From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Community Input for the Maintenance and Revision of the Ada
 Programming Language
Date: Sun, 3 Sep 2017 14:59:35 +0200
Organization: Aioe.org NNTP Server
Message-ID: <oogubm$1ijg$1@gioia.aioe.org>
References: <oludae$seh$1@franka.jacob-sparre.dk>
 <4dc188de-802b-41ad-9cdd-b8246eb9a1c7@googlegroups.com>
 <47cc6474-8b75-4644-92d0-bd1f694c20e7@googlegroups.com>
 <338b355a-dee4-4c73-b00e-09d9a8430fb1@googlegroups.com>
 <oofm8f$ull$1@franka.jacob-sparre.dk> <ooglev$c4u$1@dont-email.me>
NNTP-Posting-Host: MajGvm9MbNtGBKE7r8NgYA.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.3.0
X-Notice: Filtered by postfilter v. 0.8.2
Content-Language: en-US
Xref: news.eternal-september.org comp.lang.ada:47913
Date: 2017-09-03T14:59:35+02:00
List-Id: <comp.lang.ada>

On 2017-09-03 12:32, Alejandro R. Mosteo wrote:

> And is there a difference to using rename?:

Yes, because renaming does not touch bounds even if explicitly required 
by the subtype.

It is clear why renaming to a subtype does not slide bounds. That is 
because Ada treat a subtype as a constraint. For the same reason it is 
not illegal have such a renaming, that appears to the reading as bound 
sliding.

What was a language design bug that the new constraint is not enforced. 
The program should raise Constraint_Error when the index is outside the 
new bounds, even if wihin the original bounds.

Surely that would be useless behavior, but that is what follows from the 
Ada's subtype definition. One cannot have both.

> (I was actually surprised that you can't explicitly write 
> Zero_Array(Item) with renames).

I am in favor of changing subtypes to be more flexible with the 
representation.

An alternative would be allowing renaming to another type:

    type Array_Type is array (Integer range <>) of Float;

    procedure Foo (A : Array_Type) is
       type Zero_Array is array (Unsigned range <>) of Float;
       B : Zero_Array renames A;

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