- From: Dave Reynolds <der@hplb.hpl.hp.com>
- Date: Fri, 20 Jun 2003 17:10:05 +0100
- To: Ian Horrocks <horrocks@cs.man.ac.uk>
- CC: public-webont-comments@w3.org
Ian, Thank you for your response. It is a shame that it is not possible to close these "loopholes", and reduce the complexity levels for the two subsets, without unacceptable restrictions. However, I accept that the proposed tradeoff is the best that the working group can achieve and accept your response as satisfactory. Dave Reynolds Ian Horrocks wrote: > > Thank you for your comments. > > As you point out, the issue of OWL DL implementability is dealt with > in a separate comment, so here I will restrict my attention to your > comment regarding the complexity of reasoning in OWL Lite. > > On May 9, Dave Reynolds writes: > > > > We wish to register a comment on the implementation complexity of the selected > > subsets of OWL - Lite and DL - based on our implementation experience with Jena. > > > > We understand that there is a tradeoff between complexity of reasoner > > implementations and expresivity of the language for ontology authors. There are > > applications of OWL that just involve the exchange of ontology documents and do > > not require complete reasoning support. For this reason we do not object to > > OWL-full being undecidable. > > > > However, we understand the purpose of the defined subsets (Lite, DL) as being to > > provide interoperability points between implementations that *are* offering > > reasoning support. > > > > It has already been pointed out by working group members that OWL/DL reasoning > > is NExpTime and that practical, tractable implementations of the complete subset > > remains a research problem. It seems inappropriate to us to call out a language > > subset which is not yet effectively implementable - we cover this point, and its > > implications for CR stage, in more detail in a separate comment. > > > > Turning to OWL/Lite, the inclusion of intersectionOf together with the ability > > to define multiple complete definitions of a named class means that the language > > is not very "light". In particular, it appears to be possible to define > > equivalents to complementOf[1] and thus unionOf within OWL/Lite. Their exclusion > > would have been useful in order to facilitate low complexity rule-based > > implementations but does not seem to have been achieved. > > > > One means to simplify OWL/Lite would be to restrict class definitions to only be > > "partial". Our concern is that this would go too far - there is value in having > > complete definitions in order to support classification of individuals based on > > their properties. We wonder if a constraint of the form "each classId may only > > participate in a single axiom of the form Class(classID complete ...)" would > > remove this source of complexity. We ask those with greater knowledge of this > > field to explore whether an approach along these lines would enable OWL/Lite to > > better live up to its name. > > The design of OWL Lite is intended to maximise utility while providing > easier implementability (not just of reasoners, but also of tools such > as editors). As you point out, it is possible through "abuse" of the > syntax to express, e.g., negation, even though it is not directly > supported in the syntax. > > The working group did consider trying to close such "loopholes" via > the mechanisms you suggest (amongst others), but concluded that this > would be difficult to achieve without an unacceptable reduction in the > power/utility of the language. E.g., see the thread beginning with [1] > for a discussion on how the complexity of reasoning in Lite might be > reduced and [2] for an argument as to why eliminating complete > definitions from Lite would be unacceptable. > > As far as your suggestion to have "each classId may only participate > in a single axiom of the form Class(classID complete ...)" is > concerned, this would not work as we can easily assert (or even infer) > the equivalence of classes, allowing different classIds to be used to > be used in different axioms in order to achieve the same result. > > Please reply to this message as to whether this response is satisfactory, > copying public-webont-wg@w3.org. Again, thank you for your comments. > > Ian Horrocks > > [1] http://lists.w3.org/Archives/Public/www-webont-wg/2002Dec/0054.html > > [2] http://lists.w3.org/Archives/Public/www-webont-wg/2002Dec/0088.html > > > > > Dave Reynolds for the Jena team > > > > [1] An example construct, which Jeremy credits to Ian Horrocks, is as follows. > > > > Given a definition of a class C: > > Class(C complete <expr1>) > > > > The let P be a property which is not used elsewhere and define: > > Class(C complete restriction(minCardinality(P, 1)) > > Class(C-co complete restriction(maxCardinality(P, 0)) > >
Received on Friday, 20 June 2003 12:10:51 UTC