Re: Language checks (Was: C++0x and Threads - a poor relation to Ada's tasking model?)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 14 Aug 2009 13:24:09 +0200, Jacob Sparre Andersen wrote:

> Dmitry A. Kazakov wrote:
> 
>> * Example in Ada. If you wanted to iterate two arrays at once, you could
>> not eliminate index checks of one of them:
>> 
>>    for I in A'Range loop
>>       ... A (I) + B (I + B'First - A'First) ...
>>                              -- No way to eliminate B index check
>>    end loop;
> 
> In your example we don't know if the index check on B is needed.  An
> alternative:
> 
>    if B'Length >= A'Length then
>       for I in A'Range loop
>          ... A (I) + B (I + B'First - A'First) ...
>       end loop;
>    else
>       ...
>    end if;
> 
> Here a compiler should be able to figure out statically that there is
> no need for an index check on B in the loob.

I would prefer something more universal and binding than a compiler with
AI. For example:

declare
   B_View : array (Index range A'Range) of Element renames B;
begin
   for I in A'Range loop
      ... A (I) + B_View (I) ...
   end loop;

renaming raises Constraint_Error if something goes wrong.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de