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=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: a07f3367d7,146d9a693430fff2
X-Google-Attributes: gida07f3367d7,public,usenet
X-Google-NewGroupId: yes
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news1.google.com!goblin1!goblin.stu.neva.ru!feeder1-2.proxad.net!proxad.net!feeder2-2.proxad.net!newsfeed.arcor.de!newsspool3.arcor-online.net!news.arcor.de.POSTED!not-for-mail
Date: Tue, 21 Jun 2011 14:17:11 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US;
 rv:1.9.2.17) Gecko/20110414 Thunderbird/3.1.10
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: Ada2012 Invariants and obaque types
References: 
 <239a78ad-0937-4a7a-8163-231430fd5ffe@k27g2000yqn.googlegroups.com>
 <17t4afbmsrbm4.7llaajq91zz3.dlg@40tude.net>
In-Reply-To: <17t4afbmsrbm4.7llaajq91zz3.dlg@40tude.net>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-ID: <4e008bc8$0$6568$9b4e6d93@newsspool3.arcor-online.net>
Organization: Arcor
NNTP-Posting-Date: 21 Jun 2011 14:17:12 CEST
NNTP-Posting-Host: 81a1a104.newsspool3.arcor-online.net
X-Trace: 
 DXC=HL7Mii<>[A4L2C_`koXfC5McF=Q^Z^V384Fo<]lROoR18kF<OcfhCO;<>_5CCM><;?nc\616M64>:Lh>_cHTX3j=fN3DTSl=aJ<
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news1.google.com comp.lang.ada:19980
Date: 2011-06-21T14:17:12+02:00
List-Id: <comp.lang.ada>

On 21.06.11 14:08, Dmitry A. Kazakov wrote:
> On Tue, 21 Jun 2011 01:53:31 -0700 (PDT), Martin wrote:
> 
>> package P1 is
>>    type T1 is tagged private
>>       with Invariant => Is_Valid (T1);
> 
> Unrelated to Ada, but in theory, an invariant is a private implementation
> dependent thing. An invariant is trivially true in all public views of the
> object, i.e. between any two calls to the object's operations. From that
> follows, when mentioned in a public part then:
> 
>    type T1 is tagged private with Invariant => True;
> 
> (Again, I don't know which ideas Ada designers had about invariants, I am
> not a language lawyer.)

In another theory, the invariant may express things
such as
 Num_Green_Lights (T1) >= 3;
or
'Length < State_of_Things (T1) * 2;
where Num_Green_Lights is a publicly visible function whose
result is somehow computed.  These predicates would be informative,
and formal.

Would they be private implementation dependent things?  Or could
I expect, seeing the public view and its invariant, the possibility
of different implementations (of both the view and the invariant)?