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Re: New Proposal (6.1) for GRAPHS

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 29 Mar 2012 10:47:38 -0500
Cc: public-rdf-wg <public-rdf-wg@w3.org>
Message-Id: <95BB3B71-EB11-4F4A-9B0C-F8A4E89A7365@ihmc.us>
To: Sandro Hawke <sandro@w3.org>

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.

@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


{ <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. 

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.


> 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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Received on Thursday, 29 March 2012 15:48:14 UTC

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