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: formal array types and default values
Date: Wed, 3 Jan 2018 15:16:34 -0600
Organization: JSA Research & Innovation
Message-ID: <p2jh7j$d8g$1@franka.jacob-sparre.dk>
References: <053b2370-5c15-4662-a9e3-e1464de206a1@googlegroups.com>
 <5c75fcdd-c965-4d03-9161-3576212e674d@googlegroups.com>
Injection-Date: Wed, 3 Jan 2018 21:16:35 -0000 (UTC)
Injection-Info: franka.jacob-sparre.dk;
 posting-host="rrsoftware.com:24.196.82.226";
	logging-data="13584"; 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:49737
Date: 2018-01-03T15:16:34-06:00
List-Id: <comp.lang.ada>

"Mehdi Saada" <00120260a@gmail.com> wrote in message 
news:5c75fcdd-c965-4d03-9161-3576212e674d@googlegroups.com...
> Let's make it a proposal for the next norm then. I'm half serious: 
> depending on the difficulty,
> which I know nothing of, I am ok to help, however I can.
> Such a nice (and logical) feature, would a shame not to do.

You mean a useless feature. :-)  (See my response to Niklas about the 
difficulty.)

The primary purpose of Default_Value is to detect errors. Specifically, most 
enumeration types have (or could have) a value that means Unknown or 
Uninitialized. For instance:

    type My_Color is (Unknown, Black, Red, Green, Blue, White)
        with Default_Value => Unknown;

With this declaration, any programming errors that fail to set an object of 
My_Color will show up as a value of "Unknown". Which can be reliably 
detected.

Other uses of default values are evil, because they hide an operation that 
should always be explicit. For instance:
     type Evil_Integer is new Integer
         with Default_Value => 0;

Since 0 is a legal, useful value of type Evil_Natural, some code might work 
by accident when an uninitialized object happened to get the right value by 
default. That sort of thing will not help future readers.

                                   Randy.