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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: What is wrong with Ada?
Date: Thu, 12 Apr 2007 14:47:19 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2007-04-12T14:47:19-04:00
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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.

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

- Bob