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=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no 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!news3.google.com!feeder1.cambriumusenet.nl!feeder2.cambriumusenet.nl!feed.tweaknews.nl!194.134.4.91.MISMATCH!news2.euro.net!feeder.news-service.com!94.75.214.39.MISMATCH!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: Ada2012 Invariants and obaque types Date: Tue, 21 Jun 2011 14:31:49 +0200 Organization: cbb software GmbH Message-ID: <12jyfk40laprz.1mxczd0ze0dr7.dlg@40tude.net> References: <239a78ad-0937-4a7a-8163-231430fd5ffe@k27g2000yqn.googlegroups.com> <17t4afbmsrbm4.7llaajq91zz3.dlg@40tude.net> <4e008bc8$0$6568$9b4e6d93@newsspool3.arcor-online.net> Reply-To: mailbox@dmitry-kazakov.de NNTP-Posting-Host: FbOMkhMtVLVmu7IwBnt1tw.user.speranza.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: 40tude_Dialog/2.0.15.1 X-Notice: Filtered by postfilter v. 0.8.2 Xref: g2news1.google.com comp.lang.ada:19984 Date: 2011-06-21T14:31:49+02:00 List-Id: On Tue, 21 Jun 2011 14:17:11 +0200, Georg Bauhaus wrote: > 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. That is not an invariant, but a constraint. Constraint creates a subtype, it is a type-algebraic operation. Invariant does nothing, it is just a predicate known to be true for all instances of the type in public contexts. As such it can be removed any time without changing the program semantics. A constraint cannot be removed, because its violation is 1) possible and legal, 2) has defined effect (exception). Violation of an invariant in public context is impossible in a correct program. subtype Line is String (1..80); -- 1..80 is a constraint Line'Length = 80 -- This is an invariant P.S. Again, no idea how Ada 2012 treats this issue, differently I guess. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de