Re: Declaring super types

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.

-- 
Reinier