desperately seeking RULES help

[Conal Elliott]

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)))
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