I'm trying to do some fusion in ghc, and I'd greatly appreciate help with the code below (which is simplified from fusion on linear maps). I've tried every variation I can think of, and always something prevents the fusion. Help, please! Thanks, - Conal {-# OPTIONS_GHC -O2 -Wall -fglasgow-exts -ddump-simpl -ddump-simpl-stats #-} -- {-# OPTIONS_GHC -ddump-simpl-iterations #-} module F where -- | Domain of a linear map. class AsInt a where toInt :: a -> Int fromInt :: Int -> a {-# RULES "toInt/fromInt" forall m. toInt (fromInt m) = m #-} {-# INLINE onInt #-} onInt :: AsInt a => (Int -> Int) -> (a -> a) onInt f = fromInt . f . toInt test :: AsInt a => (Int -> Int) -> (Int -> Int) -> (a -> a) test h g = onInt h . onInt g -- The desired result: -- -- test h g -- == onInt h . onInt g -- == (fromInt . h . toInt) . (fromInt . g . toInt) -- == \ a -> (fromInt . h . toInt) ((fromInt . g . toInt) a) -- == \ a -> (fromInt . h . toInt) (fromInt (g (toInt a))) -- == \ a -> fromInt (h (toInt (fromInt (g (toInt a))))) -- == \ a -> fromInt (h (g (toInt a)))