[Haskell-cafe] mtl: Why there is "Monoid w" constraint in the definition of class MonadWriter?

[Roman Cheplyaka]

* Edward Z. Yang <ezyang at MIT.EDU> [2012-12-08 11:19:01-0800]
> The monoid instance is necessary to ensure adherence to the monad laws.

This doesn't make any sense to me. Are you sure you're talking about the
MonadWriter class and not about the Writer monad?

Roman