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RE: motivation for bNodes/existentials in RDF; note for parsers

From: Massimo Marchiori <massimo@w3.org>
Date: Fri, 5 Apr 2002 04:55:15 +0200
To: <w3c-rdfcore-wg@w3.org>
Cc: "Massimo Marchiori" <massimo@w3.org>
Message-ID: <NGBBJNKIMLOPPCFHEJEMEEOMCAAA.massimo@w3.org>
This thread is getting overloaded, as many concurring issues are
overlapping (and many replies are coming... ;).
Rather than parallelizing issues on a pile, I'll just try to focus
on the major one.


Let's look at the charter:
<charter>
Implementor feedback concerning the RDF Model and Syntax Recommendation
points to the need for a number of fixes, clarifications and improvements to
the specification of RDF's abstract model and XML syntax. There is also
considerable interest in the exploration of alternative XML serialization
mechanisms for RDF data. The role of the RDF Core WG is to prepare the way
for such work by stabilizing the core RDF specifications. The RDF Core WG is
neither chartered to develop a new RDF syntax, nor to reformulate the RDF
model. However, the group is expected to re-articulate the RDF model and
syntax specification in such a way as to better facilitate future work on
alternative XML encodings for RDF.
</charter>

What this means is that the "minimum to declare victory" has to be done (to
"prepare the way for such work",
but not to normatively *do* all such work).
If along this process, bugs or ambiguities or absolute-needs arise, then
modifications can occur, but
they must have bullet-proof justifications.

Now, back to the RDF M&S and to what it means. Forget about the "S" part,
and focus on the "M".
We all agree the word "model" is rather unfortunate in the M&S.
The "M" in M&S defines essentially an *algebraic model*. Then, it hints
on the possible meaning it could have (alas, or luckily ;), staying at
natural language level, without formalization.

So, what should be done, according to the charter, is fixing and formalizing
the
algebraic model (anonymous nodes first belong in this realm), and formally
stating what the natural language semantic descriptions present in M&S say.
Everything beyond is at risk, and ought to have very good justifications on
why
it has to go into this "core fix", and not just in the later "such work" the
charter talks about.

Now, the concern here is that the anonymous node --> existential variable
is more than the "minimal fix", and is in fact at risk to belong to the
"such work" next part.
It might not be the case, but for public accountability, there should be
very good reasons
on why we have necessarily to do this.
Quoting from one of the replies:
   The definitive statement in M&S is (para 41)

http://lists.w3.org/Archives/Public/www-archive/2001Jun/att-0021/00-part#41
   [[[
   The sentence above does not give a name to that resource; it is
anonymous,
   so in the diagram below we represent it with an empty oval
   ]]]
but in fact, the reality is more complex. Anonymous nodes are not
"first-class"
in M&S, they only carefully appear in some places (like containers and
reifications),
where in fact naming such a node would just be superfluous. The "minimal
fix" would
be therefore just to better state what/how anon nodes are/behave in the
algebraic model
(because, they then disappear at the semantic level).
Now, the anonymous node -> existential route follows three extensions that
depart from
the minimal fix of M&S:
a) grant anonymous node first-class status in the RDF algebraic model
which in turn implies that
b) you have to deal with an algebraic model where anonymous nodes can occur
everywhere
which leads to the
c) you have to define what the semantics of these "extra" (wrt M&S)
algebraic models are.

This extension chain has then to be justified as absolutely needed, in order
to be brought
back into the current charter and not just left to future extensions.
Note, not arguing one can't show it's needed, just asking somebody should
show it's
absolutely needed.
[Incidentally, note the current MT doesn't tackle containers and reification
 (useful exercise: show how much this depends by the departure from M&S with
a) ... ;) ]

Once this is clarified, we can go deeper with the other issues touched by
the thread.

Thanks,
-M

ps Sorry if this is making losing cycles to the wg. I think it's a very
serious issue, but
I also understand (by experience ;) the possible "gee, we did this analysis
already some time ago".
So, if the above question has already been analyzed and answered, I'm well
willing to accept
specific pointers, so to save other people's time (in fact, that's in
general one of the
big advantages for public accountability to have open wg's, unlike XML-Query
or P3P... sigh).

pps Brian, DanB, if you want me not to use w3c-rdfcore-wg and to use
www-rdf-comments@w3.org instead
raising a formal issue there, please tell me at any moment without any
problems (I think I'm excused as
DanC brought me in this thread, but it's also true that if this goes on
much, it might be as an abuse
of the mailing list).
Received on Thursday, 4 April 2002 21:56:06 EST

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