Re: Ada design bug or GNAT bug?

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

On Tue, 23 Jun 2015 12:14:44 -0700 (PDT), vincent.diemunsch@gmail.com
wrote:

> Le mardi 23 juin 2015 19:15:36 UTC+2, Dmitry A. Kazakov a écrit :
> 
>> On Tue, 23 Jun 2015 09:57:43 -0700 (PDT), Vincent wrote:
>>> 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.
> 
> Ok ARM 2012 says :
> Any set of types can be called a "category" of types, and any set of types
> that is closed under derivation (see 3.4) can be called a "class" of
> types. 

RM 83 calls it class of types (see 3.3). But that does not matter. I don't
care about wording RM uses, meanings is what I care.

> But I was trying to imagine a new category : a "class type". It would be something between :
> - a general access-to-class-wide type
> - a controlled object with Initialize, Adjust and Finalize.
> - a protected type for the syntax of primitive operations

What for? Any type is a set of values and operations defined on them.
T'Class is necessary because there must be a type which values are from the
closure (union of all sets of values of types derived from T). OOPL's
mixing T and T'Class are inconsistent because sets of values are different.
These two cannot be same type.

When you want to introduce a type just start with its values and its
operations.

>>>>> 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.
> 
> Ok. I should have said, very often one need a constrained object,
> and this requires an access value for polymorphic tagged objects. 

Why?

How this is even related to constraints? Constraint is a property of a set
of types when for each given constraint there is a type. Yes, Ada RM does
not call it "class", but that is RM's problem. A constrained subtype is a
type. The sets of all constrained subtypes is a class. The type with values
having different constraints is class-wide. Just another method of having
classes, nothing more.

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