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!feeder.eternal-september.org!nntp-feed.chiark.greenend.org.uk!ewrotcd!newsfeed.xs3.de!io.xs3.de!news.jacob-sparre.dk!franka.jacob-sparre.dk!pnx.dk!.POSTED.rrsoftware.com!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: A function that cannot be called?
Date: Thu, 18 Oct 2018 16:30:08 -0500
Organization: JSA Research & Innovation
Message-ID: <pqau10$us1$1@franka.jacob-sparre.dk>
References: <pq9tf7$9bf$1@dont-email.me>
 <7ab688d0-b6b8-459c-b5b7-39b6c35daad2@googlegroups.com>
 <pqanbb$hl5$1@dont-email.me>
Injection-Date: Thu, 18 Oct 2018 21:30:09 -0000 (UTC)
Injection-Info: franka.jacob-sparre.dk;
 posting-host="rrsoftware.com:24.196.82.226";
	logging-data="31617"; mail-complaints-to="news@jacob-sparre.dk"
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.5931
X-RFC2646: Format=Flowed; Response
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246
Xref: reader02.eternal-september.org comp.lang.ada:54641
Date: 2018-10-18T16:30:08-05:00
List-Id: <comp.lang.ada>

"G.B." <bauhaus@notmyhomepage.invalid> wrote in message 
news:pqanbb$hl5$1@dont-email.me...
> On 18.10.18 19:03, AdaMagica wrote:
>> Am Donnerstag, 18. Oktober 2018 14:14:32 UTC+2 schrieb G.B.:
>>> package What is
>>>
>>>       type Void (<>) is private;
>>>
>>>       function Impossible (X : T) return Void;
>>>
>>> private
>> ...
>>> end What;
>>
>> Of course this is possible. Declarations of objects of type Void must 
>> have an initial value, which can be provided via the "constructor" 
>> Impossible:
>>
>> X: Void := Impossible (T0);
>
> Impossible needs to return a value. But a returned object can
> neither be declared nor a value constructed, since type Void
> has an unknown discriminant so that no initial constraint
> can be given anywhere.

It can always be given in the body of the package, since the full type for 
Void has to have some declaration with "known" discriminants. (Unknown 
discriminants aren't allowed on full types.) And that's the point: with this 
declaration, the only objects that can be created are those created by the 
package, which is fully within the control of the creator of the 
abstraction. If you don't want any objects at all (which would be weird, as 
types have no real use until some object is created of it or a descendant), 
then don't create any.

                                        Randy.