Mapping over errors

gistfile1.hs

{-# LANGUAGE TypeFamilies, FlexibleInstances #-}
import Control.Monad.Error
import Control.Monad.Reader

type family   MappedMonad (m :: * -> *) e' :: * -> *
type instance MappedMonad (ErrorT e m)  e' = ErrorT e' m
type instance MappedMonad (ReaderT r m) e' = ReaderT r (MappedMonad m e')

class MapError m where
  type ErrorType m :: *
  mapE :: (ErrorType m -> e') -> m a -> (MappedMonad m e') a

instance Functor m => MapError (ErrorT e m) where
  type ErrorType (ErrorT e m) = e
  mapE f = mapErrorT (fmap (either (Left . f) Right))

instance MapError m => MapError (ReaderT r m) where
  type ErrorType (ReaderT r m) = ErrorType m
  mapE f = mapReaderT (mapE f)