[Haskell-cafe] Printing of asynchronous exceptions to stderr

[Simon Marlow]

On 10/11/2010 17:52, Mitar wrote:
> Hi!
>
> On Wed, Nov 10, 2010 at 4:16 PM, Simon Marlow<marlowsd at gmail.com>  wrote:
>> The right way to fix it is like this:
>
> Optimist. ;-)
>
>>   let run = unblock doSomething `catches` [
>>                 Handler (\(_ :: MyTerminateException) ->  return ()),
>>                 Handler (\(e :: SomeException) ->  putStrLn $ "Exception: " ++
>> show e)
>>               ] `finally` (putMVar terminated ())
>>   nid<- block $ forkIO run
>
> In 6.12.3 this does not work (it does not change anything, I hope I
> tested it correctly) because finally is defined as:
>
> a `finally` sequel =
>    block (do
>      r<- unblock a `onException` sequel
>      _<- sequel
>      return r
>    )

Oh, good point!  I must have tested it with 7.0.  So this is indeed 
something that is fixed by the new async exceptions API: finally itself 
doesn't raise async exceptions inside a mask.

To fix this with GHC 6.12.3 you'd have to avoid finally and do it manually:

   let run = do
         r <- (unblock doSomething `catches` [
                 Handler (\(_ :: MyTerminateException) -> return ()),
                 Handler (\(e :: SomeException) -> putStrLn $ 
"Exception: " ++ show e)
               ]) `onException` sequel
         sequel
         where sequel = putMVar terminated ()

Now *that* works - I tested it with 6.12.3 this time.

Cheers,
	Simon




> You see that unblock there? So it still unblocks so that second
> exception is delivered immediately after catches handles the first
> exception?
>
> But I agree that your explanation for what is happening is the correct
> one. Better than my "hanging threads at the end". And with my throwIO
> approach I just override MyTerminateException with ThreadKilled.
>
>
> Mitar