This proposal [1] was originally submitted to Trac by Conal Elliott, but it was apparently abandoned and closed after some time. I've picked it back up and written out a patch. Here is the text of the proposal:
I'd like to have a (,)-style writer instance alongside the (->)-based reader instance for Monad in Control.Monad.Instances.
Here's the current reader:
instance Monad ((->) r) where return = const f >>= k = \ r -> k (f r) r
and my proposed writer:
instance Monoid o => Monad ((,) o) where return = pure (o,a) >>= f = (o `mappend` o', a') where (o',a') = f a
where the return definition relies on the Applicative instance of ((,) o). Written out explicitly,
return a = (mempty,a)
Control.Monad.Instances will also need two new imports:
import Data.Monoid (Monoid(..)) import Control.Applicative (pure)