[phobos] Exception chaining

[Lars Tandle Kyllingstad]

I think this is a good rule.  As you say, it is simple and easy to
understand, and it ensures no critical exceptions are "lost" somewhere
in a chain.  I would be very dissatisfied if my program throws an Error
and it is caught (indirectly) by a catch(FileException) clause.

-Lars



On Tue, 2011-01-11 at 21:31 +0100, 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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