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Re: Graphs Design 6.2

From: Ivan Herman <ivan@w3.org>
Date: Wed, 25 Apr 2012 15:14:52 +0200
Cc: public-rdf-wg <public-rdf-wg@w3.org>
Message-Id: <4B6CDB90-F0B6-4D0C-987A-5045EC757D57@w3.org>
To: Sandro Hawke <sandro@w3.org>

On Apr 25, 2012, at 14:57 , Sandro Hawke wrote:

> On Wed, 2012-04-25 at 14:45 +0200, Ivan Herman wrote:
>> Hey Sandro,
>> 
>> On Apr 25, 2012, at 13:44 , Sandro Hawke wrote:
>> 
>>> Here's a sketch of 6.2, which is similar to 6.1, but differs in the
>>> areas where people have made me think they didn't like it.  I have not
>>> put it on a wiki page or given it test cases yet.
>>> 
>>> The differences are:
>>> 
>>> * Partial-graph semantics, instead of complete-graph semantics.  This
>>> is more quad-like, and may be seen as more in keeping with RDF's usual
>>> style of working with partial knowledge.   It makes it harder to reason
>>> about what's unsaid, but few people are doing that anyway.  
>>> 
>>> * A keyword "@union" may be given instead of the default graph,
>>> indicating the default graph is the union of all the named graphs.  This
>>> means everything in those graphs is asserted.    (Alternatively, we
>>> could have "@asserted", perhaps parameterized by "all" or the names of
>>> those graphs which are considered asserted.)
>> 
>> 
>> I am not sure I fully understand it. Is it so that I can use @union as part of the declarations meaning that the default graph is everything that is explicitly noted as default graph plus the content of the named graphs? You say 'instead', which seems to suggest that 
>> 
>> @union
>> 
>> { a b c }
>> 
>> is not legal...
> 
> The idea here is that EITHER @union OR the default graph could be given
> in a trig document.   This corresponds to SPARQL's
> http://www.w3.org/TR/sparql11-service-description/#sd-uniondefaultgraph
> and matches how some SPARQL engines (eg 4store) work.
> 
> If you want to add a few more triples to the default graph, when using a
> union dataset like this, you put them in another named graph.

Hm. I am not sure I understand this restriction. It forces the user to come up with a URI or a blank node for no good reason. Why is it a problem to say that if I say:


@union .
{ a b c }
d { e f g }
h { i j k }

then the default graph is 

{ a b c .
  e f g .
  i j k .
}

What is wrong with that? It is only a shorthand...

Ivan


> 
>>> 
>>> * A class rdf:GraphAssociate containing all the things denoted by RDF
>>> terms used as labels in datasets.     The label is an IRI or bNode, the
>>> "associate" is the thing that IRI or bNode denotes.   The associate is
>>> associated with the given graph.  This is a superclass of rdf:Graph,
>>> because graphs have themselves as associates.   (I wouldn't mind a
>>> better word, but haven't thought of one.)  
>>> 
>>> * A class rdf:GraphContainer, a subclass of rdf:GraphAssociate.  A
>>> GraphContainer differs from a Graph in that conceptually it can change
>>> over time.   [We don't say anything about how to deal with it changing
>>> over time, because (so far) RDF never talks about change-over-time.  If
>>> it did (such as with rdf:starting and rdf:ending predicates) then that
>>> solution would apply here as well.]   The trig document "{ <u> a
>>> rdf:GraphContainer} <u> { <a> <b> <c> }" is true at exactly those times
>>> that the Graph Container identified by "u" contains the triple expressed
>>> as "<a> <b> <c>".    [Note well: I did not say "contains ONLY" that
>>> triple.  Because of partial-graph semantics, the document is also true
>>> if <u> also contains some other triples.]
>> 
>> Does it also mean that dereferencing <u> through HTTP would return a serialization of a graph containing (<a> <b> <c>)? (At the moment, there is nothing about that for rdf:Graph.)
> 
> That's how I would read Web Architecture, yes.   I'm not sure if we want
> to go there explicitly or not.   I'm not sure we can forbid people using
> rdf:Graph this way. 
> 
>>> The rest of 6.1 remains the same, including global-scope bNode labels,
>>> bNodes allowed as graph labels, rdf:Graph, and rdf:hasGraph.   (I have
>>> an idea for 6.3, but I don't have time to think it through before
>>> today's meeting.)
>> 
>> Let us keep to one number a week:-)
> 
> Yeah...    But 6.3 was going to address this Graph vs GraphContainer
> point.  :-)
> 
>     -- Sandro
> 
>> Ivan
>> 
>>> 
>>>   -- Sandro
>>> 
>>> 
>>> 
>>> 
>> 
>> 
>> ----
>> Ivan Herman, W3C Semantic Web Activity Lead
>> Home: http://www.w3.org/People/Ivan/
>> mobile: +31-641044153
>> FOAF: http://www.ivan-herman.net/foaf.rdf
>> 
>> 
>> 
>> 
>> 
> 
> 
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf







Received on Wednesday, 25 April 2012 13:12:38 UTC

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