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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Language checks (Was: C++0x and Threads - a poor relation to Ada's
 tasking model?)
Date: 14 Aug 2009 13:24:09 +0200
Organization: Jacob Sparre Andersen
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Date: 2009-08-14T13:24:09+02:00
List-Id: <comp.lang.ada>

Dmitry A. Kazakov wrote:

> 1. These checks must be there, else the program is wrong
> 2. The language is wrong not allowing programmer to design these checks
> static [*]
> 
> When it is neither 1 nor 2, then it is something which we can prove
> to be OK in our program, while the compiler cannot. Is a normal
> programmer that smart?

Probably not.

> -----------
> * 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.

Greetings,

Jacob
-- 
�Later software patents have had a similar effect, they
 force us to stand on each other's toes instead of on each
 other's shoulders.�                       -- Per Abrahamsen