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From: Matthew Woodcraft <mattheww@chiark.greenend.org.uk>
Newsgroups: comp.lang.ada
Subject: Re: Quality systems (Was: Using Ada for device drivers? (Was: the
  Ada mandate, and why it collapsed and died))
Date: 25 May 2003 11:53:37 +0100 (BST)
Organization: Linux Unlimited
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Date: 2003-05-25T11:53:37+01:00
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In article <3ECFF541.1010705@attbi.com>,
Robert I. Eachus <rieachus@attbi.com> wrote:
>It also seems like a problem that is easy to fix in Ada0Y, if not just 
>by getting compilers to do the right thing.
>
>You want to be able to say:
>
>type Unsigned is range 0..2**32-1;
>for Unsigned'Size use 32;

I would certainly like to see something of the sort.

At the moment, we have:

[3.5.4]
| 9   A signed_integer_type_definition defines an integer type whose
| base range includes at least the values of the simple_expressions and
| is symmetric about zero, excepting possibly an extra negative value. 
| A signed_integer_type_definition also defines a constrained first
| subtype of the type, with a range whose bounds are given by the
| values of the simple_expressions, converted to the type being
| defined.

It seems to me that something like your solution three would be needed:

>3) Add a pragma Unsigned which has to be used immediately after a type 
>declaration, and has the semantic effect that negative values of the 
>type can raise Constraint_Error even on intermediate calculations.  In 
>other words A-B+C could raise an exception if B < A, even if B < A+C.

3.5.4.9 would presumably need to change so that for unsigned types the
base range didn't include negative integers?

-M-