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: 103376,a84eaf8fb2470909
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news1.google.com!news4.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!newsfeed00.sul.t-online.de!newsfeed01.sul.t-online.de!t-online.de!newsfeed.arcor.de!newsspool2.arcor-online.net!news.arcor.de.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada generics
Newsgroups: comp.lang.ada
User-Agent: 40tude_Dialog/2.0.15.1
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Reply-To: mailbox@dmitry-kazakov.de
Organization: cbb software GmbH
References: <1166710494.869393.108730@a3g2000cwd.googlegroups.com>
 <17fe4xfogg7p5.1dcyc5nyc2gsl.dlg@40tude.net>
 <1166805696.291429.239590@48g2000cwx.googlegroups.com>
 <wvsqamnzgasf.gfemd82fzfpz$.dlg@40tude.net> <DFQjh.1222$oo4.1118@trndny09>
Date: Mon, 25 Dec 2006 15:39:03 +0100
Message-ID: <186qujlcx6rwl.1h6eq4mbdaa5s$.dlg@40tude.net>
NNTP-Posting-Date: 25 Dec 2006 15:39:01 CET
NNTP-Posting-Host: 72859966.newsspool1.arcor-online.net
X-Trace: 
 DXC=0B[`]A_a@PI2jYf>V4L0gLic==]BZ:afN4Fo<]lROoRA^YC2XCjHcbIhWXNQgB4?MKDNcfSJ;bb[EIRnRBaCd<MN`oJj=^ljCIKL]gU`:FbBhM
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news2.google.com comp.lang.ada:8011
Date: 2006-12-25T15:39:01+01:00
List-Id: <comp.lang.ada>

On Mon, 25 Dec 2006 13:49:23 GMT, Hyman Rosen wrote:

> Dmitry A. Kazakov wrote:
>> Automatic conversions do not represent any special case. If S is
>> convertible to T, then S is a subtype of T
> 
> That simply is not the case in C++. Automatic conversions do not
> define subtypes in any meaningful or useful sense. For example,
> all the numeric types automatically convert to each other. It's
> also possible to write automatic conversions between arbitrary
> classes.

Ah, but it is not required for S<:T & T<:S be wrong. Two types can be
subtypes of each other like int and float are in C. It is a common
misconception about subtypes to believe that either must not hold. Observe,
that it is wrong in Ada either. Consider this:

   type T is range 1..100;
   subtype S is T;
   procedure Exported_To_Base (X : in out S);

   X : T;
begin
   Exported_To_Base (X);  -- It is fine!

See, the operation Exported_To_Base defined on S was exported to the base
type T. In other words T inherits it from S, which is equivalent to say
that T is a subtype of S. T and S are sub-/supertypes of each other.

>> Exactly. The idea of structural matching is wrong.
> 
> It's not wrong, and it's not even structural. It is in fact
> matching by name, not by structure.

But in foo<123>, 123 is not a name. It could be in some other language
where I could do:

int 123() const
{
   return ::124; // (:-))
}

But that would make many of instantiations ambiguous.

> It's only in the case of
> constant expressions that things go a little awry. But if you
> recall the discussions we've had here on implementing units,
> it lets C++ do things that are impossible in Ada.

You mean automatic instantiation here, but I don't want it. I also don't
want this solution for this problem. I even dare say that this is a bad
solution for *any* problem. Looking a bit more deeply into the issue, there
are two things to address:

1. compile-time computations

2. types computations (types algebra)

Generics and templates offer some quite weird and limited way to have 1.
and 2. Ask yourself, is it the only way?

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