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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Bus error
Newsgroups: comp.lang.ada
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References: <mailman.9.1183109010.1644.comp.lang.ada@ada-france.org>
Date: Fri, 29 Jun 2007 12:17:42 +0200
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Date: 2007-06-29T12:14:39+02:00
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On Fri, 29 Jun 2007 11:23:08 +0200, Grein, Christoph (Fa. ESG) wrote:

> Was this feature, which is now lost, often used and very important?

No so oft. One example I know is immutable lists of strings [actually any
unconstrained objects]. We don't have aggregates or tuples of, so let's
emulate them using a limited types:

   type List (<>) is limited private;
   function "/" (L, R : String) return List;
   function "/" (L : List, R : String) return List;

   procedure Foo (X : List);

We could call Foo as

   Foo ("A" / "B" / "C" / "D");

Let we wanted to prevent the second "/" from copying lists. In order to do
so, we would implement List as a controlled record containing a pointer to
a reference-counted implementation of the list. So "/" would actually
modify target of left argument in hope that it would vanish anyway. It
will, if there were impossible to create a variable of List.

   X : List := "A" / "B";
   Y : List := X / "C";   -- Surprise!

> Then
> we should consider making it again available in the next Ada version.

Have you an idea of how it could be done? It looks difficult to me.

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