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

From: Dan Connolly <connolly@w3.org>
Date: Tue, 28 Jan 2003 16:36:44 -0600
To: Jan Grant <Jan.Grant@bristol.ac.uk>
Cc: Jeremy Carroll <jjc@hpl.hp.com>, RDFCore Working Group <w3c-rdfcore-wg@w3.org>
Message-id: <1043793404.29622.200.camel@dirk.dm93.org>

On Tue, 2003-01-28 at 15:59, Jan Grant wrote:
> On Tue, 28 Jan 2003, Dan Connolly wrote:
> 
> > JJC wrote:
> 
> > > see:
> > > 3.6 Graph Equality
> > > http://www.w3.org/TR/2002/WD-rdf-concepts-20020829/#xtocid103648
> > > (which uses both terms)
> > >
> > > If I have two N-triple files of one line each
> > >
> > > <eg:a> <eg:b> _:x .
> > >
> > > and
> > >
> > > <eg:a> <eg:b> _:y .
> > >
> > > Do you think they are equal or unequal?
> >
> > Those two files are clearly distinguishable, hence
> > they are not equal.
> >
> > > Personally, I would say they are equal as RDF graphs, and unequal as text
> > > documents.
> >
> > "as RDF graphs"... I'm not sure what you mean by that, formally.
> > I don't think you mean that N-triples file are RDF graphs.
> > I think you mean that there's a straightforward correspondence
> > from N-triples files to RDF graphs, and that the
> > two distinct N-triples files above correspond to the
> > same RDF graph.
> 
> Jeremy's talking about *syntactic* equality (ie, "X equals Y" means
> "every expression involving X can be rewritten with Y substituted for it
> and the expression's value is preserved"),

Hmm... I wasn't familiar with that idea before... I'll have
to think it over.

But my intuition says the difference between syntactic
equality and identity matters to the RDF spec.

For example, if X and Y are distinct graphs that
art syntactically equal, what's the cardinality
of the set {X, Y}? It's 2, right?

The model theory spec does stuff like putting
graphs into sets, and I think it matters what
the cardinality of the resulting set is;
if X and Y are the graphs arising from
the n-triples document jeremy gave as
an example, I think the model theory
spec depends on the cardinality of {X, Y} being 1.

But I'm not certain. It could be that it doesn't
matter.

>  not identity (X and Y can be
> equal but not identical; if X and Y are identical then they're equal).
> This terminology is used (in my experience) by some mathematicians but
> not all; depends on the field and personal choice. In the sense it's
> used it's ok (I think) so it comes down to choosing a terminology and
> sticking to it. If this is going to cause wide-scale confusion it might
> be better to revert to couching this in terms of "isomorphism" which is
> a less overloaded term.
> 
> jan
-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 28 January 2003 17:37:26 EST

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