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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Wed, 8 May 2013 20:17:30 +0200
Organization: cbb software GmbH
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Date: 2013-05-08T20:17:30+02:00
List-Id: <comp.lang.ada>

On Wed, 08 May 2013 18:13:26 +0200, Yannick Duch�ne (Hibou57) wrote:

> Le Wed, 08 May 2013 17:29:33 +0200, Dmitry A. Kazakov  
> <mailbox@dmitry-kazakov.de> a �crit:

>> Each time you do something with a type you get another one.
> 
> Yes
> 
>> Otherwise it
>> becomes untyped. So if you wanted to disallow operations that would make
>> another interface.
> 
> That's what a subtype is to me (I don't know if it's the proper term; at  
> least it seems to match with Ada's own wording).

No. Ada subtype cannot not disallow an operation. It can at best override
one with "raise Message_Not_Understood;". In order to disallow an operation
you need a new Ada type. Disallowing is not a constraint.

>> To put it differently, there shall be no preconditions
>> on types and their operations which aren't trivially true.
> 
> I don't understand that point.

Whatever differences are, they are rendered as differences in the
interfaces rather that in the preconditions. Preconditions of arguments are
always true, or else the language becomes weakly typed.

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