Returning ranges from a function (was: How to loop (elegant) throug a vector ?)

[Stefan Bellon <bellon@software-erosion.org>]

On Fri, 20 Jun, Reinert Korsnes wrote:

> -- I want something more beautiful than this loop:
> 
>    for k in 1 .. Positive(X.Length) loop
>        do_something(X.Element(k));
>    end loop;

Sorry to high-jack this thread, but I'd like to comment on a somewhat
related topic: For a very long time I was looking for some way to
return a range from a function so that you can do something elegant
like in the following:

   for I in Get_Range loop
      Do_Something (I);
   end loop;

I always thought it to be a deficiency of Ada not being able to return
a range from a function.

A few weeks ago I realised the following solution, which I'd like to
share:

   generic
      type Index_Type is (<>);
   package Generic_Range_Objects is

      type Empty_Component is null record;
      for Empty_Component'Size use 0;

      type Range_Type is array (Index_Type range <>) of Empty_Component;
      pragma Pack (Range_Type);

   end Generic_Range_Objects;

And an instance for Integer ranges ...

   with Generic_Range_Objects;

   package Integer_Range_Objects is new Generic_Range_Objects (Integer);

Now you can do the following:

   function Get_Range
     return Integer_Range_Objects.Range_Type
   is
   begin
      return (42 .. 128 => <>);
   end Get_Range;

And then:

   for I in Get_Range'Range loop
      Do_Something (I);
   end loop;

It's not as elegant as I wished (because of the additional 'Range), but
it is simple and light-weight enough to be usable.

-- 
Stefan Bellon