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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: User-defined type attributes
Newsgroups: comp.lang.ada
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Date: Sat, 15 Mar 2008 10:33:38 +0100
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Date: 2008-03-15T10:33:38+01:00
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On Fri, 14 Mar 2008 22:04:53 -0500, Randy Brukardt wrote:

> Not if you need other generic parameters along with the type. And this is a
> way to do mixins, which otherwise can't be done in Ada 95 because type can't
> derive from a class-wide type:
> 
>    generic
>       type S is new T with private;
>    package Foo is
>       type New_S is new S with private;
>       procedure New_Operation (Op : in out New_S);
>    end Foo;
> 
> It's a nice way to add persistence, for instance. (Interfaces only allow you
> to add, umm, interfaces, not code.)

Speaking of language enhancements proper multiple inheritance would be a
better way to do this.

But actually I would like to have "supertypes" in order to be able to add
interfaces and code to existing types:

   type T is ...;  -- Defined independently on Persistent

   type Persistent is abstract...; -- Oops, we forgot to do it before T

   type Bridge is new Persistent and old T; --- (:-))

Bridge is a subtype of Persistent and a supertype of T. This injects
Bridge's implementation of Persistent into T within Bridge's scope.

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