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Re: RDF model revisited, or: How to make the most out of Reifications and Containers

From: Graham Klyne <GK@ninebynine.org>
Date: Fri, 01 Jun 2001 10:45:35 +0100
Message-Id: <5.0.2.1.2.20010601095455.00a54010@joy.songbird.com>
To: Wolfram Conen <conen@gmx.de>
Cc: www-rdf-logic@w3.org
I think your approach is constructive, and consistent with (e.g.) the 
discussion that Pat and I have had about the relationship between syntax 
and semantics.

I think the assumptions need closer examination;  your proposed approach 
means that additional assertions that reference a container- or 
reificant-resource are not possible in the extended model (or they become 
detached from the container/statement when your constructive proof of your 
proposition 2 is applied).  I think this does represent a change in the RDF 
model, but one which might be considered beneficial.

Concerning your assumption 2, it is not clear to me when you say "p relates 
s to (r1,...,rn)"  {{{NOTE:  I think that's what you meant, but you 
wrote  "[r1,...,rn]".}}} whether you mean p relates to the container, or p 
relates to the members of the container.  In existing RDF, this would apply 
to the container.

Test case (using an obviously RDF-equivalent subset of N3 as an easier syntax):

   ex:c1 a rdf:Bag ;
         rdf:_1 ex:m1 ;
         rdf:_2 ex:m2 ;
         ex:prop ex:val1.

   ex:c2 a rdf:Seq ;
         rdf:_1 ex:m1 ;
         rdf:_2 ex:m2 ;
         ex:prop ex:val2.

   ex:c1 ex:prop ex:val3.

This defines *two* containers (one rdf:Bag and one rdf:Seq) with different 
properties.
Using your proposed approach (to crudely extend the N3 syntax used above) I 
guess this might be achieved with something like:

   ( ex:m1, ex:m2 ) a rdf:Bag.
   ( ex:m1, ex:m2 ) a rdf:Seq.

The problem I see comes when one wants to *separately* make an assertion 
about one of those collections; i.e. how to say

   ex:c1 ex:prop ex:val3.

The resource 'ex:c1' that "stands for" the container has been removed in 
the extended model.  (I suppose you would say that the resource is still 
there, but the syntactic designator to which properties can be applied has 
been elided.)

** As I write, I think I've finally spotted the root of what's bothering me:
Your extended model is unclear about whether multiple occurrences of, say, 
( ex:m1, ex:m2 ) indicate the same resource of different ones.  Section 2 
seems to allow distinct occurrences to represent distinct resources R.  But 
the proof of proposition 1, footnote 5, says "a new symbol for each 
subexpression t that is not already represented in the flat model", which 
suggests that a single resource is generated for any given collection.  The 
proof of proposition 2, it is not clear to me whether or not "the 
expression <sigma> which r represents" is repeated if multiple resources 
stamnd for equivalent expressions.

#g
--

At 05:07 PM 5/31/01 +0200, Wolfram Conen wrote:
>Please, allow to point readers of this list that are interested in the
>RDF model, reification, or containers to a short discussion paper we
>wrote recently. Any comment is welcome (conen@gmx.de,
>klapsing@wi-inf.uni-essen.de). Below, you'll find the abstract and two
>additional remarks.
>
>
>Thank you for your kind consideration,
>       Wolfram and Reinhold
>
>
>TITLE: RDF model revisited, or: How to make the most out of Reifications
>and Containers
>
>AUTHORS: Wolfram Conen and Reinhold Klapsing
>
>ABSTRACT: The basic result presented is the following: with two
>(hopefully) reasonable assumptions about the intentions behind the RDF
>model, it can be shown that the RDF model and a model allowing for
>nested triple and lists of resources and triples, can be mapped to one
>another (in both directions). This allows to establish a close link
>between the RDF model and "extended" models recently suggested (Slim
>RDF, XRDF). Furthermore, it can be used to define the notion of
>essential models that allows to get rid of unneccessary triples in
>reifications and containers while still capturing the underlying
>intentions of the original RDF model.  This, in turn, may help to
>clarify some problems related to interpreting the roles of reification
>and Bag, Alt and Seq in the RDF model.
>
>LINK: http://nestroy.wi-inf.uni-essen.de/rdf/RDFmodel_revisited_v10.pdf
>
>
>Two additional remarks:
>
>* We do not propose to change the RDF model (I hope that you like to
>read this, Brian ;) -- instead, we "interpret" the RDF model in a
>certain "upwards compatible" way, allowing to "identify" reification
>with nesting and containers with lists.
>
>* Reification and containers are treated as instruments that can be used
>for automatic structural transformations. (I hope that using the word
>"structure" here is not too far fetched - Graham? Pat?). That means,
>that the more "sophisticated" uses of the "reification construct" in RDF
>would have to be defined (precisely/formally) on a semantic/logic level
>(on the "schema" level, to use RDF speak). The same is true (more or
>less) for containers (especially for the meaning of a container's type).
>In this sense, the discussion paper is an approach to "clean" RDF (not
>RDFS) from hidden and ambigous semantic implications and to interpret it
>as a way to express nested triples/lists (and that's it).

------------
Graham Klyne
GK@NineByNine.org
Received on Friday, 1 June 2001 06:12:39 GMT

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