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

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Thu, 19 Jul 2001 18:53:43 +0100
Message-Id: <5.1.0.14.2.20010719184933.03c2eec0@joy.songbird.com>
To: Brian McBride <bwm@hplb.hpl.hp.com>
Cc: rdf core <w3c-rdfcore-wg@w3.org>
At 11:27 AM 7/19/01 +0100, Brian McBride wrote:
>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.

Good, thanks!

The problem I now see is that this asserts the existence of the required 
service:

   thereExists ?X which is the object of (advert123 description ?X)
   (etc)

But the apparent intent of this is ask if such a service exists.  Do I 
detect a "gensym" error?

> >
> > 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 case asserts the existence of the un-identified individual.

> > >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?

Well, the difference I perceive is the apparent intent in one case to make 
a statement about something that may or may not exist, and in the other 
case to assert that something does exist.

> > 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.

See below.

> >
> > >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.

In the seller case, I thought Pat's note explained quite well how a skolem 
form or existential quantification gave you the same information (excerpt 
repeated below).  Maybe much hinges on "If all you know about 'genid:xyzzy' 
is some sentence that I send you"?

If one happens to know something else about "The service called U", such as 
"how to relate U to the service it denotes", then this is a different 
situation.  But I don't think it's brought about by the anonymity, but by 
the expected other knowledge.

#g
--

At 02:42 PM 7/5/01 -0700, pat hayes wrote:
[...]
 > If all you know
 >about 'genid:xyzzy' is some sentence that I send you, then all you really
 >know is (exists (?x)(<my sentence with' ?x' instead of 'genid:xyzzy'>)),
 >and in fact that is all you can infer, apart from such dumb-ass things as
 >(= genid:xyzzy genid:xyzzy). Now of course you are free to generate
 >another genid of your own, and use that as a skolem form; but then you
 >could do that anyway, if I were to give you the existential. There really
 >isnt any inferential advantage to having the existential form over the
 >skolem form.




------------------------------------------------------------
Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
------------------------------------------------------------
Received on Thursday, 19 July 2001 15:52:33 EDT

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