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Re: Issues danc-01 Re: 2 formalities in RDF concepts

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 5 Feb 2003 22:39:07 -0600
Message-Id: <p05111b02ba6793a56f85@[10.0.100.86]>
To: Jan Grant <Jan.Grant@bristol.ac.uk>
Cc: w3c-rdfcore-wg@w3.org

>On Mon, 3 Feb 2003, Dan Connolly wrote:
>
>>
>>  On Mon, 2003-02-03 at 16:13, pat hayes wrote:
>>  [...]
>>  > There are really two ways to go. We can say that RDF syntax (I'll
>>  > avoid the G-word for now) uses identifiers for blank nodes somehow;
>>  > or not, ie blank nodes are blank. If we go the first way, then the
>>  > concept of 'same graph' is effectively useless, since a graph with a
>>  > systematic change to its bound variables is semantically
>>  > indistinguishable from the original; and we have to talk everywhere
>>  > about equivalence classes of graphs, in effect, or isomorphism
>>  > between graphs. Also there are issues about scopes of these bound
>>  > variables, which we put to bed 18 months ago and which it would be a
>>  > chore, to put it mildly, to re-open at this stage. We decided in
>>  > Sebastopol to go the second route, ie to identify graphs with
>>  > indistinguishable blank nodes and to therefore not bother with
>>  > notions of renaming, bound variable and so on.
>>
>>  Yes, that's the issue as I see it.
>>
>>  > My own preference is that we retain the advantages of this second way
>>  > of doing things and re-write the concepts document wording to reflect
>>  > the decisions we made in Sebastopol and which we have been using as a
>>  > basis for our discussions ever since.
>>
>>  OK by me. That was my understanding when I sent the comment
>>  that resulted in danc-01.
>>
>>  I didn't realize we could trace it to a WG decision but upon
>>  review, it's pretty clear from the record:
>>
>>  "It was agreed that Pat would update the model theory based on the graph
>>  instead of n-triples."
>>   -- http://www.w3.org/2001/sw/RDFCore/20010801-f2f/
>>
>>  Jan? Jeremy? Graham? Others? What say you?
>
>I say, there are a bunch of equivalent representations of the same
>thing: whether you consider "a graph" to _be_ or just to be _represented
>by_ a set of triples, or an ntriples document, doesn't really make a
>huge difference, except that if you start getting really pedantic about
>it then you have to over-qualify the language in the statements and
>proofs of all your lemmas.

Well, if you can suggest a way to word things so that the proofs can 
be written without being  precise about this one way or the other, 
I'd be grateful for any suggestions. Compared to the almost exquisite 
detail we are getting into over the minutae of the syntax of XML 
literals, this seems like a pretty important issue.

>In other words: avoid extra effort which provides no real benefit.

Its not that simple. The issue is whether or not we have to 
distinguish two pieces of RDF which differ only by renaming their 
bnodes. In one definition of graph we do, in the other we do not. 
This makes a substantial difference to a lot of places in the 
semantics document. It is also a pedagogic issue: the WG spent a lot 
of time arguing in confusion about whether or not bnodes were the 
same as anonymous urirefs, for example. We got this cleared up, but 
it can easily re-open again if we are careless about it. And what is 
the problem, in any case? Defining a graph to be a set of triples is 
about the simplest definition imaginable.

Pat

>
>--
>jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/
>Tel +44(0)117 9287088 Fax +44 (0)117 9287112 http://ioctl.org/jan/
>If you have received this email in error, do whatever the hell
>you want with it. It's not like I can stop you anyway.


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Received on Thursday, 6 February 2003 12:57:52 EST

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