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
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.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: orthogonal inheritance and extension aggregates
Date: Sat, 20 Jul 2013 08:36:09 +0200
Organization: cbb software GmbH
Message-ID: <16rbbo5ajimr0.17dwtq00njx6u.dlg@40tude.net>
References: <ksbu1u$75k$1@online.de>
 <igo2b70i4ox.t5duix9weguc$.dlg@40tude.net> <ksd8dm$1l3$1@loke.gir.dk>
Reply-To: mailbox@dmitry-kazakov.de
NNTP-Posting-Host: R0SnWoufDrgXLosLSQbuFw.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: news.eternal-september.org comp.lang.ada:16446
Date: 2013-07-20T08:36:09+02:00
List-Id: <comp.lang.ada>

On Sat, 20 Jul 2013 00:49:41 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:igo2b70i4ox.t5duix9weguc$.dlg@40tude.net...
> ...
>> P.S. I don't know if Ada 2012 silently inherits from null extension. I
>> heard that there was an intention to add the kludge. Though semantically
>> broken it would save programs like yours.
> 
> That's actually an Ada 2005 feature - see 3.9.3(4/2). One that I was against 
> because it introduces a maintenance problem (null extensions tend to turn 
> into non-null extensions over time, and once that happens, the feature stops 
> working).

Right.

> It appears that GNAT failed to properly implement this feature in the case 
> described by the OP.

But the extension is null only *privately*. How can a public declaration of
the overriding depend on private declaration? It looks totally crazy to me.

> OTOH, as soon as the OP starts adding components to the extension, they'll 
> have the problem again. (I don't think it is practical to extend the 
> predefined containers, particularly because it's impossible to derive a 
> cursor type to go with the extended container -- so you either end up with 
> broken typing or you have do everything twice. And there are a lot of 
> functions that have to be overridden.)

Maintaining parallel types hierarchies is a big type system problem, which
is unclear how to resolve.

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