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Re: New Proposal (6.1) for GRAPHS

From: Eric Prud'hommeaux <eric@w3.org>
Date: Thu, 29 Mar 2012 16:11:16 -0400
To: Pat Hayes <phayes@ihmc.us>
Cc: Sandro Hawke <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org>
Message-ID: <20120329201112.GJ2834@w3.org>
* Pat Hayes <phayes@ihmc.us> [2012-03-29 10:47-0500]
> Sandro
> 
> First, congratulations on expalining the idea so elegantly (I will try to take a style lesson from you). But I don't think your neat idea for defining the class rdf:Graph actually can be made to work in the way you want. See below.
> 
> 
> On Mar 27, 2012, at 9:23 PM, Sandro Hawke wrote:
> 
> > I've written up design 6 (originally suggested by Andy) in more
> > detail.  I've called in 6.1 since I've change/added a few details that
> > Andy might not agree with.  Eric has started writing up how the use
> > cases are addressed by this proposal.
> > 
> > This proposal addresses all 15 of our old open issues concerning graphs.
> > (I'm sure it will have its own issues, though.)
> > 
> > The basic idea is to use trig syntax, and to support the different
> > desired relationships between labels and their graphs via class
> > information on the labels.  In particular, according to this proposal,
> > in this trig document:
> > 
> >   <u1> { <a> <b> <c> }
> > 
> > ... we only know that <u1> is some kind of label for the RDF Graph <a>
> > <b> <c>, like today.  However, in his trig document:
> > 
> >   { <u2> a rdf:Graph }
> >   <u2> { <a> <b> <c> }
> > 
> > we know that <u2> is an rdf:Graph and, what's more, we know that <u2>
> > actually is the RDF Graph { <a> <b> <c> }.  That is, in this case, we
> > know that URL "u2" is a name we can use in RDF to refer to that g-snap.
> > 
> > Details are here: http://www.w3.org/2011/rdf-wg/wiki/Graphs_Design_6.1
> 
> From there:
> 
> We define the class rdf:Graph such that for its instances, the rdf:hasGraph relation is the identity relation. That is, a Graph hasGraph itself.
> 
> [edit]Test
> @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#
> >
> { <u1> rdfs:comments "A good graph", a rdf:Graph. }
> <u1> { <a> <b> <c> }   # u1 *is* this graph
> <u2> { <a> <b> <c> }   # u2 merely *has* this graph
> 
> DOES NOT ENTAIL
> 
> { <u2> rdfs:comments "A good graph" }
> 
> ......
> 
> But it does entail that. The relation is on the entailed objects, not on the IRIs, right? So that first quad says that what <u1> denotes, let me write I(<u1>) for that, and the graph { <a> <b> <c> }, are actually the very same thing: I(<u1>) =  { <a> <b> <c> }. And this is so because  I(<u1>) is in the class rdf:Graph. Which is the same as saying that {<a> <b> <c>} is in that class (because these are the very same thing.) So now look at the second quad. That says that the rdf:hasGraph relation holds between I(<u2>} and {<a> <b> <c>}, and we know that the second of these is in the class rdf:Graph. So, the rdf:Graph relation on it is the identity relation, so I(<i2>) = {<a> <b> <c>} as well. 
> 
> This follows because you have made the criterion be membership of the denoted thing in a class. As soon as you do that, you lose any way to distinguish between binary cases based on one of the argument IRIs. 

What if { <a> <b> <c> } isn't the same thing as { <a> <b> <c> }? Apart from mathematical reflexes, do we have a reason to want to graphs to be the same? Taking the above Trig, in another order for the purposes of illustration:

<u1> { <a> <b> <c> }   # u1 *is* this graph
  gives me: <u1> :hasGraph Parse1{<a><b><c>} . where Parse1{<a><b><c>} is an instance of { <a> <b> <c> }

<u2> { <a> <b> <c> }   # u2 merely *has* this graph
  gives me: <u1> :hasGraph Parse2{<a><b><c>} . where Parse2{<a><b><c>} is an instance of { <a> <b> <c> }

{ <u1> rdfs:comments "A good graph", a rdf:Graph. }
  gives me Parse1{<a><b><c>} a :Graph . which magically ('cause something will have to be magical) asserts everything in { <a> <b> <c> }.

+ We never risk assigning properties to the platonic graphs, only our utterances of them.
- Every assertion we want to make about the triples in a platonic graph will need to dereference, which will look clumsy.

Is this as good as it gets?


> Contrary to what I said in the telecon yesterday, I now don't think there is any way out of this within the current RDF framework. Basically, you want to talk about the naming relation between a URI and a denotation, and you can't do that in a conventional  rdf-2004-style model theory. You need a small amount of referential opacity to make this work. We will have to change something to get that.
> 
> Pat
> 
> 
> 
> > 
> > That page includes answers to all the current GRAPHS issues, including
> > ISSUE-5, ISSUE-14, etc.
> > 
> > Eric has started going through Why Graphs and adding the examples as
> > addressed by Proposal 6.1:
> > http://www.w3.org/2011/rdf-wg/wiki/Why_Graphs_6.1
> > 
> >     -- Sandro (with Eric nearby)
> > 
> > 
> > 
> 
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> 
> 
> 
> 
> 

-- 
-ericP
Received on Thursday, 29 March 2012 20:11:48 GMT

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