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Re: New Proposal (6.1) for GRAPHS

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 29 Mar 2012 22:35:12 -0500
Cc: Sandro Hawke <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org>
Message-Id: <94F68506-88C3-4603-A1CD-69A18A1E9E36@ihmc.us>
To: Eric Prud'hommeaux <eric@w3.org>
OK, I got it. But I don't like it. With this picture, we *never* name a graph. We only name a particular "expression" of the graph, and every time we write out the graph, we get a different expression. I am left wondering what the point is of even having this? SPARQL datasets don't even have graphs in them any more. 

Pat

On Mar 29, 2012, at 9:01 PM, Eric Prud'hommeaux wrote:

> * Pat Hayes <phayes@ihmc.us> [2012-03-29 17:34-0500]
>> 
>> On Mar 29, 2012, at 3:11 PM, Eric Prud'hommeaux wrote:
>> 
>>> * 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> }?
>> 
>> Not sure I follow. An RDF graph is a set of triples. That is the same set of triples. OK, I guess we can call them graph-primes and say they are distinct. But now, when are graph-primes *ever* the same? 
> 
> Sorry, I should have edited (or at least read) my text. I intend them to be graph-primes, so they are never the same. When we want to assert equivalence, e.g. to say that the graph had by <u1> has the same triples as the graph had by <u2>, we need a predicate which includes that dereference.
>  { <u1> :isExpressionOfSameTheGraphAs <u2> }
> 
> 
> 
>>> 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> }
>> 
>> Again I don't follow. What is an instance of a set?
> 
> Hmm, s/instance/expression/.
> 
> Parse1 was also a poor choice; it's just some expression of a graph. In Trig, we associate labels with the representations of graphs. In the default graph, we make some assertions about those representations, in the form a a type arc on the label (sure, why not). One of those assertions is that the author of the Trig document intends those statements in the graph which has a representation which has the label <u1> to carry the same strength of assertion as the triples in the default graph.
> 
> 
>>> <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> }.
>> 
>> Why would that parse of it be parse1 rather than parse369 ? The graph is mentioned three time in the Trig in total, so if the second two are magically different, how come the first one is the same as the third? 
> 
> Ahh, I should not have used a small number. It's not any particular identifier, it's yet another representation of { <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?
>> 
>> You lost me a while ago, Im afraid. 
> 
> Let's see if this one helps.
> 
> 
>> Pat
>> 
>>> 
>>> 
>>>> 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)
>>>>> 
>>>>> 
>>>>> 
>>>> 
>>>> ------------------------------------------------------------
>>>> 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
>>>> phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>> 
>>> -- 
>>> -ericP
>>> 
>>> 
>> 
>> ------------------------------------------------------------
>> 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
>> phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
>> 
>> 
>> 
>> 
>> 
>> 
> 
> -- 
> -ericP
> 
> 

------------------------------------------------------------
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
phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
Received on Friday, 30 March 2012 03:35:45 GMT

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