Re: Where DAML+OIL deviates from the RDF-Schema spec.

On March 1, Jim Hendler writes:
> At 1:37 PM -0500 3/1/01, Dan Brickley wrote:
> >I agree with [2] and [3], and could live with [1]. My main concern w.r.t.
> >using loops in the class and property hierarchies to indicate synonyms is
> >with end-user comprehensibility and with user interface generation. I can
> >see that there's a _logical_ story to tell about why loops are OK; I'm not
> >so sure there's a modelling and usability story. But then it's not up to
> >the core RDFS system to guarantee that folk can't make goofy modelling
> >decisions, I guess.
> >
> >Dan
> >
> 
> I'm with DanB on this one.  I originally proposed that if we wanted 
> to use loops to assert equality, we'd lose the ability to (1) 
> distinguish intentional from unintentional loops, and (2) force 
> developers to understand the logical relationship (which I know from 
> experience ain't always easy). In fact,
> I was reminded later by one of my former postdocs that this situation 
> has come up in practice in our experience -- we developed some of our 
> KBs for the Parka-DB project using web-scrapers from online 
> taxonomies and thesauri.  We saw a number of cases where loops 
> existed - unintentionally, so we had to write loop breaking code in 
> our systems -- under [1] we'd have trouble distinguishing the 
> accidental from the intentional.
> 
>   I argued we should have a language construct that was explicit to 
> assert equality.  I was overruled 

Err, hang on a minute. Let me quote from daml+oil.daml:

<Property ID="sameClassAs">
  <rdfs:label>sameClassAs</rdfs:label>
  <comment>
    for sameClassAs(X, Y), read X is an equivalent class to Y.
    cf OIL Equivalent
  </comment>
  <rdfs:subPropertyOf rdf:resource="#equivalentTo"/>
  <rdfs:subPropertyOf rdf:resource="http://www.w3.org/2000/01/rdf-schema#subClassOf"/>
  <rdfs:domain rdf:resource="#Class"/>
  <rdfs:range rdf:resource="#Class"/>
</Property>

So we don't want/need to use subClassOf cycles to assert
equality. However, we have to decide what to do with such cycles when
they are detected - it is no use just saying that they are "forbidden",
because by the standard argument they will occur out there on the
web. The choices are either:

a. barf - declare the ontology to be illegal/broken

b. accept the facts as presented and draw the appropriate conclusions
(and possibly issue some kind of warning as to the consequences).

I much prefer b. Here are just a few reasons:

1. There is no semantic justification for declaring such an ontology
to be broken - we are just guessing that it contains one or more
errors. This seems to be setting a very dangerous precedent - we could
declare many other DAML+OIL constructions illegal on the grounds that
they are often used in error.

2. It is not, in general, possible to "fix" cyclical ontologies
automatically as there is no way to know where the cycle should be
broken. So it is hard to do anything sensible when a cycle is detected
other than to just reject the ontology. 

3. Given that we have to detect cycles anyway, surely it is much
better to deal with them and simply warn users that a two or more
classes have collapsed into a single class.

4. In DAML+OIL we can easily create ontologies with cycles that
include implicit subClassOf relations that cannot be detected
syntactically. Are these kinds of ontology also illegal? 

Ian

> on the grounds that having two ways 
> to do the same thing in a language was bad (Dan Connolly) and that 
> DAML+OIL had to have the cycles for their associated logic engine  to 
> handle this stuff (Ian Horrocks/Peter Patel-Schneider) -- as a 
> result, my personal theorem prover was faced with
>   a. Two equivalent solutions are bad
>   b. We had to have solution [1]
> and therefore, using the stuff that will power the semantic web
> I was forced to conclude
>   c. We should have [1] and only [1]
> 
> If someone is willing to remove assumption a or b, I would think we'd 
> end up with something that makes more sense to the logically 
> challenged folks (like me) who really want to use this stuff do to 
> "frame"-like reasoning more than logical inference, or who would make 
> errors that would cause whole chains of subclass relationships to 
> collapse by accident.
>    cheers
>   Jim H
> p.s. Parka-DB: http://www.cs.umd.edu/projects/plus/Parka/parka-db.html
> 
> 
> At 11:53 PM +0100 2/24/01, Frank van Harmelen wrote:
> >
> >[1]
> >"Warning: The RDF Schema specification demands that the 
> >subclass-relation between classes
> >must be acyclic. We believe this to be too restrictive, since a 
> >cycle of subclass
> >relationships provides a useful way to assert equality between 
> >classes. Consequently,
> >DAML+OIL places no such restriction on the subClassOf relationship 
> >between classes;"
> 
> Dr. James Hendler		jhendler@darpa.mil
> Chief Scientist, DARPA/ISO	703-696-2238 (phone)
> 3701 N. Fairfax Dr.		703-696-2201 (Fax)
> Arlington, VA 22203		http://www.cs.umd.edu/~hendler
>

Received on Saturday, 3 March 2001 12:52:38 UTC