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module Main where
import Control.Concurrent
import qualified Control.Exception as E
trapHandler :: MVar Int -> MVar () -> IO ()
trapHandler inVar caughtVar =
(do E.mask_ $ do
trapMsg <- takeMVar inVar
putStrLn ("Handler got: " ++ show trapMsg)
trapHandler inVar caughtVar
)
`E.catch`
(trapExc inVar caughtVar)
trapExc :: MVar Int -> MVar () -> E.SomeException -> IO ()
-- If we have been killed then we are done
trapExc inVar caughtVar e
| Just E.ThreadKilled <- E.fromException e = return ()
-- Otherwise...
trapExc inVar caughtVar e =
do putStrLn ("Exception: " ++ show e)
putMVar caughtVar ()
trapHandler inVar caughtVar
main :: IO ()
main = do
inVar <- newEmptyMVar
caughtVar <- newEmptyMVar
tid <- forkIO (trapHandler inVar caughtVar)
yield
putMVar inVar 1
threadDelay 1000
throwTo tid (E.ErrorCall "1st")
takeMVar caughtVar
putMVar inVar 2
threadDelay 1000
throwTo tid (E.ErrorCall "2nd")
-- the second time around, exceptions will be blocked, because
-- the trapHandler is effectively "still in the handler" from the
-- first exception. I'm not sure if this is by design or by
-- accident. Anyway, the trapHandler will at some point block
-- in takeMVar, and thereby become interruptible, at which point
-- it will receive the second exception.
takeMVar caughtVar
-- Running the GHCi way complains that tid is blocked indefinitely if
-- it still exists, so kill it.
killThread tid
putStrLn "All done"
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