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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2005 puzzle
Date: Fri, 20 Jul 2012 10:27:15 +0200
Organization: cbb software GmbH
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Date: 2012-07-20T10:27:15+02:00
List-Id: <comp.lang.ada>

On Fri, 20 Jul 2012 01:09:45 -0700 (PDT), Maciej Sobczak wrote:

> W dniu pi�tek, 20 lipca 2012 04:41:46 UTC+2 u�ytkownik Randy Brukardt napisa�:
> 
>> For example, you can't put limited objects into containers.
> 
> Then we have poor containers.
> 
> Let's take existing Query_Element-like operations for inspiration. What about:
> 
>    procedure Insert (C : in out Container;
>       Constructor : not null access function return T);

The problem is that a *new* limited object cannot be result of a function,
this is conceptually broken. The factory of limited objects passed to
Insert must provide means to:

1. determine the constraints of the object (using some set of arguments);

2. construct the object in place (using the arguments again);

3. optionally, construct the class-wide object upon the specific object
(using the arguments)

1-3 cannot be effectively fused into single operation, e.g. constructing
function wrapping an aggregate. This is the root problem.

P.S. I think you could probably go further with

   procedure Insert (C : in out Container;
      Constructor : not null access function return not null access T);

But that is not we all wanted to get from a language like Ada.

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