Re: Clean Object Class Design -- Circle/Ellipse

On Sun, 22 Jul 2001 00:42:02 -0400, "Bob Badour" <bbadour_at_golden.net> wrote:

>>My original point was to show Date's mistake
>>of confusing sets and types.
>
>He does not do that. In his work, he very explicitly points out that
>relations are sets and not types. Domains are types, which consist of a set
>of values and a set of operators on those values.
>
>

The distinction between relations and domains is not the same as the distinction between sets and types.

In particular, I was reacting to Date's
" Type Inheritance: Is a Circle an Ellipse?" ( www.dbdebunk.com), where he writes:
<quote>
In my opinion, therefore, any "model" of inheritance in which type CIRCLE is not considered to be a subtype of type ELLIPSE can hardly be said to be a good model of reality. ...
(in Date/Darwen model) the set of update operations that apply to circle variables is neither a subset nor a superset of the set of such operations that apply to ellipse variables. </quote>

Since operations on circles and elipses are hard to agree on (in absence of a context of a specific application area and a formal specification of these ops), I suggested considering an equivalent question of:
 Is IA=<I,{+,*}> a subtype of RA=<R,{+,*}> in the "real" world (math)?

The "real" world answer is no, but Date's reasoning (?) would lead to qualified yes (integers are reals, but the set of update operations that apply to integer variables is neither a subset nor a superset of the set of such operations that apply to real variables).

What does that mean? Are we talking about +, * or := ?

Is it still true that "any model of inheritance in which type Integer is not considered to be a subtype of type Real can hardly be said to be a good model of reality" ?

To put bluntly, " Type Inheritance: Is a Circle an Ellipse?" documents lack of understanding of types (and OO), and the usefulness of Date's inheritance model approaches zero. No wonder that "such a model seems to be conspicuous by its absence in the industry at large, at least at the time of writing."

Marc Gluch
Mindtap Inc. Received on Mon Jul 23 2001 - 00:24:52 CEST