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Re: A use case for anon nodes - action from telecon

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Tue, 17 Jul 2001 16:48:33 +0100
Message-Id: <5.0.2.1.2.20010717160927.03317ec0@joy.songbird.com>
To: Brian McBride <bwm@hplb.hpl.hp.com>
Cc: rdf core <w3c-rdfcore-wg@w3.org>
At 08:23 PM 7/16/01 +0100, Brian McBride wrote:
>I asked my colleagues if they had any use cases for anon resources
>that might help us with this issue.  What follows is an (edited)
>response I received.
>
>
>Consider the case of describing services and being able to place adverts and
>offers for such services. For example here is an (massively simplified) advert
>requesting a service (all uri prefixes dropped for clarity):
>
>#advert123 :role "buyer";
>            :description [:product :roses;
>                          :quantity [:units :kg; :minValue "100"]].

What does this RDF actually mean?  What statement does it make?  I think 
it's something like:

    There may exist an X such that:
      Someone wants to buy X AND
      There exists a Y such that:
        X :description Y AND
        Y :product :roses AND
        There exists a Z such that:
          Y :quantity Z AND
          Z :units :kg AND
          Z :minValue "100".

There seems to me to be no way of rendering this statement using just 
existential quantification.

This may be a compelling use-case, but I don't see any sanction for this 
usage in M&S 1.0, and as such would suggest it be deferred to V2.0.

>And here is a supplier who can offer a range of services:
>
>#advert456 :role "seller";
>            :description [:product :roses;
>                          :quantity [:units :kg; :maxValue "500"]].

I think this case can be expressed adequately using just existential 
quantification

    There exists an X such that:
      X is for sale AND
      There exists a Y such that:
        X :description Y AND
        Y :product :roses AND
        There exists a Z such that:
          Y :quantity Z AND
          Z :units :kg AND
          Z :maxValue "500".

In this case, I think the meaning can be conveyed using either of the 
approaches we have discussed on the list and in the last teleconference [1].

>Now. If we don't have anonymous nodes then we have the following problems.
>
>(1) In the seller advert it would appear that the seller is only advertising a
>single specific (but under-specified) service, #anon12345 or whatever, which
>would be hard to distinguish from an actual service instance like #service42.

I would refer to Pat's explanation, copied in [1].  Skolemization seems to 
work just fine here.

>(2) Similarly in the buyer advert instead of describing a template, giving the
>service a URI would make it appear that I am looking for a specific 
>service with
>that URI.

I still have not heard a argument that this is sanctioned by the current 
M&S 1.0.  If this use-case is to be captured, I think the argument for it 
needs to be made.

>This is clearly similar to DanC's book buying example.

Yes, I think so.

#g

[1] My response to [2], and apparent convergence on two approaches to 
handling anonymous resources; viz. Skolemization/genid, and scoped 
variables. 
http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jul/0131.html

[2] Frank argues that the main goal sanctioned by M&S is statements about 
resources that exist but which are not named: 
http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jul/0124.html


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Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
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Received on Tuesday, 17 July 2001 13:08:40 EDT

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