[phobos] Exception chaining

Don Clugston dclugston at googlemail.com
Wed Jan 12 08:07:59 PST 2011 

On 12 January 2011 16:00, Sean Kelly <sean at invisibleduck.org> wrote:
> I think scope(failure) may need some work though, since it's modeled as a catch block.

Yeah, if it throws an exception it will clobber any existing ones,
instead of chaining with them. I think it should be implemented as a
finally{} which is bypassed in normal execution.


>
> Sent from my iPhone
>
> On Jan 12, 2011, at 12:46 AM, Don Clugston <dclugston at googlemail.com> wrote:
>
>> No, that's unaffected by the scheme. The scheme only affects
>> situations when an exception is thrown from inside a finally clause,
>> when the finally clause is being executed because an exception had
>> been thrown. It doesn't affect catch clauses, because once you're
>> inside the catch, the first exception is no longer in flight.
>>
>>
>> On 12 January 2011 09:27, Max Samukha <maxsamukha at gmail.com> wrote:
>>> Will one be able to replace exceptions? A common C# scenario:
>>>
>>> class SystemException : Exception
>>> {
>>>     this(string msg, Exception innerEx) { this(msg, innerEx); }
>>> }
>>>
>>> class SubsystemException : Exception
>>> {
>>>      this(string msg) { this(msg); }
>>> }
>>>
>>> void system()
>>> {
>>>     try
>>>     {
>>>          subsystem();
>>>     }
>>>     catch (SubsystemException ex)
>>>     {
>>>         // subsystem exception is replaced with system exception and linked
>>> to the latter
>>>         throw new SystemException("a system exception", ex);
>>>     }
>>> }
>>>
>>> void subsystem()
>>> {
>>>     throw new SubsystemException("a subsystem exception");
>>> }
>>>
>>> void main()
>>> {
>>>       try
>>>       {
>>>           system();
>>>       }
>>>       catch (SystemException ex)
>>>       {
>>>            // catch system exceptions and subsystem exceptions are available
>>> via innerException property
>>>            writeln("system: ", ex, ", subsystem: ", ex.innerException);
>>>
>>>       }
>>> }
>>>
>>> As far as I understand, your scheme makes the above problematic.
>>>
>>> On Wed, Jan 12, 2011 at 1:50 AM, Andrei Alexandrescu <andrei at erdani.com>
>>> wrote:
>>>>
>>>> I don't think that's helpful. It complicates the flow a lot because now
>>>> understanding how the program acts depends not on the types anymore, but on
>>>> what happens dynamically. Makes it more difficult, not easier, to write
>>>> robust code.
>>>>
>>>> If I throw a FileException, I must catch a FileException with
>>>> catch(FileException) regardless of what collateral exceptions have happened.
>>>>
>>>>
>>>> Andrei
>>>>
>>>> On 1/11/11 12:31 PM, Don Clugston wrote:
>>>>>
>>>>> I've thought about this a bit more. Another simple rule is, that an
>>>>> exception chain can be caught if  and only if every exception in that
>>>>> chain can be caught.
>>>>> So, for example,
>>>>> catch(FileException) will catch multiple file exceptions.
>>>>> catch(Exception) will catch any exception (but not Errors).
>>>>> catch(Throwable) catches Errors as well.
>>>>>
>>>>> I went ahead and implemented this. Everythings seems to Just Work.
>>>>> Will check it in shortly.
>>>>>
>>>>>
>>>>> On 11 January 2011 18:30, Andrei Alexandrescu<andrei at erdani.com>  wrote:
>>>>>>
>>>>>> Wow, this is incredible news!
>>>>>>
>>>>>> Thanks Don for working on this. Solid exception handling is a huge
>>>>>> selling
>>>>>> point for D.
>>>>>>
>>>>>> Regarding collateral throwables that are not Exception, good point (and
>>>>>> I
>>>>>> agree that the solution should be simple). TDPL doesn't discuss that
>>>>>> issue,
>>>>>> but it says that the initially-thrown exception is the "boss" and that
>>>>>> everybody follows, so a possible design is to simply make the Throwable
>>>>>> part
>>>>>> of the chain.
>>>>>>
>>>>>> I'd want to have chained exceptions still catchable by catch (Exception)
>>>>>> because it would be a first to have the contents of the data influence
>>>>>> its
>>>>>> type. As far as the type system is concerned, catch (Exception) should
>>>>>> catch
>>>>>> Exceptions, whether or not they have a tail.
>>>>>>
>>>>>> One possibility would be to move the Throwable to the front of the list.
>>>>>> This also has its issues, for example the stack is unwound for a while
>>>>>> and
>>>>>> then not anymore (a Throwable is allowed to respect fewer rules than an
>>>>>> Exception).
>>>>>>
>>>>>> Ideas please?
>>>>>>
>>>>>>
>>>>>> Andrei
>>>>>>
>>>>>> On 1/11/11 1:57 AM, Don Clugston wrote:
>>>>>>>
>>>>>>> I believe I have got TDPL exception chaining working correctly using
>>>>>>> Windows Structured Exception Handling.
>>>>>>> (This was far from easy!)
>>>>>>> Central to making chaining work correctly, is that chaining must only
>>>>>>> occur
>>>>>>> when a collision occurs (not merely when two exceptions are in flight,
>>>>>>> because one may be caught before it has any effect on the other). This
>>>>>>> means that multiple chains of exceptions
>>>>>>> may be in flight at any given time.
>>>>>>> My code works in all nasty corner cases I've tested, including
>>>>>>> multi-level collisions,
>>>>>>> where two exceptions collide in a function, then collide again with an
>>>>>>> even earlier exception chain in a finally block in a different
>>>>>>> function.
>>>>>>>
>>>>>>> So the general scheme appears to work.
>>>>>>> But, there's something I'm unclear about. When should chained
>>>>>>> exceptions be catchable?
>>>>>>> They are very nasty creatures, and you really want to know when they
>>>>>>> happen.
>>>>>>> Presumably, an AssertError which occurs while processing an
>>>>>>> FileException, should not be silently chained
>>>>>>> and caught in the FileException.
>>>>>>> In fact, should a chain containing an Error be catchable at all?
>>>>>>> (If not, it still has to at least be catchable in the catchall handler
>>>>>>> that wraps main()).
>>>>>>> Many other schemes are possible, but I think it's important that the
>>>>>>> rules remain simple.
>>>>>>>
>>>>>>> One simple solution would be to make chained exceptions only catchable
>>>>>>> by catch(Throwable).
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