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From: Richard Riehle <richard@adaworks.com>
Newsgroups: comp.lang.ada
Subject: Re: Another Idea for Ada 20XX
Date: Wed, 05 Dec 2001 20:14:24 -0800
Organization: AdaWorks Software Engineering
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Date: 2001-12-06T04:15:50+00:00
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Wes Groleau wrote:

> Larry Kilgallen wrote:
> > If they simply used common type declarations, a lot of errors would
> > be detected.  If Meters is a new type and is used throughout, and
> > Feet is another new type, it becomes difficult to add them.
>
> But that's not enough to prevent 2 Meters * 2 Meters = 4 Meters
> nor to allow 10 minutes * 5 KPH = 833 Meters
>
> --
> Wes Groleau
> http://freepages.rootsweb.com/~wgroleau

Correct.  However, a simple type declaration is just part of the
solution.
I don't believe it requires a change to the language.  Rather, it begs
for a
simple object-oriented programming solution, along with some
old-fashioned
encapsulation.

         package Metric_Number is
              type Metric_Type is private;
              -- export services for arithmetic on this type
              function "+"(L,  R : Metric_Type) return Metric_Type;
              -- more arithmetic and boolean functions
         private
              -- full definition for the Metric_Type;
              -- helper type to avoid recursion during implementation
         end Metric_Number;

In this way, we can control the behavior of each function with some
exactness
instead of depending only on the predefined behavior of ALRM Chapter 4
for
numeric types.   Of course, if we also had pre- and post-conditions as
part of
the language, this contract could be made even more robust.   In that
case, there
would definitely be no need for a specialized language feature.

Richard Riehle