[Haskell-cafe] Correct way to "catch all exceptions"

John Lato jwlato at gmail.com
Wed Jul 10 12:01:55 CEST 2013

On Wed, Jul 10, 2013 at 5:02 PM, Erik Hesselink <hesselink at gmail.com> wrote:

> On Wed, Jul 10, 2013 at 10:39 AM, John Lato <jwlato at gmail.com> wrote:
> > I think 'shouldBeCaught' is more often than not the wrong thing.  A
> > whitelist of exceptions you're prepared to handle makes much more sense
> than
> > excluding certain operations.  Some common whitelists, e.g. filesystem
> > exceptions or network exceptions, might be useful to have.
> You'd think that, but there are common use cases. For example, if you
> have a queue of work items, and a thread (or threads) processing them,
> it is useful to catch all exceptions of these threads. You can then
> log the exception, remove the item from the queue and put it in some
> error bucket, and continue on to the next item. The same goes for e.g.
> socket listening threads etc.
> The thing here is that you are *not* actually handling the specific
> exception, but instead failing gracefully. But you still want to be
> able to kill the worker threads, and you don't want to handle
> exceptions that you cannot recover from even by moving on to the next
> work item.

I think that's a particularly niche use case.  We have some similar code,
and our approach is to have the thread re-throw (or terminate) after
logging the exception.  There's a separate thread that monitors the thread
pool, and when threads die new ones are spawned to take their place (unless
the thread pool is shutting down, of course).  Spawning a new thread only
happens on an exception and it's cheap anyway, so there's no performance

As Haskell currently stands trying to sort out thread-control and
fatal-for-real exceptions from other exceptions seems rather fiddly,
unreliable, and prone to change between versions, so I think it's best
avoided.  If there were a standard library function to do it I might use
it, but I wouldn't want to maintain it.
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