Re: types and subtypes

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 13 Mar 2006 10:08:45 -0800, ada_student@yahoo.com wrote:

> A type also defines the set of values that an object can have.
> If an Ada subtype adds a constraint to an Ada type, then the
> subtype is a type that is a derivative(derivative as in the Ada
> sense) of the Ada type and is different from that type.

Yes it is, but that does not prevent the new type to have the same
representation and be substitutable for the base one. You can consider a
subtype as a new type for which the compiler routinely applies identity
conversions. Downward conversions additionally check the constraint. So it
is consistent with strong typing, and the notion of subtype is nothing but
just a type relation. There is nothing except types.
 
> [I know you can say "type T2 is new T1 ..." in Ada to denote
> derivation]

That was IMO a bad design decision. It should be "subtype T2 is T1 with".
But that's my personal opinion.

> Why doesnt Ada subtyping also denote derivation as in the
> sense C++ base classes and derived classes do?
> 
> Why was Ada's subtyping defined to exclude derivation?

Probably because it seemed conceptually simpler. Ada 83 had subtype = same
representation + constraint. Ada 95 added derived tagged extension = same
representation + extension part. If we drop "same representation", which is
clearly an implementation detail, we will see that there is no difference.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de