Re: Why can't you create a out of order subtype?

[Martin Dowie <martin.dowie@btopenworld.com>]

Marius Amado Alves wrote:
>> ...
>>
>>> Another option that I think is much, much worse then the boolean array
>>> would be create an overloaded type...
>>> type msg is ( msg_a, msg_b, msg_c, msg_d, msg_e);
>>> type vowelmsg ( msg_a, msg_e);
>>> But as you know, msg.msg_e = 4 and vowelmsg.msg_e = 1 so an unchecked
>>> conversion is wrong on so many levels.
>>
>>
>> Yes, this may be worse, but you can at least get Unchecked_Conversion 
>> to work:
>>
>> for Vowelmsg use (Msg_A => 0, Msg_E => 4);
>> for Vowelmsg'Size use Msg'Size;
>>
>> This would still be a run-time check:...
> 
> 
> For run-time I find the "Value of Image" idiom a better one. It's 
> readily available. No Unchecked_Conversion required.
> 
> declare
>    M : Msg := Whatever;
>    V : Vowelmsg;
> begin
>    V := Vowelmsg'Value (Msg'Image (M));
>    -- if control reaches here then M is a vowel
> exception
>    when Constraint_Error => -- M is not a vowel
> end;

That's very, very, very slow...

Cheers

-- Martin