[Haskell-cafe] Applicative Monoid instance

This may be a stupid question, but I'm wondering why the following instance isn't in prelude: instance (Applicative m, Monoid a) => Monoid (m a) where mempty = pure mempty mappend a b = mappend <$> a <*> b ? I ask, because being able to use a WriterT (IO ()) STM a to debug complex STM expressions would be really nice. But it depends upon IO () being a monoid. Brian