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From: "Pat Rogers" <progers@classwide.com>
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Subject: Re: Refactoring and Ada
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"Richard Riehle" <richard@adaworks.com> wrote in message
news:3C7324BF.996E182B@adaworks.com...
<snip>
> It turns out that 'Valid is not as useful as many of us thought it was. For
> example,  the result of an erroneous result from unchecked conversion,
> which we originally thought was harmless if we checked it ourselves
> with 'Valid before using it is a problem.   Consider the following,
>
>                type X is ... ;
>                type Y is ... ;
>
>                function Convert is new Ada.Unchecked_Conversion
>                                          (Source => Y, Target => X);
>
>
>                 X := Convert (Y);     -- suppose X is erroneous or not valid
>                 if X'Valid then ...      -- We used to think this was OK.
>                                                  -- Some compilers fail on
this because of an
> interpretation
>                                                  -- of the ALRM rules.
>
>  At present, one must wonder about the usefulness of 'Valid.

What's the exegesis for this?