Re: Class-wide types algebra

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

On 13/09/2016 13:46, rieachus@comcast.net wrote:

> IMHO the biggest problem in Ada is the namespace pollution of types.

That is not a problem either because resulted types are anonymous.

> Dotted notation is acceptable when package names hide each other,
> but  I hate having to write "subtype Foo is Bar.Foo."

Well that is another language problem that types cannot orderly be 
renamed. But I don't see how it is related to class-wide types. And type 
renaming is only a problem in generics which are beyond any hope anyway.

> Subprograms can be
> overloaded, but adding a (new) type name during maintenance can break
> something down a chain of derivations.

No, it cannot break anything, it is just like "access T", T'Base etc.

> A long time ago, Lori Clarke and others wrote: "Nesting in Ada is
> for  the Birds." http://dl.acm.org/citation.cfm?id=948651 There are a few
> missing pieces when you go to mix class wide types into Ada. I've
> started numbering the often never mentioned again types that appear when
> using interfaces to do mix-ins.

Yes, I have long proposed to introduce T'Interface to denote the type 
interface stripped of implementation. That will spare you many explicit 
interface declarations and fix language library problems. E.g.

    type Root_Stream_Type is ...

OK, that was a mistake, it should have been

    type Root_Stream_Interface is limited interface;
    type Root_Stream_Type is abstract new Root_Stream_Interface
       with private;

With T'Interface you could still work it around:

    type DB_Stream is new Data_Base_Connector -- I need my base type here
       and Root_Stream_Type'Interfrace; -- But it is a stream still

[Of course T'Interface will override abstract all non-abstract not null 
primitive operations inherited by T]

Of course:

T'Immediate_Parent to denote the parent type.
T'Element to denote array element type
T'Index to denote array index type
T'Member(Name/Position) to denote record member type
T'Constraints to denote a null record type with constraints of T
T'Target to denote the pointer's target type

Nothing of this would be any difficult, but it would make usage of 
generics a lot easier.

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