First attempt

first.hs

loginUser
  :: ByteString
      -- ^ Username field
  -> ByteString
      -- ^ Password field
  -> Maybe ByteString
      -- ^ Remember field; Nothing if you want no remember function.
  -> (AuthFailure -> Handler b (AuthManager b) ())
      -- ^ Upon failure
  -> Handler b (AuthManager b) ()
      -- ^ Upon success
  -> Handler b (AuthManager b) ()
loginUser unf pwdf remf loginFail loginSucc =
    runEitherT (loginUser' unf pwdf remf)
    >>= either loginFail (const loginSucc)


loginUser' :: ByteString
           -> ByteString
           -> Maybe ByteString
           -> EitherT AuthFailure (Handler b (AuthManager b)) AuthUser
loginUser' unf pwdf remf = do
    mbUsername <- lift $ getParam unf
    mbPassword <- lift $ getParam pwdf
    remember   <- lift $ liftM (fromMaybe False)
                    (runMaybeT $
                    do field <- MaybeT $ return remf
                       value <- MaybeT $ getParam field
                       return $ value == "1")


    password <- maybe (left PasswordMissing) return mbPassword
    username <- maybe (left UsernameMissing) return mbUsername

    -- Lifting already get the "success" value for me,
    -- and store it inside loginStatus
    loginStatus <- lift $ loginByUsername username (ClearText password) remember
    EitherT $ return loginStatus

I used left for the maybe trick:

left :: l -> EitherT l m a