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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Is the Writing on the Wall for Ada?
Date: Fri, 03 Oct 2003 22:30:02 +0200
Organization: At home
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Date: 2003-10-03T22:30:02+02:00
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Robert I. Eachus wrote:

> Dmitry A. Kazakov wrote:
> 
>  > Which only means that there is a deficiency in array design. They
>  > have to be allowed to have discriminants...
> 
> You can't have thought about this.  Ada does allow Multidimensional
> arrays, arrays of arrays, and ragged arrays.  But they are different,
> and vivé la difference.  I could even create an ISAM type in Ada, but
> why bother, ragged arrays are usually better.  The point is though that
> all of these structures have structure invariants which are different.
>
> An array is a structure where all the elements are alike.  Ada lets you
> play games by "hiding" properties of elemental types to get this
> alikeness.  You can have arrays of types with discriminants, but if you
> do, all the elements in an array have the same discriminant.

This is what I want. And this is what arrays cannot:

type Element (Constraint : Natural) is ...;
type Array_Of_Elements is array (...) of Element;

This is not allowed, because I cannot constrain Element, and there is no way
to do it other than create a constrained subtype of Element.

Even if I pack the array into a record to have a discriminant, even so, it
will be still illegal:

type Array_Of_Same_Elements (Constraint : Natural) is record
   Body : array (...) of Element (Constraint);
end record;

Note that it is *not* a ragged array. All elements are of exactly *same*
size. What is required is:

type Array_Of_Same_Elements (Constraint : Natural) is
   array (...) of Element (Constraint);

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