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,FREEMAIL_FROM,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,3885b7fd66a1db28
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2003-01-11 10:13:50 PST
Path: 
 archiver1.google.com!news1.google.com!newsfeed.stanford.edu!bloom-beacon.mit.edu!nycmny1-snh1.gtei.net!news.gtei.net!newsfeed.mathworks.com!cyclone.swbell.net!bos-service1.ext.raytheon.com!bos-service2.ext.raytheon.com.POSTED!not-for-mail
From: Wes Groleau <wesgroleau@despammed.com>
Reply-To: wesgroleau@despammed.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US;
 rv:1.1) Gecko/20020826
X-Accept-Language: en,es-MX,es,pt,fr-CA,fr
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: Why is Ada NOT a good choice for a beginner to programming?
References: <au0nld$b0$1@bunyip.cc.uq.edu.au>
   <wccd6nsjuzu.fsf@shell01.TheWorld.com>
 <d7ch0vsi7ek3n1occ3l6ulve15j1qmatj6@4ax.com>
 <wccfzso9rbb.fsf@shell01.TheWorld.com>
 <BrXQ9.2323$c6.2543@bos-service2.ext.raytheon.com>
 <8ivo1vo5ir3piqsck4ondj0cuo47g426kf@4ax.com>
 <9gMS9.483613$GR5.189928@rwcrnsc51.ops.asp.att.net>
 <ft_S9.2369$c6.2643@bos-service2.ext.raytheon.com>
 <Kf%S9.3568$yi6.79862@newsfep1-gui.server.ntli.net>
 <Ga1T9.2375$c6.2314@bos-service2.ext.raytheon.com>
 <avk4l9$19i$1@a1-hrz.uni-duisburg.de> <3E200981.DC3A4A37@adaworks.com>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <y9ZT9.2398$c6.2558@bos-service2.ext.raytheon.com>
Date: Sat, 11 Jan 2003 13:13:49 -0500
NNTP-Posting-Host: 151.168.133.155
X-Complaints-To: news@ext.ray.com
X-Trace: bos-service2.ext.raytheon.com 1042308830 151.168.133.155 (Sat,
 11 Jan 2003 13:13:50 EST)
NNTP-Posting-Date: Sat, 11 Jan 2003 13:13:50 EST
Organization: Raytheon Company
Xref: archiver1.google.com comp.lang.ada:32919
Date: 2003-01-11T13:13:49-05:00
List-Id: <comp.lang.ada>

Richard Riehle wrote:

> We can still achieve compile-time safety using generic formal package
> parameters.   The following code does not directly address the derived
> type issue,  but it does illustrate a model that can be modified to
> provide a solution to this problem.  In some ways, it is like putting
> a wrapper around the data type, as suggested in the C++ example
> from Hyman Rosen.

Except the multiplying operators package should have three formal
types (and a conversion factor that defaults to 1.0) so that it
can provide

   TA * TB = TC      TC / TA = TB
   TB * TA = TC      TC / TB = TA

And we still haven't made length * length = length illegal.
Can this be done as follows?

Application:

    package Length_Types is
            new Dimensioned_Float (Significant_Digits => 12);

    type Length is new Length_Types.Dimension;

    package Area_Types is
            new Dimensioned_Float (Significant_Digits => 12);

    type Area is new Area_Types.Dimension;

    package Multipliers is
            new Multiplying_Operators (Term_One, Term_Two => Length,
                                       Product            => Area);

Library:

    generic
       ... Significant_Digits ...
    package Dimensioned_Float is

       type Dimensioned is digits Significant_Digits;

       -- "+" and "-" are now available just as we want them

       -- Now we must declare the default "*" and "/" illegal:
       function "*" (Left, Right : Dimensioned)
                     return        Dimensioned
          is abstract;

       function "/" (Left, Right : Dimensioned)
                     return        Dimensioned
          is abstract;

       ....

Or will the compiler _insist_ that the abstraction be implemented?