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

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Fri, 5 Apr 2002 09:30:34 -0600
Message-Id: <p05101535b8d3701e14ed@[65.217.30.94]>
To: "Massimo Marchiori" <massimo@w3.org>
Cc: Brian McBride <bwm@hplb.hpl.hp.com>, www-rdf-comments@w3.org
>Hi Massimo,
>
>Let me try to address your concerns here:
>
>At 04:55 05/04/2002 +0200, Massimo Marchiori wrote:
>>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.
>
>This would appear to be an assertion about your interpretation of 
>the charter.  Fair enough.  Does clarifying the meaning of M&S's 
>"anonymous resources" fall within your class of "bugs, ambiguities 
>or absolute needs"?  i.e. do you think we had to say something about 
>them?
>
>>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*.

Really?? That is the first time I have ever heard that view 
expressed. In your view, then, RDF should be seen as an algebra 
rather than as an assertional language, is that what you are saying? 
Can you give us any more details of the kind of structure you feel it 
should have?

>>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",
>
>Says you.  However, others disagree.
>
>>  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.
>
>The properties of the so called "anonymous nodes" of m&s had long 
>been under discussion.  Clarification of what they meant was 
>necessary and the job of the RDFCore WG.  In clarifying the 
>specification, the WG took into account the various readings of M&S 
>and also what the WG thought the best answer was.  After long and 
>difficult discussion, the answer reached is the one described in the 
>current model theory WD.
>
>>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.
>
>Where in M&S does it state the limited roles that anonymous 
>resources can play?  Various implementors, based on their 
>understanding of M&S, have implemented systems (Jena, Redland, 
>SiRPAC) which allow more general use of blank nodes than your 
>interpretation would allow.
>
>>  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).

Again, there seems to be a strong but unstated assumption here about 
the relationship between 'algebra' and semantics. Anything in the 
algebra disappears at the semantic level?? So you are not talking 
here about an algebraic semantic theory in the style of Tarski, I 
take it.  You are also not talking about syntax as an algebra. I am 
even more at a loss to follow what you mean by 'algebraic' in this 
context.

>>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

What does this mean? For example, what would it mean to NOT have 
'first-class status'? Anonymous nodes are part of the RDF syntax, 
surely that is clear from even a casual reading of the M&S. You seem 
to think that the WG took a decision here, but I see no alternative 
path we could possibly have taken.

>>which in turn implies that
>>b) you have to deal with an algebraic model where anonymous nodes can occur
>>everywhere

We did not think of the language as 'algebraic' in any sense that I 
can understand.

Any clarification of your views would be useful. Right now it is hard 
for me to evaluate your arguments.

Pat Hayes
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Received on Friday, 5 April 2002 10:31:32 GMT

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