Re: subtype of enumeration type

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Maciej Sobczak <no.spam@no.spam.com> writes:

> Consider this:
> 
>     type Day is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
> 
> and the following standard example:
> 
>     subtype Weekday is Day range Mon .. Fri;
> 
> Now imagine that in some country children don't have to go to school on
> Wednesdays, they go there only Monday, Tuesday, Thursday and Friday.
> 
> I want to say it in Ada.
> 
>     subtype Schoolday is Day <what goes here?>;

You can't do that.

You could put "pragma Assert(Some_Day /= Wednesday)"
all over the place, but that's not quite the same thing.

Eiffel allows arbitrary conditions as invariants,
but Ada has a fairly restricted set of constraints
(ranges for discrete types, a single value for
discriminant constraints, etc).

- Bob