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{-# LANGUAGE DoRec, ScopedTypeVariables #-}
import Control.Concurrent
import Control.Exception
import Control.Monad
import Prelude hiding (catch)
main = do
m <- newMVar 1
t1 <- forkIO $ thread m
t2 <- forkIO $ forever $ killThread t1
threadDelay 1000000
takeMVar m
thread m = run
where
run = (unblock $ forever $ modifyMVar_ m $ \v -> if v `mod` 2 == 1 then return (v*2) else return (v-1))
`catch` \(e::SomeException) -> run
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