mplus' :: MonadPlus m => Maybe a -> m a -> m a mplus' m l = maybeToMonad m `mplus` l
maybeToMonad :: Monad m => Maybe a -> m a maybeToMonad = maybe (fail "Nothing") return
In general, however, this operation can't be done. For example, how would you write:
mplus' :: IO a -> [a] -> [a]
Perhaps the question should be: is there an interesting structure that would allow us to capture when this kind of merging Monads is possible? We can convert every 'Maybe a' to a '[] a', but the other way round is partial or loses information, so lets focus on the first direction. Should there be a type family Up m1 m2 type instance Up Maybe [] = [] so that one could define mplusUp :: m1 a -> m2 a -> (m1 `Up` m2) a ? Well, we'd need the conversions, too, so perhaps {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, TypeFamilies, TypeOperators #-} import Control.Monad class Up m1 m2 where type m1 :/\: m2 :: * -> * up :: m1 a -> m2 a -> ((m1 :/\: m2) a, (m1 :/\: m2) a) instance Up m m where type m :/\: m = m up ma1 ma2 = (ma1, ma2) instance Up Maybe [] where type Maybe :/\: [] = [] up m1a m2a = (maybe [] (:[]) m1a, m2a) instance Up [] Maybe where type [] :/\: Maybe = [] up m1a m2a = (m1a, maybe [] (:[]) m2a) mplusUp :: (m ~ (m1 :/\: m2), Up m1 m2, MonadPlus m) => m1 a -> m2 a -> m a m1a `mplusUp` m2a = mUp1a `mplus` mUp2a where (mUp1a,mUp2a) = up m1a m2a Whether or not that is interesting, or whether it needs to be defined differently to correspond to an interesting structure, I'll leave to the residential (co-)Categorians!-) Claus