Thanks Daniel, Yves and Edward for explanation. Two things come to my mind now. 1. It should be unified. Why? Because conceptually: runIdentity (runErrorT (fail "msg")) :: Either String Int Left "msg" and fail "msg" :: Either String Int *** Exception: msg Should be the same as Identity monad should not add structure. 2. I need a Failure monad that works well with pattern match failures (that call fail). I'd like to use it like this: runErrorT $ do Active <- getStatus -- ensure proper status Just elm <- lookup stuff there -- lookup element when (condition) $ fail "wrong!" -- check condition return 1234 -- return useful value sort of... Any ideas what could be used in place of Either monad? Basically I need working pattern match failures (I guess that means I need working fail method that is not equal to error). -- Gracjan