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

>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

That says that it buys roses in a (singular) quantity of at least 
100, not ANY (plural) such quantity. It would be made true by a 
single purchasing act.

What you want it to mean is that the service will buy *any* quantity 
of roses with at least 100 roses in it, but that's not expressible 
using existential quantifiers.

>
> >
> > 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

That example refers to a single creator. It doesn't say that 
http://www.w3.org/Home/Lassila  has creator *everything* and they all 
have name Ora and Email lassila@w3.org.

> >
> > >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?

I tend to agree that this should be expressed using a universal as 
well, given the intended meaning, if it is an assertion.

>
> >
> > 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.

Well, strictly speaking it does mean something different, but its a 
miniscule difference. Certainly not enough to support any kind of 
binding to an anonymous node.

> > >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".

Then that involves a *universally* quantified assertion, not an 
existential. Think: would this description be satisfied just by a 
single act of selling less than 100 roses? Or does it assert that 
*any* such act of selling is on offer here?

>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.

Well, we could give RDF that semantics. But if we do, then the 
example in 
http://lists.w3.org/Archives/Public/www-archive/2001Jun/att-0021/00-pa 
rt#41
must mean that *everything* is the creator of 
http://www.w3.org/Home/Lassila , for example.

Pat

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Received on Monday, 23 July 2001 23:55:04 UTC