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

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.