[phobos] Exception chaining

[Andrei Alexandrescu]

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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