- From: Dan Connolly <connolly@w3.org>
- Date: 16 Apr 2002 11:12:59 -0500
- To: Pat Hayes <phayes@ai.uwf.edu>
- Cc: Jeremy Carroll <jjc@hplb.hpl.hp.com>, w3c-rdfcore-wg@w3.org, "Peter F. "Patel-Schneider <pfps@research.bell-labs.com>
On Mon, 2002-04-15 at 20:07, Pat Hayes wrote: > >ACTION 2002-04-12#7, jjc: post message to rdfcore with motivating example > >(for dark triples?) > > > >The basic problem can be expressed in DAML+OIL as: > > > ><rdf:Description rdf:about="#John"> > > <rdf:type> [...] > Um. I don't think this quite captures the essential point. Would you please given an example that does? > The > entailment given is indeed wanted, but its only one of a whole class > of entailments. Now, those entailments themselves are not the > problem: after all, OWL can impose its own semantic conditions on the > graphs, to make sure its entailments come out the way it wants. In > these cases the RDF in effect says less than OWL requires, but that > is to be expected. The problem (that dark triples is intended to > address) is almost the inverse of this: it is that the RDF graphs > that encode DAML (more generally, OWL) might say *more* than what is > intended: in particular, they assert the *existence* of certain > containers, corresponding to the syntax of some DAML (OWL) > expressions, eg the use of daml:collection to encode some DAML > expressions in RDF. When DAML was designed, RDF had no semantics, I disagree; RDF has had existential/conjunctive semantics all along. > so > it was quite OK for DAML to use RDF triples freely and assign its own > meaning to them. But now that RDF does have a semantics, those > triples have a definite RDF meaning (they assert the existence of a > thing called a daml:collection, with relationships to its contents) > and since DAML (OWL) is supposed to be a 'layered' extension of RDF, > they should have the same meaning in DAML as they have in RDF. quite. > But > DAML doesn't want them to have that meaning no? I think it does. (and I helped design DAML+OIL). > (the DAML MT gives them a > different meaning); I don't think that's entirely clear. The DAML axiomatic semantics gives them exactly this meaning. > and in OWL, Peter has shown I'm not convinced. > that if we insist > that OWL interpretations preserve all the meanings that the RDF MT > assigns to all the 'syntax-encoding' triples that will be needed to > ensure an adequate layering, then those extra things that are > asserted (by RDF) to exist, can actually produce inconsistencies in > the OWL, basically rendering what would otherwise be perfectly good > OWL assertions into nonsense. I still think we should work on fixing that; i.e. being careful about what entailments follow in OWL so that paradoxes don't arise. > In general, it usually isn't a good > idea to impose, as a semantic condition on a language (particularly a > 'class', ie set-theoretic, language like DAML or OWL), that it is > obliged to talk about its own expression syntax: I'm not sure what you mean by that. DAML+OIL is just a collection of terms for use in RDF syntax, no? > but that is what the > 'layering' conventions basically do, if the higher layers are obliged > to take the RDF semantics seriously. So, we need a way to say > something like: *these* triples are being asserted (and DAML accepts > them as assertions and agrees with their meanings) but *these other* > triples are just being used to encode syntax, and are not being > asserted. The 'dark' triples are the latter kind, and all that is > required is that DAML (and OWL, and probably almost every other > layered extension of RDF) has some way to make the distinction. > > I would emphasize that this issue really is a semantic non-issue for > RDF: it doesn't fundamentally change the language at all, other than > asking it to provide a way for a 'higher' layer to make this > distinction, and be willing to record it and preserve it under > inferences. Until I can look at some specific proposals and examples, I don't have any way of evaluating that claim. > The distinction is already in the RDF MT, in fact ('dark' > = 'unasserted'). So it just requires some kind of syntactic > convention or addition to allow an RDF graph/document to actually > record the distinction so that it can be accessed and preserved. This > would be easy in N-triples, but seems to require some new idea in > RDF/XML. (Eg one idea that was mooted was to allow <rdf> ...</rdf> > brackets inside an RDF/XML document, the understanding being that > anything inside them was 'dark'.) > > If the WG accepts this task, the basic decision to be taken is > whether an RDF document with dark triples should be designed to be > acceptable to current parsers or whether it should, as a matter of > policy, be designed to break current parsers. I would suggest the > former, if some way can be found to do this while preserving the > distinction. I would also suggest the easiest way to go would be to > provide some way to say that a given namespace is 'dark'; Peter and I > agree that, while somewhat less flexible than allowing an arbitrary > distinction, this would be sufficient to allow WOWG to proceed. > > >In the discussion WOWG appeared to like this entailment so much, that it was > >worth paying the cost of having two separate RDF documents to express the > >light and dark triples. This was the mechanism discussed at greatest length > >in the webont f2f. (I took it that the motive for discussing that mechanism > >is that this was the smallest possible change required of RDFCore: simply > >permission to have an RDF/XML document that is interpreted as a "dark > >document"). > > That would indeed be the minimal change to RDF, ie no change at all, > and I did seriously suggest it to the WOWG, since that is what Euler > does. But it was felt that mechanism would be rather hard to use in > OWL, since it would probably lead to a proliferation of multiple dark > documents from any reasonably complex piece of OWL. > > >In terms of RDF collections, we could imagine trying to make the same > >construct with a closed bag. > > That would not achieve the required goal, however, since the triples > describing the bag would be present in the graph and hence the bag > would be asserted to exist in the MT, and hence would be required to > exist in any OWL interpretation; but that is exactly what we are > trying to avoid. > > Pat > > PS. A pre-emptive strike: reification doesn't do it, either. > > -- > --------------------------------------------------------------------- > IHMC (850)434 8903 home > 40 South Alcaniz St. (850)202 4416 office > Pensacola, FL 32501 (850)202 4440 fax > phayes@ai.uwf.edu > http://www.coginst.uwf.edu/~phayes -- Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 16 April 2002 12:14:12 UTC