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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Is this expected behavior or not
Date: Mon, 25 Mar 2013 09:32:22 +0100
Organization: cbb software GmbH
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Date: 2013-03-25T09:32:22+01:00
List-Id: <comp.lang.ada>

On Sun, 24 Mar 2013 09:27:02 +0100, J-P. Rosen wrote:

> Le 24/03/2013 09:17, Dmitry A. Kazakov a �crit :
>> Moreover, you want to tell that the behavior of an operation would depend
>> on a variable? Do I read it right? Not on the type or value it has. This is
>> how you want to deal with simple scalar types?
> Yes, the behaviour of assignment depends on the SUBTYPE of the target
> variable.

Merely in order to warn other readers of c.l.a. It is not a variable
property and there exist cases when this matters. E.g.

   X : Integer := Positive'(-1);

This must raise Constrant_Error, though X as an Integer variable does not
have any constraint; though Positive allegedly has the value -1. But
qualified expression disagrees.

Another example where it does matter, that the constraint is not a property
of the variable, is the behavior of X'Valid. We can trick the compiler to
assign -1 to a Positive through the hole in the semantics of renaming and
this will become a "valid" value of:

   with Ada.Text_IO; use Ada.Text_IO;
   procedure Test is
      X : Integer;
      Y : Positive renames X;
   begin
      Y := -1;
      Put_Line (Boolean'Image (Y'Valid));
   end Test;

TRUE is printed.

[I don't know if Ada 2012 is going to fix this, e.g. considering range a
"predicate"]

> Now, I think we have reached a point where nothing new can be added, and
> we are boring people. Let's agree that we don't agree and stop the thread.

Yes.

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