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: two questions on allocators
Date: Wed, 28 Feb 2018 17:37:57 -0600
Organization: JSA Research & Innovation
Message-ID: <p77egm$k05$1@franka.jacob-sparre.dk>
References: 
 <93229821-ae3d-4e47-91d9-a20ff3c1f1a7@googlegroups.com><55fda761-55f8-4b25-b8ab-0125acf16b05@googlegroups.com>
 <wcczi3t6qqh.fsf@TheWorld.com>
Injection-Date: Wed, 28 Feb 2018 23:37:58 -0000 (UTC)
Injection-Info: franka.jacob-sparre.dk;
 posting-host="rrsoftware.com:24.196.82.226";
	logging-data="20485"; 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; Original
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246
Xref: reader02.eternal-september.org comp.lang.ada:50727
Date: 2018-02-28T17:37:57-06:00
List-Id: <comp.lang.ada>

"Robert A Duff" <bobduff@TheWorld.com> wrote in message 
news:wcczi3t6qqh.fsf@TheWorld.com...
> Mehdi Saada <00120260a@gmail.com> writes:
>
>> I saw that since I (tried to) read the AARM, but didn't really got
>> it. Now it's ok. If the problem of these null forbidding hypothetic
>> subtypes is that can't have the default null value, why not just require
>> the user to give one ?
>
> Don't use "not null"; it's broken as you see.  Instead, do:
>
>    subtype S is T with Predicate => S /= null;

??? You'd get the same problem with the allocators (that they always raise 
an exception) with this formulation.

If you did want a subtype to use for an initialized allocator (which the OP 
specifically wasn't using), then

    subtype S is not null T;

works just fine. And this at least is guaranteed to be checked everywhere 
necessary, that's not true for a Dynamic_Predicate (as you know).

Probably in most cases, they are equivalent, but when they are not 
equivalent, it is just because you have pushed static checks to runtime, 
which hardly seems to be an advantage.

                                              Randy.