Monad instance for pairs

Question

This question shows an instance definition for (,) a b, where a is an instance of Monoid.

However, I don\'t know how to write sim

Answer 1

We could write, for now synonyms works inappropriate for this case, so we use newtype:

newtype RevTuple b a = RevTuple { totuple :: (a , b) }

instance Monoid b => Monad (RevTuple b) where
    return a = RevTuple (a,mempty)
    (RevTuple (a,b)) >>= f = 
                 let RevTuple (c,b1) = f a in RevTuple (c,b `mappend` b1)

Answer 2

You cannot. It's crucial for Monad to be polymorphic in its parameter, so if a particular monadish thing only makes sense for monoids, it's not a monad.

For example, return :: Monad m => a -> m a. There's no way you can add an extra constraint to the signature of return.