Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!ncar!boulder!gore!jacob
From: jacob@gore.com (Jacob Gore)
Newsgroups: comp.lang.eiffel
Subject: Re: The indexing clause
Message-ID: <120001@gore.com>
Date: 14 Jul 89 19:24:56 GMT
References: <180@eiffel.UUCP>
Reply-To: jacob@gore.com (Jacob Gore)
Organization: Gore Enterprises
Lines: 22

/ comp.lang.eiffel / bertrand@eiffel.UUCP (Bertrand Meyer and Philippe Stephan) / Jul 13, 1989 /
     + Include positive information  only.  For  example,  a
       ``representation''  index  is  used  to  describe the
       choice of representation  (linked,  array,  ...).   A
       deferred  class  does  not have a representation. For
       such a class the clause should not contain an   entry
       ``representation: none'' but simply no entry with the
       index ``representation''. A reasonable query language
       will make it possible to use a query pair of the form
       <representation, $NONE$>, where $NONE$  is  a special
       value indicating absence.
----------

Why?

It is not clear to me that in a database that the combined indexing entries
will form lack of information must imply negation.  Why is it better to
assume that the lack of "representation:" means "$NONE$" rather than
"$UNKNOWN$" (or, perhaps, simply "$UNSPECIFIED$")?

--
Jacob Gore	Jacob@Gore.Com		{nucsrl,boulder}!gore!jacob