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

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Wed, 3 Apr 2002 11:46:14 -0600
Message-Id: <p0510150eb8d0ee7f506c@[]>
To: "Massimo Marchiori" <massimo@w3.org>
Cc: w3c-rdfcore-wg@w3.org, "Lynn AndreaStein" <las@olin.edu>
>  > -----Original Message-----
>>  From: Dan Connolly [mailto:connolly@w3.org]
>>  Sent: Saturday, March 23, 2002 3:59 AM
>>  To: Pat Hayes
>>  Cc: w3c-rdfcore-wg@w3.org; Massimo Marchiori; Lynn AndreaStein
>>  Subject: motivation for bNodes/existentials in RDF; note for parsers
>>  Pat, Dave,
>>  I just explained to another colleague of
>>  mine, Massimo, that RDF formulas are not
>>  just sets of ground facts...
>>    (and
>>      (p1 s1 o1)
>>      (p2 s2 "lit2")
>>      ...
>>     )
>>  but that RDF can express existential quantification...
>>   (exists (?b1 ?b2 ?b3 ...)
>>    (and
>>      (p1 s1 ?b1)
>>      (p2 ?b2 "lit2")
>>      ...
>>     )
>In these kind of sentences I always have to argue with Dan, as
>he always says so and I always reply in the usual way...
>So, restating the above, the *current version of the
>RDF Model theory* states that the interpretation of RDF
>ought to be ... bla.
>This is important to remember, as it's a fundamental design choice
>that it's going to be decided, but it's not present in the
>"normative RDF" (M&S) document.

Well, the normative M&S seems pretty clear about the intended status 
of anonymous nodes. I can see why, for some purposes, it would be 
convenient if anonymous nodes had hidden labels; but that 
interpretation is wishful thinking imposed on the M&S, which is about 
as unambiguous as it is possible to be without actually giving a 
formal model theory. Those blank nodes in the diagrams really are 
blank, right?

>  > Have you added some explanation about that
>>  to the model theory spec?
>I think what concerns me most, in this context, is the appropriate
>explanation of why this is the choice to make, versus the most
>obvious choice we have at our disposal (i.e., skolemization).

I fail to see why you think that skolemization is an 'obvious' 
choice, or why you think it is 'easier'. Logically, skolemization is 
quite complicated. It isn't valid, for example, and it blocks several 
useful intuitive inferences. It would be impossible to express 
queries in RDF if it had no variables, for example.

>That is, what are the pro's and con's that favour the existential
>approach vs the skolem one?
>AFAIK the second one has been so far the natural choice (the
>"understood standard" if you want ;), for some good reasons.

Which are? (I know some, but I wonder if you have others.)

Pat Hayes

>So, summing up, since this is a fundamental architectural decision
>(not just syntactic sugaring) concerning RDF, what is most interesting
>is to give the reasons for this interpretation vs the easiest skolem one.
>Yes, it's a classic "last call" fundamental question, because that spawns
>into the data model, on which there are many things to discuss, but well,
>Dan brought the matter up early ;)
>ps Apologies if the pro-con analysis and motivation has already been posted,
>if so I probably missed it (not an unlikely possibility).

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Received on Wednesday, 3 April 2002 12:46:14 UTC

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