Re: Declaring super types

On Apr 23, 3:27 pm, r..._at_raampje.lan (Reinier Post) wrote:
> Tegiri Nenashi wrote:
> >[...] More importantly, I don't quite follow your
> >definition. Suppose you have two relations
>
> >Circles = [centerX centerY radius]
> >            0        0       10
> >            10      0       20
> >;
>
> >Ellipses = [centerX centerY axisX axisY]
> >            0        0       10       10
> >            10      0       20       20
> >            0       10      10       50
> >;
>
> >these don't match your definition.
>
> True.
>
> > Or perhaps you want to correct the
> >"radius" attribute name to match say "axisX", then I still fail to see
> >how it would match your definition.
>
> It wouldn't.  Remove radius from Circles or add it
> to Elliupses, and these two particular instances will match,
> but in general it still won't be a case of "is-a" because
> in general, the same Circle may correspond to
> different Ellipses, which my second clause forbids.
>
> I also gave the rationale for restricting "is a" in this way:
> it doesn't involve any reasoning about domain values.
> (Except reasoning involving equality.)
>
> I didn't invent this notion of "is a", I have it from
> a textbook and I believe it is pretty standard.

I still don't follow. First, let's reiterate that we are considering the case when both relations have different set of attributes: if attribute set is the same, then subtyping is trivially subset relation. In a way we are after generalization of subset onto arbitrary pair of relations.

Again, I have a problem with Circles and Ellipses. First, if one removes the Radius attribute from a circle, then it becomes a Point! Second, why would I add redundant attributes to a Circle? If the idea is to make both relations to have the same set of attributes, then we go back to the previous paragraph: I'm interested to see a convincing example of two relations with different sets of attributes that fits your definition. Received on Sat Apr 24 2010 - 01:30:36 CEST