An Error transformer monad

gistfile1.hs

class Monad m => MonadError e m | m -> e where
    catchError :: e -> m a
    throwError :: m a -> (e -> m a) -> m a

data ErrorT e m a = ErrorT {
    runErrorT :: m (Either e a)
    }

instance MonadTrans (ErrorT e m) where
    lift m = ErrorT $ do
        a <- m
        return $ Right a

instance Monad (ErrorT e m) where
    return a = ErrorT $ return (Right a)
    m >>= k = ErrorT $ do
        a <- runErrorT m
        case a of
            (Left v) -> return $ Left v
            (Right v) -> runErrorT (k v)

instance MonadError (ErrorT e m) where
    throwError e = ErrorT $ return (Left e)
    catchError m handler = ErrorT $ do
        a <- runErrorT m
        case a of
            (Left v) -> runErrorT (handler v)
            (Right v) -> return a