- From: Dan Brickley <danbri@w3.org>
- Date: Mon, 3 Sep 2001 18:32:25 -0400 (EDT)
- To: <w3c-rdfcore-wg@w3.org>
FWIW I've no objections to making the change. I still thing that folk designing schemas with subclass loops are making a usability error, but am quite happy with the idea of removing this constraint from RDFS itself. Dan ---------- Forwarded message ---------- Date: Mon, 03 Sep 2001 22:56:16 +0200 From: Frank van Harmelen <Frank.van.Harmelen@cs.vu.nl> To: www-rdf-logic@w3.org Subject: cycles in subclass hierarchy (was: RDFCore Update) Resent-Date: Mon, 3 Sep 2001 17:00:15 -0400 (EDT) Resent-From: www-rdf-logic@w3.org Brian McBride <bwm@hplb.hpl.hp.com> wrote: > The minutes of the RDFCore WG face to face meeting > > http://www.w3.org/2001/sw/RDFCore/20010801-f2f/ > > have just been approved The following is a summary of the discussion in the DAML+OIL joint committee as input to the decision on allowing cycles in the RDFS class hierarchy or not. The message is: allowing cycles is mandatory for DAML+OIL, and not allowing them in RDFS will seriously break the layering of the Semantic Web architecture. In more detail: 1. in DAML+OIL, subclass-relations can be inferred even if they are not explicitly stated (note that this is an important difference from RDF-S, where A is only a subclass of B if >*and only if*< it is either explicitly stated, or follows from subsumption in the class-hierarchy). 2. as a result of point 1, acyclicity of the subclass-relation in DAML+OIL cannot always be detected by purely syntactic means, since computationally expensive inference may be needed to detect implicit ("implied") subclass-relations 3. because of point 2, DAML+OIL cannot require acyclicity of the subclass-relationship, since that requirement could only be enforced at great computational expense. 4. Point 3 means that if RDF-S will enforce acyclicity of rdfs:subClassOf, then DAML+OIL can no longer use rdfs:subClassOf. In other words, DAML+OIL will be forced to introduce daml:subClassOf 5. Point 4 would mean that much (all?) backward compatability between RDF-S and DAML+OIL would disappear: an RDF-S processor would be unable to catch any of the semantics of a DAML+OIL ontology (whereas currently, all the explicitly stated subclass relationships in a DAML+OIL ontology are accessible to an RDF-S agent, since DAML+OIL uses rdfs:subClassof 6. An important design rationale behind DAML+OIL (and in fact much other stuff on the Semantic Web so far) has been a layered approach, where languages are stacked on top of each other, with as much partial interpretation between the layers as possible. Tim BL has even argued in [1], [2], [3] and many other places that such "partial understanding is an essential design principle of the Web in general, Semantic or not. The decision to make rdfs:subClassOf acyclic will force DAML+OIL to introduce daml:subClassOf, and will therefore lead to an almost total loss of partial understanding between these two closely related ontology languages... Frank van Harmelen. ---- Frank.van.Harmelen@cs.vu.nl http://www.cs.vu.nl/~frankh Department of AI, Faculty of Sciences, Vrije Universiteit Amsterdam de Boelelaan 1081a, 1081HV Amsterdam, The Netherlands tel (+31)-20-444 7731 fax&voicemail (+31)-84-8722806 [1] Evovability, WWW7 keynote speach, http://www.w3.org/DesignIssues/Evolution.html [2] "Web Architecture from 50,000 feet" http://www.w3.org/DesignIssues/Architecture.html [3] "Web Architecture: Extensible languages" http://www.w3.org/DesignIssues/Extensible.html
Received on Monday, 3 September 2001 18:44:53 UTC