Re: Preventing type extensions

["(see below)" <yaldnif.w@blueyonder.co.uk>]

On 21/09/2010 18:37, in article
b1409cbd-97c5-4f7b-bbc4-15721a8f6ff7@v23g2000vbi.googlegroups.com, "Cyrille"
<comar@eu.adacore.com> wrote:

> On Sep 21, 4:50�pm, "(see below)" <yaldni...@blueyonder.co.uk> wrote:
>>> all OO languages I'm aware of, have the notion of classwide types...
>>> They just don't make the distinction between a type on its own and a
>>> type with its derived types. If you think about it, it is a peculiar
>>> distinction at the "design" level. A horse is an animal... When I deal
>>> with animals, I must be ready to accept that maybe the animal I'm
>>> dealing with may be a horse, or may be something else. It's peculiar
>>> to expect it to be the pure notion of "animal" and nothing else...
>> 
>> What is peculiar about abstraction?
> 
> Abstraction is a good thing. Differentiating between Animals and
> Animals'Class doesn't particularly help abstraction, as far as I am
> concerned. This differentiation was a neat way to introduce OO into an
> existing language such as Ada 83, I don't think there is more to it
> than that.

NOT distinguishing between Animals and Animals'Class causes forced
polymorphism: it makes it impossible to have an object that is certain to be
of type Animals and not any of the descendent types of Animals.

This is a Bad Thing in my view.

-- 
Bill Findlay
<surname><forename> chez blueyonder.co.uk