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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Issue with GNAT GPL 2009 and GtkAda
Newsgroups: comp.lang.ada
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Date: Fri, 26 Jun 2009 10:52:27 +0200
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Date: 2009-06-26T10:52:27+02:00
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On Fri, 26 Jun 2009 10:17:49 +0200, Georg Bauhaus wrote:

> Dmitry A. Kazakov wrote:
> 
>> Access types are much overused in Ada in order to overcome language
>> problems unrelated to access types.
> 
> Another reason to overuse _anonymous_ access types is
> probably outside the language, but is present with language
> users:  anonymity provides for a reduction in perceived
> programmer typing cost which is a convincing motive
> to not use names in practice. I.e.
> 
>       type R is record
>         C1 : access T;   -- quick & risky
>         C2 : Float;
>       end record
> 
> is thought to bring less work.

One of the language problems is that the container types do not well
compose unconstrained types. Considering this one:

   type R is record
      C1 : T'Class;   -- does not work, considering an access type
      C2 : Float;
   end record

it should be allowed to do without access types, like:

   type R (Constraint : T'Range) is record
      C1 : T'Class (Constraint); -- Still class-wide, but constrained
      C2 : Float;
   end record

T'Range is a subtype of Ada.Tags.Tag:

   subtype T'Range is T'Tag...<the last tag of types derived from T>;

> Experience apparently shows that
> it doesn't, since the anonymity virus becomes activated sooner
> or later. (Judging by the number of reports of anonymous access
> issues.)

Yes. I think it is related to a more general issue of structural vs.
nominal type equivalence. I suspect that structural equivalence never pays
off. Ada, initially designed with nominal equivalence, swayed to the
structural one. Anonymous access types is the most prominent example of.
 
> Does "access T :=" really have 80/20 merits?

I would remove anonymous access types altogether, fixing the real language
problems, which led to introducing them in first place.

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