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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Half Constrained Array Types and Slices
Date: 10 Mar 2006 12:38:58 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2006-03-10T12:38:58-05:00
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"Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org> writes:

> I think the index type should be discrete. But surely it doesn't need to
> be an integer:
> 
> with Ada.Text_IO;
> procedure Null_Range is
>     type A is (B, C);
> 
>     type D is array (A range <>) of Integer;
> 
>     E : D (C .. B);
> begin -- Null_Range
>     Ada.Text_IO.Put_Line (Item => A'Image (E'Last) );
> end Null_Range;
> 
> This is valid Ada.

Indeed it is.  I don't think it's very useful, however.
My claim is that if you want empty arrays, you almost
certainly want to index them by (non-modular) integers.
Which is why I'm happy with the restriction in the Containers
library that Vector indices must be signed integers.

>... With GNAT, it outputs B. However, if D were a string
> type, "" would not be valid for it.
> 
> > The idea that 'Range is equivalent to 'First..'Last seems reasonable to
> > me...
> 
> I'm not saying it's a good idea. I'm just wondering what the
> consequences of it are.

I suppose the consequence is bugs -- rare cases where 'First and/or
'Last surprisingly raise Constraint_Error.

- Bob