- From: Pat Hayes <phayes@ai.uwf.edu>
- Date: Mon, 15 Apr 2002 21:07:39 -0400
- To: "Jeremy Carroll" <jjc@hplb.hpl.hp.com>
- Cc: w3c-rdfcore-wg@w3.org, "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
>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> > <daml:class> > <daml:intersectionOf rdf:parseType="daml:collection"> > <daml:class rdf:ID="Student"/> > <daml:class rdf:ID="Employee"/> > <daml:intersectionOf> > </daml:class> > </rdf:type> ></rdf:Description> > >entailing > ><rdf:Description rdf:about="#John"> > <rdf:type> > <daml:class> > <daml:intersectionOf rdf:parseType="daml:collection"> > <daml:class rdf:ID="Employee"/> > <daml:class rdf:ID="Student"/> > <daml:intersectionOf> > </daml:class> > </rdf:type> ></rdf:Description> > >==== > >In English, the premise reads John is in the intersection of Student and >Employee. >The conclusion reads John is in the intersection of Employee and Student. > >WOWG believes that this entailment is appropriate (although that has not >been formally decided). >However, in RDF the structure of the first collection and the structure of >the second collection is fairly different and so it is difficult/impossible >to account for the relationship. (I think the exact level of difficulty is >disputed). One aspect of this difficulty is that the meaning of the triples >according to the RDF Model theory is order preserving. > >WOWG considers that this issue can be addressed by using dark triples. >I have not understood the exact mechanism of the solution. Um. I don't think this quite captures the essential point. 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, 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. But DAML doesn't want them to have that meaning (the DAML MT gives them a different meaning); and in OWL, Peter has shown 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. 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: 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. 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
Received on Monday, 15 April 2002 21:07:42 UTC