- From: Brian McBride <bwm@hplb.hpl.hp.com>
- Date: Wed, 29 Aug 2001 12:17:26 +0100
- To: pat hayes <phayes@ai.uwf.edu>
- CC: w3c-rdfcore-wg@w3.org
I'll ask Jema to put it on the agenda. You haven't met Jema yet. I'm hoping to introduce her on Friday. Brian pat hayes wrote: > > Brian, could we put this item on the agenda for the next telecon, > please. I would like to get this issue settled before we publish the > model theory, if possible. > > --------------- > > rdfs:subClassOf loops and consistency with DAML+OIL. > > At the F2F we discussed a number of pieces of feedback from the > DAML+OIL JC. The only one that gave rise to any significant > discussion was their recommendation that the M&S be modified to allow > subclass loops, ie that combinations such as > > A rdfs:subclassof B > B rdfs:subclassof A > > not be considered illegal, but be thought of as a way to assert that > A and B are the same class. Two voices were raised against this > proposed change. One was the opinion that disallowing subclass loops > was accepted practice in the OO community, the other was that it > would be incompatible with Java usage, and that if subclass loops > were permitted then Java-based RDFS engines would need to perform > expensive loop-detection checks to avoid syntax errors when mapping > RDFS to Java. (I hope I have this more or less right, it is based on > memory.) I was to report this feedback to the DAML JC and get their > reaction, which I did today and here it is, summary first and more > details later. > > Summary > 1. DAML+OIL absolutely requires subclass loops, and cannot operate > with the current rdfs:subclassof restriction. > > 2. Therefore, if RDFS retains its current no-loops restriction, the > only viable option for DAML+OIL would be to remove subClassOf from > the rdfs namespace and to adopt its own daml:subClassOf with the > DAML+OIL interpretation. > > 3. But this would have the unfortunate effect of breaking the only > remaining substantial link between RDFS and DAML+OIL, which would be > unfortunate for a number of reasons. > > 4. Therefore, DAML+OIL respectfully urges the RDFCore WG to > reconsider the matter more carefully. > ------ > > Elaborations. > 1. DAML+OIL requires subclass loops because: > 1a. When merging information from several sources, one needs to be > able to discover during the reasoning process that two classes are > equal (even when nobody has explicitly said they are). This ability > is central to the class-heirarchy reasoning that is at the core of > DAML+OIL. > 1b. In any case there is no effective way to limit the language so > that such conclusions (that one subclass is identical to another) are > somehow forbidden. > 1c. It is also not feasible to consider the no-subclass-loops > restriction a syntactic wellformedness constraint (as it is in Java > and OO more generally) since to check it requires general inference > machinery. Since it can't be parse-time checkable in any reasonably > expressive class-inference language, there is no advantage to > imposing it as a semantic constraint. > > 2. If DAML+OIL introduces its own subClassOf, the only relationship > between daml:subClassOf and rdfs:subClassOf would be that the latter > entails the former. But this means that RDF/S engines would not be > able to make any use of DAML subclass information, in effect > rendering the DAML class heirarchies invisible to RDFS. (DAML could > make use of rdfs:subClassOf information, but only by also using other > information which is not expressible in RDFS, eg that A is *not* a > subclass of B. ) It would set up a one-way flow of information from > RDFS into DAML. In practice, one would expect that DAML+OIL usage > would simply become detached, in practice, from information expressed > in RDFS, since the class heirarchy is the only nontrivial semantic > connection between DAML+OIL and RDFS. > > 3. The development of DAML+OIL was predicated on the assumption that > the languages would be 'layered' in some sense, so that each layer > provided more expressive functionality than lower layers. The > possible option in which DAML+OIL uses a more inclusive notion of > subclass but RDFS uses a more restictive notion would make this > 'layering' so distant as to be effectively meaningless. Moreover, > this is not an arbitrary or capricious decision, since almost any > reasonably expressive extension of RDFS is going to have similar > properties. > > 5. As a postscript, the JC offers the following observations. > > 5a. In general, the ability to say that one class is identical to > another is valuable, and probably essential when putting together > information from several different sources. Allowing subclass loops > is an elegant way to anable the language to say this without > introducing any new primitives. (This was one of the main motivations > for the DAML+OIL usage, in fact, but the reasoning seems to apply to > RDFS with just as much force.) > > 5b. DAML+OIL inference engines would need to detect subclass loops > during their normal inference operations, and could (and indeed > currently often are) be required to flag such loops when detected, > allowing other engines, or users, to take aappropriate actions if > required. In other words, don't feel that all is lost if the language > allows subclass looping. > > ----- > > Pat Hayes > --------------------------------------------------------------------- > (650)859 6569 w > (650)494 3973 h (until September) > phayes@ai.uwf.edu > http://www.coginst.uwf.edu/~phayes
Received on Wednesday, 29 August 2001 07:20:57 UTC