[Haskell-cafe] fmap vs. liftM

Hi there, Reading http://www.haskell.org/haskellwiki/Things_to_avoid I found an interesting saying: "By the way, in the case of IO monad the Functor class method fmap and the Monad based function liftM are the same." I always tought that prop :: (Functor m, Monad m, Eq (m b)) => (a -> b) -> m a -> Bool prop f x = fmap f x == liftM f x was True regardless of 'm'. Is there any exception? If so, why? I've even done s/fmap/liftM/g and s/liftM/fmap/g in the past for consistency =). Thanks! -- Felipe.