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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Exceptions
Newsgroups: comp.lang.ada
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Date: Sat, 8 Dec 2007 15:01:02 +0100
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Date: 2007-12-08T15:01:03+01:00
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On Sat, 08 Dec 2007 07:26:21 -0500, Peter C. Chapin wrote:

> The problem often quoted is that there are cases where an exception
> looks like it could happen, but for reasons the compiler doesn't know
> can't actually happen. As a simple example consider the code fragment
> 
> if Has_Acceptable_Value(X) then
>   P1(X);
>   P2(X);
>   P3(X):
> end if;
> 
> Suppose P1 raises an exception if X is negative. Depending on what
> Has_Acceptable_Value does, it is quite possible that can't happen in the
> code above. If you specify exception contracts the way Java did, then
> the programmer in the above case ends up having to handle an exception
> that won't happen or claim to propagate and exception that won't get
> propagated. Either option is undesirable.

The problem here is not exception contracts but firstly lack of subtypes in
C++. Ada has subtypes, so the code above could be rewritten [properly]:

if X in Safe_For_P1 then
   declare
      Checked_X : Safe_For_P1 renames X;
   begin
      P1 (Checked_X); -- This does not raise exception
 
The actual problem for Ada is that there is no syntax sugar for "if X in S
then X indeed in S" (:-)) This is not about exceptions, we also do

if Ptr /= null then
   Foo (Ptr.all);

No way to assure that Ptr cannot be null in Ptr.all to require the compiler
to drop checks.

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