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Re: defaults

From: Enrico Motta <e.motta@open.ac.uk>
Date: Tue, 22 Jan 2002 13:38:04 +0000
Message-Id: <p05100300b8731871ea08@[212.67.113.158]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, schreiber@swi.psy.uva.nl
Cc: www-webont-wg@w3.org
Aside from all the technical problems, the problem with ``input 
completion'' is that even when it works, it does not really implement 
the idea of a default value.  Defaults are essentially a mechanism 
for stating typical cases, which can be overridden in those 
statistically rare examples in which teh default information does not 
apply.   ``input completion'' is really a completion mechanism, which 
is a different thing. The point about defaults is that they provide a 
mechanism for dealing with incomplete knowledge.  For instance, "I am 
told there is a new type of bird to be entered in teh KB, I have 
never seen it, but I will assume that it can fly".  One can use 
``input completion'' in this scenario to ask the user: "Do you want 
also to state that this new bird flies".  In many domains the end 
result will be the same as using defaults. In other domains it won't 
work, because it may not be a good idea to enforce completeness, 
where completeness does not exist (i.e., the user may not have a clue 
about whether the new type of bird flies, therefore he/she will be 
unwilling to state the information explicitly in the KB, although 
he/she may be happy to use such information as a default, in the 
absence of contrary information).

I agree with Guus that defaults are a good thing to have, given that 
they are ubiquitous in applications, but of course only if some 
'clean' mechanism can be designed for OWL.

Enrico



At 6:44 am -0500 22/1/02, Peter F. Patel-Schneider wrote:
>From: Guus Schreiber <schreiber@swi.psy.uva.nl>
>Subject: defaults
>Date: Mon, 21 Jan 2002 17:06:24 +0100
>
>>  Peter,
>>
>>  During the f2f you mentioned that some limited representation of
>>  defaults could be possible in OWL. Could you elaborate on that? That would
>>  be very helpful.
>>
>[...]
>>
>>  Thanks, Guus
>
>There is a very common idiom, which I like to call ``input completion'',
>that works as follows.  I'll use frame terminology to describe input
>completion for two reasons: 1/ the processing is much clearer in frame
>terminology, and 2/ I hope that not using RDF terminology will help prevent
>misunderstandings.
>
>
>An ``input completion'' directive is something like
>	the normal value for slot S in class C is V
>You process the definition of an object O in the input stream.  The next
>thing that you do is to see if O belongs to C.  If it does not, then the
>directive does not apply.  Then you check to see if O has a value for S.
>If it does, then the directive does not apply.  If, however, O belongs to C
>and also does not have a value for S, then give it the value V for S.  This
>value is not asserted by default, i.e., information discovered later can
>not override it.
>
>
>A coherent story for input completion is much harder to make for RDF, as
>RDF does not have definitions of objects, nor does it have ordered inputs.
>A coherent story for input completion is also difficult to make for
>description logics, and thus would be hard to make for OWL.
>
>peter
Received on Tuesday, 22 January 2002 08:38:16 GMT

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