Re: [Haskell-cafe] Asynchronous exception wormholes kill modularity

On Fri, Mar 26, 2010 at 3:43 PM, Gregory Collins wrote:

Matthew Brecknell writes:

...

And is confirmed by a simple test (with GHC 6.10.4 on Linux):

import Prelude hiding(catch) import Control.Concurrent import Control.Exception

main = do   chan <- newEmptyMVar   done <- newEmptyMVar   kill <- block $ forkIO $ do     (takeMVar chan >>= putMVar done)       `onException` putMVar done "Exception received during takeMVar" ...

Should this be "forkIO $ block" instead of "block $ forkIO"?

No. First of all note that forked threads inherit the blocked state of their parents: http://haskell.org/ghc/docs/latest/html/libraries/base-4.2.0.0/Control-Excep... The reason Matthew first blocks then calls forkIO is that he needs to install an exception handler for the forked thread: `onException` putMVar done "Exception received during takeMVar" If he does not call block or only later calls it in the forked thread a chance exists that an asynchronous exception is thrown to the forked thread before the exception handler is installed. regards, Bas