Re: Ada design bug or GNAT bug?

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

On Tue, 23 Jun 2015 09:57:43 -0700 (PDT), Vincent wrote:

> Le mardi 23 juin 2015 16:38:43 UTC+2, Dmitry A. Kazakov a écrit :
>  > > For the OOP concept is based on an interface that is the same and
>>> instances that are different realizations of the same concept.
>> 
>> This is an inconsistent concept if taken literally. OOP never talk about
>> types being same. It does about same *behavior*. This is miles over miles
>> different. T and T'Class behave as if they were same in some contexts, but
>> they are not same from any point of view. Remember that class is a set of
>> types. A set is never equal to its instances. T'Class is a closure of
>> class. Closure of a set is not equal to an element of the set.
> 
> A would rather call a set of types a category [of types], like Integers or
> dicrete types.

Integers is a class of types, so discrete types are.

> A class type is an accessor to a category consisting of a given type and
> all types derived from it.

That is a parent type, which is just a type as any other.

The set of types bound by some relation (e.g. inheritance) is a class. The
class type as T'Class, is a type corresponding to the closure of the set
using the relation.

>>> One may think that the distinction between class type and tagged type is
>>> subtle, but the implications of it are huge : without class object one
>>> needs as replacement access to a class wide type.
>> 
>> Why?
> 
> Because one need a concrete object!

tagged object is a concrete object.

> With generics you can have a formal
> type because it will be instantiated during the early phase of
> compilation, but with OOP the code is generated before one knows the type
> of the object that will be called at runtime.

Ada supports shared generic bodies. For them code is generated before
instantiation.

> Strictly speaking this is not possible.

I don't know what you mean. The difference between static and dynamic
polymorphism is not when and how the code is being generated.

> The only possible solution is to use a unique object as an
> accessor for all possible objects.

A solution for what? Why there should be common ancestor of a polymorphic
class (set of types). There could be type systems when two types are mutual
ancestors and descendants of each other. The inheritance graph is not
necessarily a tree.

> The simplest way was to use an access
> to a class wide type.

Access type is not an ancestor of target types. In a better Ada it could be
descendant or a subtype, but not in present Ada.

> But Ada 83 limited the use of access types to pool specific types...

Ada 83 didn't mentioned pools.

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