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

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Thu, 19 Jul 2001 11:27:48 +0100
Message-ID: <3B56B624.DA245C10@hplb.hpl.hp.com>
To: Graham Klyne <Graham.Klyne@Baltimore.com>
CC: rdf core <w3c-rdfcore-wg@w3.org>
Hi Graham,

Graham Klyne wrote:

> >
> >#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".

Loosely in English it means advert123 is for a service that will
buy roses in quantities of at least 100.

                     advert123 role buyer
and  thereExists ?X  advert123 description ?X
                     ?X        product      roses
     thereExists ?Y  ?X        minQuantitiy ?Y
                     ?Y        units        kg
                     ?Y        minValue     100


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

As  you see, I've made an attempt.

> 
> 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.

What is the difference between this and the example in:

  http://lists.w3.org/Archives/Public/www-archive/2001Jun/att-0021/00-part#41

> 
> >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".

Can you account for the fact that both adverts are very similar in
structure, yet you assign them different semantics?

> 
> 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].

This is the essential point.  My colleagues believe that if a resource
is not anonymous they will process it differently - i.e. it means
something different.


> 
> >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.

I have reread that message and it has not helped me to understand.

The issue is really very simple.  If an anonymous node is used, it means
"a service selling roses in quantities of at least 100".  If a node with a URI U
is given it means "The service called U selling roses in quantities of least
100".  The difference is that a processor of the advert is expected to know how
to relate U to the service it denotes.  At least that's how my colleagues
are using it.

Brian
Received on Thursday, 19 July 2001 06:30:38 EDT

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