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Re: Comments on RDF Concepts and Abstract Data Model

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 20 Nov 2002 09:58:30 -0500 (EST)
Message-Id: <20021120.095830.45193145.pfps@research.bell-labs.com>
To: jjc@hpl.hp.com
Cc: www-rdf-comments@w3.org

From: Jeremy Carroll <jjc@hpl.hp.com>
Subject: Re: Comments on RDF Concepts and Abstract Data Model
Date: Mon, 18 Nov 2002 15:59:12 +0100

> Hi Peter
> I am responding to some of your comment
> http://lists.w3.org/Archives/Public/www-rdf-comments/2002OctDec/0053.html
> in particular:
> [[
> Major comment:
> The RDF graph is syntax.  As such it makes no sense to define a notion
> of equality over literals, which are pieces of syntax.  It is just as
> if one wanted to defined equality in C by defining it over pieces of a
> C program.  Similarly, it makes no sense to define equality of nodes
> or triples.
> ]]
> The new version
> http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/
> continues to define equality over literals.
> I believe this is helpful and do not intend to change it, but am open to
> further discussion.
> The uses that the WG has found for such notions are:
> + in the test cases
>   Without a defined notion of equality between literals, we would not have a
> defined notion of equality between graphs, which is necessary for the test
> cases.

First, the new test cases working draft (of 20021112) does not even contain
the string 'equal', so I don't see how a notion of equality helps in the
test cases.  

Second, I note that the test cases working draft does, however, talk about
graph isomorphism.  I maintain that this notion is a *much* better notion
for use here, particularly as it cannot be abused by developers and users
to give cover to illegal uses of syntactic (un-)similarity as semantic

> + in the semantics. Without clarity about the nature of the syntactic
> objects that the semantics are defined over, it seems difficult to know what
> the semantics may be about. 

The semantics is defined on syntactic structures sure, so it needs to know
what these syntactic structures are.  However, there is generally no need
to know whether two syntactic structures are identical - instead all that
is needed is the mapping from syntactic structures to semantic meaning.

> Your example of a C program is uncompelling
> because it is usually taken as unproblematic what the underlying syntactic
> objects are. All programming languages have to decide whether they are case
> sensitive or not, which is the sort of level at which I perceive the literal
> equality rules.

Not so, programming languages have to provide a mapping from their syntax
to their semantics (however this is couched).  If, for example, a
programming language had case-insensitive *strings*, then the only notion of
string equality it would support should be this case-insensitive one - any
notion of comparing the surface syntax of strings would be meaningless for
this language.  Similarly, RDF should not define a notion of equality
(which has strong semantic connotations) over its syntactic structures,
particularly if this notion is not the same as semantic identity.  If RDF
needs some notion of identity for its syntactic structures, then this
notion should be referred to by some other word, such as isomorphism.

> I fear that this message is at cross-purposes with your point. If so, please
> clarify.

> Thanks greatly for your interest, and for your timely comments on our work.
> Jeremy

Peter F. Patel-Schneider
Bell Labs Research
Received on Wednesday, 20 November 2002 09:58:40 UTC

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