Hello,

ErrorT instance of MonadPlus define that in case of fail both arguments of mplus a value of Error will be the Error of second computation:
    m `mplus` n = ErrorT $ do
        a <- runErrorT m
        case a of
            Left  _ -> runErrorT n
            Right r -> return (Right r)
Could it be changed in this way: 
    m `mplus` n = ErrorT $ do
        a <- runErrorT m
        case a of
            Left  e -> do


                  b <- runErrorT n


                  case b of
                      Left e' -> return $ Left (e `eplus` e')


                      r -> return r
            r -> return r


where `eplus` could be placed in class Error a:


eplus :: a -> a -> a 


eplus x y = y -- default implementation



In this case we could combine Errors more flexible.



Best regards,


Dmitry