[phobos] Exception chaining

[Don Clugston]

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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