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Fwd: summary: acyclic class hierarchy

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 29 Aug 2001 14:49:48 -0700
Message-Id: <v04210108b7b3137fca01@[130.107.66.237]>
To: w3c-rdfcore-wg@w3.org
Folks, Frank Harmelen sent me his own perception of the discussion of 
this issue on the DAML telecon this week. I'm forwarding it (with his 
OK) to give a better impression to the WG.

Pat Hayes

>X-Authentication-Warning: mail.daml.org: majordom set sender to 
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>Date: Wed, 29 Aug 2001 00:31:15 +0200
>From: Frank van Harmelen <Frank.van.Harmelen@cs.vu.nl>
>Organization: Vrije Universiteit Amsterdam
>X-Accept-Language: en-US,nl
>To: joint-committee@daml.org
>Subject: summary: acyclic class hierarchy
>Sender: owner-joint-committee@mail.daml.org
>
>
>For what it's worth, here is my summary/understanding of what we 
>could say to the RDF Core group in response to their proposal to 
>enforce acyclicity of subClassOf.
>
>Frank.
>   ----
>
>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*< there 
>is an explicit statement to that effect).
>
>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.
>   ----
>
>[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

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Received on Wednesday, 29 August 2001 17:48:41 EDT

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