How does Haskell infer correct type classes in MaybeT implementation?

Question

How does Haskell know which is correct monad instance for each return expression?

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

instance Monad m => Monad (MaybeT m) where
    return  = MaybeT . return . return

Answer 1

It's actually unambiguous from the context.

Let's play the typechecker,

-- From the signature
MaybeT . return . return :: a -> MaybeT a 
-- From the type of MaybeT
return . return :: a -> m (Maybe a)
-- From the type of `.`
(return :: Maybe a -> m a) . (return :: a -> Maybe a)

And once we have the type of each return, the "instance selection algorithm" will correctly choose the first to use ms return and the second to be Maybe.