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module Main where
import Control.Concurrent
import Control.Exception
-- Raise an exception in another thread. We need a lot of synchronisation here:
-- - an MVar for the second thread to block on which it waits for the
-- signal (block)
-- - an MVar to signal the main thread that the second thread is ready to
-- accept the signal (ready)
-- - an MVar to signal the main thread that the second thread has received
-- the signal (ready2). If we don't have this MVar, then the main
-- thread could exit before the second thread has time to print
-- the result.
main = do
block <- newEmptyMVar
ready <- newEmptyMVar
ready2 <- newEmptyMVar
id <- forkIO (Control.Exception.catch (putMVar ready () >> takeMVar block)
(\e -> putStr (show (e::SomeException)) >> putMVar ready2 ()))
takeMVar ready
throwTo id (ErrorCall "hello")
takeMVar ready2
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