buzzard(131)% cat Bug6.hs
{-# OPTIONS -fglasgow-exts #-}
import Control.Monad.State
data S1 = S1 (M1 Int)
newtype M1 a = M1 { unM1 :: StateT S1 IO a } deriving (Monad)
data S2 = S2 (M2 Int)
newtype M2 a = M2 { unM2 :: S2 -> (a, S2) }
instance Monad M2 where
return a = M2 $ \s -> (a, s)
m >>= k = M2 $ \s -> let (a, s') = unM2 m s in unM2 (k a) s'
main = return ()
buzzard(132)% ghc -c Bug6.hs
Bug6.hs:6:
Can't make a derived instance of `Monad M1'
(too hard for cunning newtype deriving)
When deriving instances for type `M1'
buzzard(133)% ghc --version
The Glorious Glasgow Haskell Compilation System, version 5.04.2
Why is the newtype deriving too hard here?
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