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: Ada 2012 Constraints (WRT an Ada IR)
Date: Wed, 14 Dec 2016 12:25:30 +0100
Organization: Aioe.org NNTP Server
Message-ID: <o2ra79$1ns2$1@gioia.aioe.org>
References: <c06ee4e6-d616-4e01-ba2e-cb111a81f6f6@googlegroups.com>
 <o24om5$vfr$1@franka.jacob-sparre.dk> <o24phv$1ou6$1@gioia.aioe.org>
 <o27gpr$o8k$1@franka.jacob-sparre.dk> <o28non$1fn8$1@gioia.aioe.org>
 <o2a2ad$vnp$1@franka.jacob-sparre.dk> <o2b79r$1dit$1@gioia.aioe.org>
 <o2b9rg$ejl$1@dont-email.me> <o2bb4o$1jpb$1@gioia.aioe.org>
 <o2c93i$rpp$2@dont-email.me> <o2du2t$1ghu$1@gioia.aioe.org>
 <o2jcpd$75p$1@dont-email.me> <o2jgom$18jg$1@gioia.aioe.org>
 <o2jkdm$bt$1@dont-email.me> <o2js2d$1qh4$1@gioia.aioe.org>
 <o2ke6d$gcs$1@dont-email.me> <o2ln13$et0$1@gioia.aioe.org>
 <o2mfpc$ttp$1@dont-email.me> <o2mncc$gjn$1@gioia.aioe.org>
 <o2mroh$dsk$1@dont-email.me> <o2n2o1$1515$1@gioia.aioe.org>
 <o2o73n$ci1$1@dont-email.me> <o2obed$qkk$1@gioia.aioe.org>
 <o2oj2u$i8h$1@dont-email.me> <o2olfi$1ca9$1@gioia.aioe.org>
 <o2p9bb$bcf$1@dont-email.me> <o2po6a$1hnq$1@gioia.aioe.org>
 <999c67b0-4478-4d2b-8108-32ac48fe6316@googlegroups.com>
 <o2r0lo$175u$1@gioia.aioe.org> <o2r8tr$bc9$1@dont-email.me>
NNTP-Posting-Host: vZYCW951TbFitc4GdEwQJg.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 6.1; WOW64; rv:45.0) Gecko/20100101
 Thunderbird/45.5.1
X-Notice: Filtered by postfilter v. 0.8.2
Xref: news.eternal-september.org comp.lang.ada:32811
Date: 2016-12-14T12:25:30+01:00
List-Id: <comp.lang.ada>

On 14/12/2016 12:04, G.B. wrote:
> On 14/12/2016 09:42, Dmitry A. Kazakov wrote:
>>
>> The precondition here is *not* (Y in Positive), it is (Y in Integer).
>> It is legal to call X on any Integer. Checking Y>=0 is semantically a
>> part of the X's body, even if the compiler could inline this part at
>> the caller's side or optimize it or the rest of the body away.
>
> And this is where Design by Contract, by intent, would differ
> from current choice of Ada's defensive style:
>
>   The programmer is NOT allowed to just call an operation
>   when there is a Pre aspect that says: Don't!

So long the program remains legal that is not a contract from the 
language point of view.

> So, by intent, if you declare
>
>   procedure X (Y : Positive)
>      with
>         Pre => Y > 0;
>
> then a client programmer is required, by contract, to not
> call X with a non-positive Integer.  If he or she does,
> then the programmer is violating the contract, and held
> responsible for any damage caused by the body of X.

It is about formal language-supported contracts and all sorts of other 
contracts.

 > This is what contract based programming is all about.

That is the problem with implied contracts. Is this legal:

begin
    loop
       X (Read (Stream));
    end loop;
exception
    when Constraint_Error =>
       null;
end;

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