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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Generics vs. O-O?
Date: Fri, 2 Aug 2013 18:15:47 +0200
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Date: 2013-08-02T18:15:47+02:00
List-Id: <comp.lang.ada>

On Fri, 2 Aug 2013 08:08:38 -0700 (PDT), Shark8 wrote:

> On Friday, August 2, 2013 8:09:46 AM UTC-6, Dmitry A. Kazakov wrote:
>> 
>>> For example, if
>>> you'd want a callback to be passed somewhere. Then, dismissing generics
>>> might entail the introduction of one level of indirection (subprogram
>>> pointer),
>> 
>> There is no need in using pointers in order to achieve downward closures.
>> It was a kludge of Ada 95 design. Generics is a kludge of Ada 83 design.
> 
> No, it was a well thought-out and justified feature; see the Ada83 rationale:

Generics were fashionable back in 80's. We are 30 years older and hopefully
wiser.

>> Yet another language design
>> problem that there is no simple way to declare such singleton types in Ada.
> 
> Really?
[...]
> Type K sure looks like a singleton to me.

I mean singleton types vs. closures. It is

   type Visitor_Interface is limited interface;
   procedure Meet (State : in out Visitor_Interface; Item : Element'Class)
      is abstract;
   procedure Enumerate
       (  Collection : Container;
         Visitor : in out Visitor_Interface'Class);

vs.

  procedure Enumerate
      (  Collection : Container;
         Action : not null access procedure (Item : Element'Class);

There is no need to be it really a singleton either for the object or for
the closure. Just it is the most common case in practice. 

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