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RE: Comments on the new RDF Test Cases draft

From: Massimo Marchiori <massimo@w3.org>
Date: Wed, 22 May 2002 09:58:13 -0400
Message-Id: <200205221358.JAA10254@tux.w3.org>
To: cmjg@engarde.ioctl.org, phayes@ai.uwf.edu, www-rdf-comments@w3.org
Cc: massimo@w3.org

Replying to both Jan's and Pat's in one email.
There were essentially two points expressed in [1], 
all coming from the fact that probably the test cases draft has been underestimated,
as it touches an extremely important issue like conformance to RDF.
The two points can be so summarized:
a) lack of formalization
b) problem of what is the correct notion of conformance

On a), once raised the point (and noted that even, according to the standard
definition, the previous statement was not only inaccurate, but even incorrect),
there's not much to say. Whatever we decide as per b), it has to be formalized,
unless we want to go back to the errors made with the previous RDF (....).

[incidental note: this comes from the fact that despite "graph" is of common
use for RDF, the only formalization of RDF graph is the one currently in the MT, 
which is not what we meant here, and that certainly necessitates of a formalization
of the concept of isomorphism. This is related to the "rectangles and ovals" point
raised in [2].]

[second incidental note on formalization attempts: please be careful to formalize
really everything, it's easy not to be complete...]

Then, onto b), which is the real core of the issue.
The "easy fix", apparently, is just to define a classic isomorphism on rdf graphs
"as we mean it", i.e., up to blank node renaming (like Pat and Ian suggests, and as
it was already in the intention of the original Test Cases text).
But there's a deeper issue here, and it is whether it is in fact more appropriate
or not to define conformance as semantical equivalence (cf. [1] and [3]).
Like Pat realizes in [4]: 
<quote>
The trouble with using semantic criteria in the syntax is that you 
have to specify WHICH semantics you are using. Does it have to be 
simply equivalent, rdf-equivalent, or rdfs-equivalent?
</quote>
and that's, yes, one of the parts I was hinting at when raising the issue in [1], cf
<quote>
There are possible related issues of conformance here, but that's the 
only major error (or issue, depending... ;) I could find.
</quote>

So bare-bones, suppose an RDF parser digests one of the test cases, and produces
all the triples we expect to (as per the "minimal interpretation" currently 
understood in the Test Cases), plus the following triple:
[rdf:type] [rdf:type] [rdf:Property] .
Is it compliant to RDF, or not?
The set of triples that have been produced are virtually indistinguishable 
according to the Model Theory (which is the *meaning* of a graph, independently
of its syntax). 
So, if we limit ourselves to the current Test Cases interpretation, we are
relying on the syntactical structure rather than on the semantical one, 
which is something I find very unelegant, and to some extent even logically broken.
Said this, yes, this is opinionable and no one (included me) will scream loud 
if the "syntactical equivalence" is used rather than the semantical one.
However, I do think it's more elegant to go the semantic way, and can't really
see many advantages to go for the syntactic way.

Now, if (if) the semantical way is the chosen one, there's the further issue
of what semantical equivalence are we talking about: RDF or RDFS. 
But this is part of another big issue (a la [1]) that depends on whether we go syntactic
or semantics, so there's no point in expand this email any further now ;)

Syntactic or semantics?
-M

[1] http://lists.w3.org/Archives/Public/www-rdf-comments/2002AprJun/0081.html
[2] http://lists.w3.org/Archives/Public/www-rdf-comments/2002AprJun/0077.html
[3] http://lists.w3.org/Archives/Public/www-rdf-comments/2002AprJun/0083.html
[4] http://lists.w3.org/Archives/Public/www-rdf-comments/2002AprJun/0094.html
Received on Wednesday, 22 May 2002 09:59:34 GMT

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