[phobos] Exception chaining

[Don Clugston]

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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>>>>> phobos at puremagic.com
>>>>> http://lists.puremagic.com/mailman/listinfo/phobos
>>>>
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