- From: Graham Klyne <Graham.Klyne@Baltimore.com>
- Date: Thu, 19 Jul 2001 18:53:43 +0100
- 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 UTC