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

[Jeffrey Carter <spam@spam.com>]

brett_gengler@yahoo.com wrote:

> I created a type whose order can not be rearranged and I want to create
> a subtype of that type.  So,
> 
> type msg is ( msg_a, msg_b, msg_c, msg_d, msg_e);
> 
> subtype vowel_msg is msg (msg_a, msg_e); --wrong, but why?

Ada only allows subrange subtypes of enumeration types. Sure, there are 
a lot of things it would be nice to be able to do, but the language only 
allows certain things.

> Is it true that I can't define a a subtype that's not a ordered
> subrange of a type?  My issue is that I want to enforce the subtype at
> compile time instead of creating an array of booleans and doing a run
> time check. (This is for an interface and I don't trust people to send
> me the right types).

It's true for elementary types.

> 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:

function To_Vowel is new Ada.Unchecked_Conversion
    (Source => Msg, Target => Vowelmsg);

It_Is_A_Vowel : constant Vowelmsg := To_Vowel (Any_Msg);

if It_Is_A_Vowel'Valid then -- It's a vowel!

-- 
Jeff Carter
"You cheesy lot of second-hand electric donkey-bottom biters."
Monty Python & the Holy Grail
14