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Re: graphs and documents Re: [ALL] agenda telecon 14 Dec

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 13 Dec 2011 22:43:37 -0600
Cc: David Wood <david@3roundstones.com>, Guus Schreiber <guus.schreiber@vu.nl>, RDF WG <public-rdf-wg@w3.org>
Message-Id: <CB4689C8-3EE4-4489-A985-3F3A6DF85A2E@ihmc.us>
To: Tim Berners-Lee <timbl@w3.org>
Hmm, slight quibble in the middle, and a big question at the end.

On Dec 13, 2011, at 9:59 PM, Tim Berners-Lee wrote:

> I'm afraid I must correct this.
> Apologies to those who have heard my definitions many times.
> 
> On 2011-12 -13, at 20:36, Pat Hayes wrote:
> 
>> 
>> On Dec 13, 2011, at 5:29 PM, David Wood wrote:
>> 
>>> Hi all,
>>> 
>>> I had a lengthy conversation with TimBL about named graphs at the LEDP Workshop [1] last week.  Briefly, he feels that the semantics for named graphs should work like this:
>>> 
>>> - An RDF Graph is named via a URI.
>> 
>> OK so far...
> 
> Well, actually the URI denotes a document, but there is a 1:1 relationship (log:semantics)
> mostly between documents and graphs here.

Right, but blech for that choice of name for the relationship. But whatever. 

> 
> The same URIs can be used I think in SPARQL after the "GRAPH" keyword
> because the GRAPH keyword uses the document's URI to
> indicate which graph.
> 
> In my language,  (1 2) is a list,  { ex:s ex:p ex:o }  is a graph, "foo bar" is a string, and 3.14159 is
> a number

Right

> and I don't  say that URIs formally denote any of those immutable data values.

Well, they *could* denote them. I think you mean (?) we can't rigidly attach a URI to a specific immutable value like this. (Other than by using equality and a literal, or some similar construction.) 

> 
> You can say
> 
> 	 ex:pi  =  3.145926                  
> 
> which means that whatever ex:pi denotes it is equal to 3.145926.

Which is just another way of saying, 'ex:pi' denotes 3.145926, right? The number, that is. The URI denotes the number (in every interpretation which makes the equation true, to be ever so exact about it.)

> (Now, for systems which understand =, this means they can use ex:pi
> most places instead of  3.145926 in mathematical formuale
> and so in fact can treat ex:pi as denoting 3.1415926,
> even though in the basic RDF graph language, ex:pi doesn't denote
> 3.1415926.

I dont think you should say that it DOESNT denote it. It might denote it, and it might denote something else. But its not prohibited from denoting a value any more than it is forced to denote it. 

> )
> 
> and you can say 
> 
> 	<#g1> =  {  ex:s ex:p ex:o }
> 
> which you can read loosely as "in this document we use local symbol
> g1 to denote [something which is equal to] the graph {  ex:s ex:p ex:o }.
> 
> I would NOT say
> 
> 	<> =  {  ex:s ex:p ex:o }                                       X NO
> 
> because <> is this document and  {  ex:s ex:p ex:o } is a graph,

Right, but...

> nor would I say 
> 
> 	<http://www.w3.org/2011/12/13-foo.n3> =  {  ex:s ex:p ex:o } .           X NO

...why not? Isnt this just like the ex:pi = 3.145962 case? 

Or do you want to *never* say that a URI names a graph 'directly', but only via an intervening document? That is, we always have 

URI  ---names/denotes--->  Document ---log:semantics---> RDF graph

never 

URI ---names/denotes---> RDF graph

?? But we can have 

graph literal ----names/denotes----> RDF graph

is that right? Because this is different from the original  'named graph' proposal, which was designed to allow the second case above (ie 'direct' naming of a graph) as the primary case. 

Is there any reason why we should not allow 'direct' naming of graphs? It seems like a useful thing to be able to do. For example, if I sign a graph, I would rather like to be sure that it can't get altered later on. 

> 
> I would say
> 
> 	<http://www.w3.org/2011/12/13-foo.n3> log:semantics {  ex:s ex:p ex:o } .  
> 
> where log:semantics is the relationship between a document
> and the n3 graph whose meaning is the meaning of the document
> and which on a good day you can get by looking up the document
> on the web and parsing which you get back. 

OK, so 'document' in all this has a 'fixed' or 'static' connotation that is different from what we have been calling a 'graph container' or a g-box, because a g-box can have its state altered without changing its identity, so its log:semantics mapping is labile and time- or state-dependent. Right? Or do you want to allow 'dynamic' documents which are (of course) resources, rather like the NYTimes which sends back a different graph, sorry, front page, every day ?  Put another way, is a document a g-box or a g-text?  Or maybe can it be either?? 

Pat



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Received on Wednesday, 14 December 2011 04:46:54 GMT

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