Re: Operation can be dispatching in only one type

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

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:18wh86jvjvoe0.cofxcc8udm6q$.dlg@40tude.net...
> On Thu, 19 Nov 2009 17:54:40 -0600, Randy Brukardt wrote:
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>> news:1iipp3bn16fe2.yqa1gz1ru17a$.dlg@40tude.net...
...
>> That might be true in general, but definitely not in this case: a handler
>> for Program_Error is *always* wrong (unless it is a global handler for 
>> *any*
>> exception), as Program_Error always represents a program bug. So I don't 
>> see
>> how "existing" handlers can be confused.
>
> exception
>   when Error : others =>
>      some cleanup  -- This usually becomes a problem
>      raise;

If that's the case, "some cleanup" is seriously flawed. An "others" handler 
has to always work no matter what condition come into it, and in particular 
cannot depend on the code/data it surrounds to be in any particular state. 
The clean up code needs to be aware that data structures may be corrupt; if 
it isn't it of course will cascade errors and generally make a mess -- but 
in that case it is just junk code doing more harm than good.

I believe this particular pattern ("others" handler containing explicit 
cleanup) usually represents a bad design. Clients (or even the implementers 
of ADTs) often forget to include the cleanup where it is needed. These days, 
I try to put all needed cleanup into the ADTs, so that they get cleaned up 
no matter what -- finalization is pretty much the only certainty in Ada. And 
in that case, you don't need these kind of exception handlers.

>> It is necessary to treat all Adjust and Finalize routines like task 
>> bodies
>> in that they need a "when others" handler -- otherwise the exceptions are
>> sucked up and you get completely lost. Our programming standard requires
>> such handlers (generally, they output Exception_Information to the 
>> logging
>> facility - and almost every significant Ada system needs some sort of
>> logging facility).
>
> Yes, but this does not help. Upon exception propagation 
> (Constraint_Error),
> you get some objects finalized. This in turn causes a snowball of
> exceptions in finalized objects, because no design is robust to hold any
> error at any place. In the end you have a huge log of meaningless 
> messages,
> which only complicate debugging.

That can happen, but it usually indicates too much coupling between objects 
and/or too little concern about the implications of errors. Finalize 
routines should never, ever propagate an exception, and need to be written 
to avoid that - that means avoiding assumptions about other objects. If 
that's not done, of course you get messy cascades of errors. I realize that 
it isn't completely possible to avoid these things, but it can be minimized.

And the huge log of messages doesn't matter, you only need the first one 
(presuming every "others" handler logs the data). That's the place the 
exception was raised, and the information tells you how it got to that 
point; just ignore the rest of the log (in that sense it is just like fixing 
syntax errors in most compilers -- the correction isn't good enough to find 
more than the first error if it is at all significant).

> Sometimes I which Ada had "halt at once"
> statement which would stop all tasks and hold any I/O.

Janus/Ada does have such a thing (always did, it was the first thing 
implemented on CP/M back in 1980). It works because (to date) we don't use 
operating systems facilities for tasking, exception handling, or 
finalization. But it is pretty dangerous, and really can be used only to 
instantly shut down a misbehaving system -- but of course if there is any 
real resource management being handled by finalization, you could leak 
resources such that you don't get them back easily (the obvious example is 
temporary files, which won't be cleaned up in that scenario).

> But of course the proper solution would be contracted exceptions.

I don't see how that would help. The problem is used "others" when you 
really need to list the exceptions that you are expecting. If the compiler 
is smart enough to be able to prove that no bounded errors occur (and no 
constraint violations either), maybe that would do some good, but I doubt 
that you will see such compilers anytime soon. In the absence of that, you 
have to put Program_Error, Constraint_Error, and Storage_Error into every 
contract, so you don't gain much.

>>> The difference is that for string bound there is a way to do it safe and
>>> for 'Access there is none (and I also agree with Robert's response.)
>>
>> Well, Ada 2012 (or whatever it will be called) should help that out by
>> giving you a way to compare accessibilites (via memberships). So at least
>> you will be able to make checks to avoid the problem. Better than 
>> nothing,
>> but still not ideal. The better way to avoid the problem is to never, 
>> ever
>> use anonymous access types. (Named access types have checks that are 
>> always
>> made at compile-time for most compilers -- but not Janus/Ada, because of
>> generic sharing.)
>
> The problem is not bound to only anonymous types. It also appears when you
> convert access types. The source type (or both) might be a formal generic
> parameter, so you cannot statically ensure that a "local" pointer is
> converted no a "less local" one. Pool specific pointers need to be
> converted though one target object is derived from another. Here 
> everything
> is broken: generic contract, meaningless conversion, meaningless check,
> meaningless exception.

In this case, whether a check will fail is known statically for any compiler 
that generates generics via macro substitution (that is all Ada compilers 
other than Janus/Ada). I can hardly imagine that there exists a compiler 
that will generate "raise Program_Error" unconditionally without generating 
a warning! So this is an academic concern at best (I don't think you're 
using Janus/Ada) -- yes, you can ignore the warnings and run such a program, 
but there isn't any reason to actually do so unless you're running an ACATS 
test. (And if you do the conversion in the generic specification, the 
compiler is required to reject the program.)

                       Randy.