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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: What would be the Ada solution?
Date: Fri, 04 Feb 2011 12:47:52 -0500
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mockturtle <framefritti@gmail.com> writes:

> A solution that I saw used somewhere
>
>    type Extended_Values is new integer range -1 .. 1023;
>    subtype Valid_Values is Extended_Values range Extended_Values'first + 1 .. Extended_values'last;
>  
>    No_Value : constant Extended_Values := Extended_Values'first;
>
> A variable that should always have a valid value is declared of type
> Valid_Values and you should get an exception when you try to assign
> No_Value to it.

Yes.  Or:

    type Valid_Values is range 0 .. 2**10 - 1;
    subtype Extended_Values is Valid_Values'Base range -1 .. Valid_Values'Last;

The base range is guaranteed to contain -1, because base ranges are
always symmetric about zero (usually with one extra negative value).

But these solutions involve implicit run-time checks.
Using two types instead of subtypes moves most of the
checking to compile time.

- Bob