Re: Contracted exceptions for Ada (was: Exceptions)

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:7m9wkymyi5h7.1235e72is9mp9.dlg@40tude.net...
> On Fri, 7 Dec 2007 21:30:05 -0600, Randy Brukardt wrote:
...
> > The ARG has discussed "exception contracts" a couple of times, but there
> > didn't seem that there was enough interest. The problem was mainly that
they
> > simply don't look very valuable
>
> My observation is that a big deal of debugging Ada code is about tracing
> down unexpected exceptions. I cannot give any figures, but it is extremely
> frequent that a bug manifests itself as an exception propagation.
Typically
> Constraint_Error propagates where you didn't expect it. So the temptation
> is to catch most of such things at compile time.

That's not going to be useful, since virtually any code that you can write
*might* raise Constraint_Error, and a lot of code *could* raise
Program_Error, and everything *could* raise Storage_Error. There is no way
to write a contract that doesn't contain those exceptions.

...
> > Another one is what to do if a contract is violated. The obvious answer
of
> > raising Program_Error doesn't do anything other than lose information
about
> > an exception, so that isn't very satisfying.
>
> Yes, in my view exception contracts have to be static. (It makes no sense
> to have run-time exception contracts. As well as any other contracts,
there
> is no any authority body to judge contract violations at run-time.)

OK, but that isn't going to happen in Ada. What you are looking for is a
static analysis tool, like SPARK. That's especially true as you'll need
static preconditions to have any hope of eliminating the Constraint_Errors
(subtypes aren't powerful enough). It is highly unlikely that the language
is going to go in that direction; the language has been very careful to
avoid requiring analysis beyond that of a single expression, and trying to
get vendors to re-architect their compilers is probably a good way to cut
the number of Ada implementations to zero.

             Randy.