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import Control.Concurrent
import Control.Exception
type S = String
newtype M s a = M { unM :: s -> (s,a) }
instance Monad (M s) where
(M m) >>= k = M $ \s -> case m s of
(s',a) -> unM (k a) s'
return a = M $ \s -> (s,a)
errorM :: String -> M s a
errorM s = M $ \_ -> error s
runM :: M s a -> s -> a
runM (M m) s = case m s of (_,a) -> a
main = print (runM (bar ["a","b"]) "state")
bar :: [String] -> M s [String]
bar xs = mapM foo xs
foo :: String -> M s String
foo x = errorM x
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