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: 103376,43f6bd9b498b66d0
X-Google-Attributes: gid103376,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news3.google.com!out02b.usenetserver.com!news.usenetserver.com!in04.usenetserver.com!news.usenetserver.com!nx02.iad01.newshosting.com!newshosting.com!newsfeed.icl.net!newsfeed.fjserv.net!newsfeed.freenet.de!feeder.news-service.com!newsfeed-fusi2.netcologne.de!news.netcologne.de!newsfeed-hp2.netcologne.de!newsfeed.arcor.de!newsspool1.arcor-online.net!news.arcor.de.POSTED!not-for-mail
Date: Thu, 06 Mar 2008 10:32:26 +0100
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
Reply-To: rm.tsoh+bauhaus@maps.futureapps.de
User-Agent: Thunderbird 2.0.0.12 (Windows/20080213)
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: default formal parameters in generic declarations
References: 
 <9b3bac4d-5ae1-4a1b-a81e-9aa9ae1843e0@e31g2000hse.googlegroups.com>
 	<wccskz93tv5.fsf@shell01.TheWorld.com>
 <a520f018-097c-406b-a3e3-387a28d38216@s8g2000prg.googlegroups.com>
 	<fqigiv$foi$1@jacob-sparre.dk>
 <d84f0434-bace-4b99-b10d-f4a56807eacd@d62g2000hsf.googlegroups.com>
 	<fqk57t$l10$1@jacob-sparre.dk>
 <0078161e-64c7-4fa8-9794-b840d855a88b@q78g2000hsh.googlegroups.com>
In-Reply-To: 
 <0078161e-64c7-4fa8-9794-b840d855a88b@q78g2000hsh.googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-ID: <47cfba2a$0$14412$9b4e6d93@newsspool3.arcor-online.net>
Organization: Arcor
NNTP-Posting-Date: 06 Mar 2008 10:32:26 CET
NNTP-Posting-Host: 171cd799.newsspool3.arcor-online.net
X-Trace: 
 DXC=cA47`:n:i6P\9P[:DUn00QMcF=Q^Z^V3X4Fo<]lROoRQ4nDHegD_]RU>@L]d:eD9:]A:ho7QcPOVS8Y_O[Ki:BkS3LIHH:B:PP_
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news1.google.com comp.lang.ada:20191
Date: 2008-03-06T10:32:26+01:00
List-Id: <comp.lang.ada>

Eric Hughes wrote:

> Even with just two layer of instantiation this is feasible, but, in
> the phrase I used earlier, "horribly klunky" (particularly with non-
> linear references).  So here's the biggest problem: You cannot get
> simultaneously both arbitrarily deep layering and concision in formal
> parameters for ordinary usage.

By way of speculation, can nesting, to the extent it is absent from C++,
lead to at least a partial solution of this problem?

package News4 is

   type Performance_Tuning is interface;

   function Null_Tuning return Performance_Tuning'Class;

   generic
      Tuner: Performance_Tuning'Class := Null_Tuning;
   package B is

      generic
         Tuner: Performance_Tuning'Class := B.Tuner;
      package A is end;

   end B;

end News4;


To some extent, this view would require building software with "towers"
of building blocks in mind, rather than patterns of cobblestone, so to
speak.


> As a coda, I wish to add that this practice is not hypothetical.  The
> sophistication of some of the generic (template) programming in the
> Boost C++ library is excellent and worth imitating.

Again by way of speculation, and as an example, the Boost
parsing facilities have a counterpart in Ada that just works
differently, using access to nested functions.
GNAT.Spitbol.Patterns solves the same practical problems
as the spirit parsers, and more. There might be a small
overhead because pattern construction does not occur at
compile time, but then Spitbol.Patterns is actually more powerful.

I'm noting all this only because I should think there is some value
in nesting that, just maybe, is easily overlooked.