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{-# LANGUAGE GADTs #-}
{- This code, courtesy of Markus Lauer (markus.lauer-2006@lauerit.de)
was rejected by the sophisticated wobbly-type impl in 6.4.1, and
with a terrible error message:
Sample.hs:22:40:
Couldn't match `MayFail' against `MayFail'
Expected type: Result s b
Inferred type: Result MayFail a
In the first argument of `Return', namely `Fail'
In a case alternative: Fail -> return Fail
Strangely it is accepted by the simplified impl in GHC 6.5. -}
module Sample where
data Safe
data MayFail
data Result s a where
Ok :: a -> Result s a
Fail :: Result MayFail a
newtype M s a = M { unM :: IO (Result s a) }
instance Monad (M s) where
return x = M (return (Ok x))
{- this one gives a type error in 6.4.1 -}
M m >>= k = M (do res <- m
case res of
Ok x -> unM (k x)
Fail -> return Fail
)
{- while this one works -}
-- M m >>= k = M (f m (unM . k))
-- where
-- f :: IO (Result s a) -> (a -> IO (Result s b)) -> IO (Result s b)
-- f m k = do res <- m
-- case res of
-- Ok x -> k x
-- Fail -> return Fail
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