Re: disjoint ranges ?

[Lao Xiao Hai <laoxhai@ix.netcom.com>]

Robert A Duff wrote:

> Lao Xiao Hai <laoxhai@ix.netcom.com> writes:
>
> > So, making a feature more powerful might be something such as being
> > able to overload the "in" membership test.
>
> It might be in the abstract, but in the context of Ada, probably not.
>
> >...   For some reason, this
> > operator is not really a first class operator in Ada.   I have often
> > wondered about the rationale for that decision.
>
> What is the type of the right-hand side of "in" here:
>
>     if X in Integer range 1..10 ...
>
> ?  Ada has no way of describing that type.  In order to make "in" a
> normal operator, we would need to invent a way to pass types as
> parameters, and talk about the type of a type (and subtypes).  This is
> not entirely unreasonable in the abstract, but it would be an enormous
> change to Ada.
>
> Consider:
>
>     type T is range 1..100;
>     function "in"(X: T; Y: type-of-T?) return Boolean;
>
> Or maybe it should be T'in?  Anyway, it raises all kinds of questions.
> What *is* the range of T (given that "in" changes it)?

I see your point about the magnitude of the change required to pass a type
as
a parameter.    I suppose I did not think this through as carefully as I
should
have.   Still, it would be nice to be able to do something like this.
On the
other hand.   I don't think there is much virtue in making wrenching
changes
in the language design.

Thanks,

Richard