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