I have a solution so this is for interest only. It is not normally the case that two monads compose to give another monad. Monad transformers capture when this is possible. However, when there is a "swap" function satisfying some commutative diagrams then it can be proved that the monads compose to produce a monad. Is there such a swap function in a library? I had a look in the comprehensive category-extra package but couldn't find anything. Here's two possible implementations. Big caveat: I haven't proved that these satisfy the commutative diagrams but I am confident that they will. Option 1 define swap :: (Functor m, Monad m) => Either String (m a) -> m (Either String a) swap (Left s) = return (Left s) swap (Right x) = fmap Right x Option 2 use Traversable (EvilTerran's suggestion) instance F.Foldable (Either String) where foldMap f (Left s) = mempty foldMap f (Right x) = f x instance T.Traversable (Either String) where traverse f (Left s) = pure (Left s) traverse f (Right x) = Right <$> f x and now you can use T.sequence to swap the monads around. For further information see Composing Monads by Mark Jones and Luc Duponcheel and Toposes, Triples and Theories by Barr & Wells (Section 9.2). Dominic.