Re: disjoint ranges ?

[dmitry6243@my-deja.com]

In article <wccr95fobf6.fsf@world.std.com>,
  Robert A Duff <bobduff@world.std.com> 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)?

IMO, to make it consistent one should introduce built-in index types (or
better to say, make them visible). Compare:

type A is array (Integer range 1..100) of ...;

and

type B is index (Integer range 1..100);

Array slice is a subarray. Index slice is a range. Array indexed by a
value of its index type is a subarray. Then

type T is range 1..100;
type T_Index is index (T);

function "in" (X: T; Y: T_Index) return Boolean;

Surely indices can be multi-dimensional:

type Square_Index is index (Integer range 1..100, Integer 1..100);

Surely one needs an attribute to get an anonymous index type from array.
Ada already has it! It is 'range. Unfortunately 'range = 'range(1) for
multi-dimensional arrays.

Static values of index types can used where ellipsis ".." appear.
Actually "range N..M" is a literal of an anonymous index type
(or maybe, of universal index type).

--
Regards,
Dmitry Kazakov


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