Re: and then... (a curiosity)

[stefan-lucks@see-the.signature]

On Thu, 4 Sep 2008,  wrote:

> > Very strange that the Ada compiler is able to compile stuff if you write
> > "and then" instead of "and". Following your point, the designers of Ada
> > did overestimate the smartness of compilers. ;-)
> 
> My objections are targeted at the thing that you propose (elsewhere called
> 1+2). That beast would either allow bugs to hide, or would require whole
> program analysis and perhaps mind-reading capabilties.

Firstly, I have been very clear that I didn't "propose" that. 

Secondly, it is easy to implement. Replace "if A and B then ... else ... 
end if" by:

declare 
  T_A := A;
begin
  if T_A then 
    return B; -- may raise an exception
  end if;
exception
  when others =>
    if B -- may raise an exception
    then 
      raise; -- propagate the exception raised by A 
    else
      return False;
    end if;
end;

Hey, do you need mind-reading capabilities for that? I would consider 
this an exercise for Students -- and a really easy one.;-)

Now, a really ugly issue is that both the evaluation of A and the 
evaluation of B might raise an exeption, and we cannot re-raise both 
exceptions. If not for other reasons, that suffices not to propagate that. 
Well, one could raise a Program_Error in that case, but that is ugly 
anyway. But note that even in Ada-as-it-is, it is not defined which of 
both exceptions is raised!

> > is that the language "and" actually attempts to mimic the mathematical
> > "and".
> 
> Yes, I agree and that is very unfortunate. But while we are at it. The
> 'designers' of logic made the same mistake: they have chosen an
> interpretation that difference from the word "and" in English. 

OK, here we disagree. They wanted the "and", because they wanted to mimic 
the mathematical and -- as much as the designers of most other languages. 
As much as integer addition mimics the addition over the integers ... 

> Quite possible. Perhaps a logic with:
> 
>    false and undefined  <=> undefined and false <=> false
> 
> If that is the definition, then that would be possible. But this is not the
> nave definition from Boolean algebra. This definition must be learned, but
> then you could equally well learn both well-defined definitions as from Ada.
> 
> But you may still prefer above definition from a three-valued logic. I
> agree, the logic is well-defined and mathemtically elegant. However, it is
> an engineering disaster. We write millions of lines of code and we must deal
> with violations, exceptions, and validations errors. 

The better it would be to focus on actual violations. Instead of hunting 
an exception raised in fragments such as "if X/=0 and Y/X<Z".


-- 
------ Stefan Lucks   --  Bauhaus-University Weimar  --   Germany  ------
               Stefan dot Lucks at uni minus weimar dot de
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------