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

From: Graham Klyne <GK@NineByNine.org>
Date: Tue, 04 Feb 2003 17:24:11 +0000
Message-Id: <>
To: pat hayes <phayes@ai.uwf.edu>
Cc: Dan Connolly <connolly@w3.org>, w3c-rdfcore-wg@w3.org

I agree with Pat here.  Updating the concepts document description seems 
like an easier, and less error-prone, option here.


At 04:13 PM 2/3/03 -0600, pat hayes wrote:

>>On Tue, 2003-01-28 at 12:07, Dan Connolly wrote:
>>>  On Tue, 2003-01-28 at 11:22, Jeremy Carroll wrote:
>>>  > Hi Dan,
>>>  >
>>>  > before the WG discusses this issue I wanted to understand your concern.
>>>  >
>>>  > I believe that you think the concept described in
>>>  >
>>>  > 6.3 Graph Equality
>>>  > http://www.w3.org/TR/rdf-concepts/#section-graph-equality
>>>  >
>>>  > is one that is relevant to RDF and should be described in RDF 
>>> Concepts, but
>>>  > that you take issue with it being labelled "Equality". Earlier 
>>> drafts called
>>>  > this concept "Isomorphism"
>>>  Well, that's a different way to resolve this matter, but
>>>  the semantics doc doesn't use that notion of graph;
>>>  it uses the notion of graph where the n-triples
>>>  documents below definitely correspond to the
>>>  identically same graph, even if the sets
>>>  of triples don't share a blank node.
>>>  So going that way would involve a change to
>>>  the semantics doc that might be quite significant.
>>After discussion with JanG, I think perhaps the change
>>to the semantics doc might not be so significant.
>>Just strike this bit:
>>Graphs with isomorphic pictures in this sense are considered to be
>>identical; this means that we do not distinguish sets of triples which
>>differ only in the identity of their blank nodes. This slight abuse of
>>terminology allows us to simplify the presentation by ignoring questions
>>of re-naming of bound variables.
>>since, in fact, in the definition of merge, we do in fact treat
>>re-naming of bound variables.
>Peter was right. I can make this change if y'all prefer but it will have 
>other consequences throughout the document where I use this assumed 
>identity. I will have to re-state and re-prove several of the lemmas. I 
>honestly think this would be a mistake. Still, if it helps to keep things 
>straight then I am willing to do this. Expect it to add about a week to 
>the time to a final version of the semantics doc.
>BTW, if we make this change then we will effectively have restored the 
>'problem' which gave us such trouble at the California F2F about 18 months 
>ago, and which we solved by adopting the graph syntax as definitive. In 
>effect, we will have tossed away the graph syntax and replaced it with the 
>N-triples syntax, thereby re-introducing bound variables and the need to 
>refer to re-naming operations on merging. We then ought to go back through 
>all the documents and purge all references to blank nodes as having no 
>names, since they will have names.
>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.
>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.
>>Also, add graph to the list of terms imported from concepts.
>>We use the following terminology defined there: uriref, literal, plain
>>literal, typed literal, blank node and triple.
>I am willing to do this IF the concepts doc is rewritten to say clearly 
>and unambiguously that an RDF graph *is* a set of RDF triples, rather than 
>any kind of 'graph' in any sense from mathematical graph theory. Otherwise 
>I would prefer that the definition in the concepts document be replaced 
>with a reference to the semantics document.
>>I do think the term "Graph Equality" is misleading; I'd prefer
>>Graph Equivalence for the RDF Concepts heading.
>I disagree. If we stick to our guns here, we really do mean graph 
>equality. It is up to us to define 'graph' so that equality comes out 
>right, and we know how to do that.
>We have had this worked out now for a very long time. Why screw it up at 
>this stage?
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Graham Klyne
Received on Tuesday, 4 February 2003 12:48:10 UTC

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