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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Counter-proposal for variable arrays
Date: Fri, 03 Oct 2003 22:29:53 +0200
Organization: At home
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Date: 2003-10-03T22:29:53+02:00
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Wes Groleau wrote:

> Dmitry A. Kazakov wrote:
>> Which only means that there is a deficiency in array design. They have
>> to be allowed to have discriminants. Then:
> 
> Even better (if it doesn't open some can of worms)
> would be
> 
>     (sub)type Variable (Lower : <discrete> := <default;
>                    Upper : <discrete> := default)
>          is array (Lower .. Upper) of <ANY_type>;
> 
>     Varies : Variable;

This is another thing. You want to expose the array bounds as discriminants
it would be also possible. But the main problem with arrays is an inability
to constrain elements together with the array:

type Line_Buffer (Height, Width : Positive) is
   array (Integer range 1..Height) of
      String (1..Width); -- Illegal

Moreover, if type tags were considered as discriminants (and they should),
one could even have the arrays of class-wide elements, which could save
rather painful and clumsy attempts to design generic containers "a la" STL.
After all the whole idea behind STL containers is to provide arrays which
C++ does not have!

Yet another issue is the defaults for discriminants. I think that the idea
to tie an existence of the defaults with treating the type constrained (by
the maximal size) was altogether wrong. These issues are unrelated. There
should be a different and a better way to specify this. Especially, to have
constrained subsets of X'Class. For instance, by specifying the whole range
of tags X'Class might have.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de