2011/rdf-wg/wiki/Graphs_Design_6.1#Blank_Nodes (ie, that blank nodes have a file scope) mean eg, in terms of semantics. 
> I don't know how the scope of blank node labels is expressed in the RDF
> semantics.   If you want, I can try to figure it out.

Blank nodes themselves do not have any notion of scope associated with them. They are just as "global' as IRIs. Graph node *identifiers* in a given concrete interchange syntax have scopes which are (presumably) defined by the rules of that particular syntax. 

>> If I look at the more abstract level
>> (D, (<u>,G), (<v>,H))
>> with G and H being different graphs, what does it mean that they share a blank node?
> If I understand right, in the RDF Semantics, two "graphs" can't share a
> blank node.  

Wrong!! They certainly can so share a blank node.  That is why we had to have the distinction between unioning and merging, and all the resultant mess. (Go through the slides in my RDF Redux talk http://www.slideshare.net/PatHayes/rdf-redux to see how we SHOULD have defined the RDF abstract syntax to avoid this glitch.)

> That's why in 6.1 I said you can/should think about D, G,
> and H as subgraphs of some other graph that contains them all.   

A graph is a set. Of course D, G and H are subsets of a larger set their union). ANY collection of sets are all subsets of their union, that is a tautology. 

> I could
> draw a diagram that made it pretty clear how they shared a node --
> they'd have arcs going to/from the same little node-circle in the
> diagram, and the different graphs would be indicated by D being drawn
> with blue arcs, G with green arcs, and H with yellow arcs. 

RIght, that would work.


>> Put it another way: if you have a TriG file  
>> <u1> { _:x <b> <c> }
>> <u2> { _:x <q> <r> }
>> what is the abstract RDF dataset for this? 
> I don't know how to type that dataset in ascii.  As a diagram, it looks
> like the diagram for { [] <b> <c>; <q> <r> } except that the "b" arc is
> colored so we know it's in u1 and the "q" arc is colored so we know it's
> in u2.
>> Unless of course all blank nodes are skolemized by TriG before generating a dataset
> This global-scope-blank-nodes rule would show up in a skolemizing test
> case.   I'm not sure how we're going to write down these test cases, or
> if we're going to define Skolemizing for trig.  But if we did:
> your example TriG file COULD be Skolemized to:
>  <u1> { <http://example.org/.well-known/genid/1 <b> <c> }  
>  <u2> { <http://example.org/.well-known/genid/1 <b> <c> }
> but it COULD NOT be Skolemized to:
>  <u1> { <http://example.org/.well-known/genid/1 <b> <c> }  
>  <u2> { <http://example.org/.well-known/genid/2 <b> <c> }
>    -- Sandro

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Received on Monday, 30 April 2012 07:37:22 UTC

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