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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: What is wrong with Ada?
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Date: Thu, 12 Apr 2007 21:39:40 +0200
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Date: 2007-04-12T21:38:51+02:00
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On Thu, 12 Apr 2007 14:47:19 -0400, Robert A Duff wrote:

> Brian May <bam@snoopy.apana.org.au> writes:
> 
>>...The only way you can
>> be sure the code is bug free is too regularly test every possible
>> path, ...
> 
> Well, yeah, static checking is helpful, but I think you overstate the
> case:
> 
> There is no way to be sure the code is bug free!  Not in any language.
> Not using testing, nor any other method (including so-called
> proof of correctness).
> 
> Testing every possible path is not enough.  Here's a function that
> detects prime numbers (except that it has a bug):
> 
>     function Is_Prime (X: Integer) return Boolean is
>         -- Returns True if and only if X is prime.
>     begin
>         if X = 2 then
>             return True;
>         else
>             return False;
>         end if;
>     end Is_Prime;
> 
> If I test with inputs 2 and 4, it gives the right answer for both,
> and I have covered all paths.  But I have failed to detect the bug.

Tests should cover all program states. Covering all paths is a rough
approximation of.

> And it is impossible to cover all paths in a program containing
> unbounded loops or recursion, because there are an infinite number of
> them.

But the number of program states is finite, or else the program is wrong
anyway? However the inputs and outputs of a program can be infinite,
uncountable, like hardware random generators or clocks.

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