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Re: Making progress on graphs

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 22 May 2012 09:10:23 -0500
Cc: Guus Schreiber <guus.schreiber@vu.nl>, RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <DBC71588-DEC3-4124-A25C-420EBDA1779A@ihmc.us>
To: Richard Cyganiak <richard@cyganiak.de>

On May 21, 2012, at 11:32 AM, Richard Cyganiak wrote:

> On 21 May 2012, at 16:23, Pat Hayes wrote:
>>>  ISSUE-28 Syntactic nesting of g-texts http://www.w3.org/2011/rdf-wg/track/issues/28
>>>  PROPOSED: Close ISSUE-28 ("Do we need syntactic nesting of graphs (g-texts) as in
>>>  N3?"), saying No, we do not -- they are useful, but we can provide the same
>>>  functionality with datasets.
>> 
>> Can you (or anyone) explain how this functionality is provided with datasets? A simple example will suffice. 
> 
> Let's assume the abstract syntax supported nested graphs. Then we can convert that to a “flat” dataset like this: For any nested graph G, replace it by a new IRI i, and add a new pair <i,G> to the dataset.

I was afraid of that. That works only if we give the datasets a semantics in which the graph 'name' really does denote the named graph, in the 2004 semantics sense of 'denote'. So, I am cool with this PROVIDED that we all understand, and accept, that this is now an absolute requirement on any proposed dataset semantics. This rules out Antoine's proposal, for example. 

> 
>>>  ISSUE-29 Do we support SPARQL's notion of "default graph"?
>>>  http://www.w3.org/2011/rdf-wg/track/issues/29
>>>  PROPOSED: Close ISSUE-29 (Do we support SPARQL's notion of "default graph"?'),
>>>  Yes, we do.
>> 
>> I would like us to clarify what we mean by "support" here. If it just means that datasets have an optional default graph in them, then I am fine with that decision. If it means more, I'd like to know what more. 
> 
> Datasets have a possibly empty default graph.
> 
>>>  ISSUE-33 Mechanism to refer to sub-graphs and/or individual triples
>>>  http://www.w3.org/2011/rdf-wg/track/issues/33
>>>  PROPOSED: Close ISSUE-33 ("Do we provide a way to refer to sub-graphs and/or
>>>  individual triples?"), with the understanding that datasets can be used to refer to
>>>  sub-graphs and individual triples.
>> 
>> Again, how does this work? Consider the graph G = { <:a :b :c> <:d :e :f> <:g :h :i>} and the subgraph G1 consisting of just the first two triples, and then the first triple alone, call that T. That is a total of three triples and two (or perhaps three) graphs. How would these relationships be represented using datasets? 
> 
> <G> { :a :b :c. :d :e :f. :g :h :i. }
> <G1> { :a :b :c. :d :e :f. }
> <T> { :g :h :i. }
> 
> Meaning: If you want to talk about a subgraph or individual triple, you need to stuff the subgraph or triple into a named graph of its own. (What exactly <G>, <G1> and <T> name, identify, refer to or denote is still open; the above just means that these IRIs are somehow associated with those triples.) One could then define entailment regimes where the following triples can be inferred:
> 
>  <G> ex:subgraph <G1>. <G> ex:subgraph <T>. <G> ex:unionOf (<G1> <T>). <G> ex:entails <G1>.
> 
> And so on. But whether this WG actually wants to define any such relationships remains an open question.

OK, so this is pretty trivial. I guess I am left wondering what ISSUE-33 was supposed to be about, since if this is all it means, why was it ever an issue in the first place? But never mind, lets not get into historical reconstruction. 

Thanks

Pat

> 
> Best,
> Richard
> 
> 
> 
>> 
>> Pat
>> 
>> 
>>> 
>>> Also, I would like to focus your attention to Sandro's document [1]. I tried to get discussion on the fact there is lot of stuff there we appear to agree on [2], but I miserably failed because my inadvertent subject line dominated the debate. Please comment on Sandro's document if you have time.
>>> 
>>> Best,
>>> Guus
>>> 
>>> [1] http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-spaces/index.html#
>>> [2] http://lists.w3.org/Archives/Public/public-rdf-wg/2012May/0372.html
>>> 
>>> On 13-05-2012 16:59, Richard Cyganiak wrote:
>>>> All,
>>>> 
>>>> We've been talking our way up and down the design space for multigraphs for a year now, with not much to show for it. We still have not settled on a basic design.
>>>> 
>>>> Once we do settle on a basic design, the real work only starts since we need to nail down the details. This will take time. Our charter says that all documents should go to LC *this month*, and obviously we are nowhere near ready for this.
>>>> 
>>>> So I think it's time to stop exploring the design space, and start collapsing it by making decisions.
>>>> 
>>>> Obviously there is still strong disagreement on many things when it comes to multigraphs, but it seems to me that all proposals on the table accept a basic *abstract syntax* that is quite similar to the RDF datasets in SPARQL, and even the most adventurous experiments don't really stray from that forumla. Therefore:
>>>> 
>>>> 
>>>> PROPOSAL: The abstract syntax for working with multiple graphs in RDF consists of a default graph and zero or more pairs of IRI and graph. This resolves ISSUE-5 (“no”), ISSUE-22 (“yes”), ISSUE-28 (“no”), ISSUE-29 (“yes”), ISSUE-30 (“they are isomorphic”), ISSUE-33 (“no”).
>>>> 
>>>> 
>>>> RATIONALE: All proposals on the table are based on an abstract syntax very similar to SPARQL's notion of an RDF dataset, although there is no consensus on the semantics and the terminology. Making a decision on the basic abstract syntax would unblock the work, and allow various strands of required detail work to proceed independently, hopefully leading to additional resolutions to remaining questions, such as:
>>>> 
>>>> • What's the formal semantics of the abstract syntax?
>>>> • Definition of the concrete syntaxes (N-Quads, etc.)
>>>> • Describing how to work with this in the Primer
>>>> • What do call the pairs? “Named graphs” or something else?
>>>> • What to call the entire thing? “RDF dataset” or something else?
>>>> • Can blank nodes be shared among graphs?
>>>> • What additional terminology (rdf:Graph etc) needs to be defined?
>>>> 
>>>> Best,
>>>> Richard
>>> 
>>> 
>> 
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>> 
>> 
>> 
>> 
>> 
> 
> 
> 

------------------------------------------------------------
IHMC                                     (850)434 8903 or (650)494 3973   
40 South Alcaniz St.           (850)202 4416   office
Pensacola                            (850)202 4440   fax
FL 32502                              (850)291 0667   mobile
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Received on Tuesday, 22 May 2012 14:11:32 GMT

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