[Haskell-cafe] Where is the pairing-with-monoid monad instance?

[Sean Leather]

On Wed, Jan 11, 2012 at 03:23, Conal Elliott wrote:

> Is the standard pair-with-monoid monad instance in some standard place? I
> see the Applicative instance in Control.Applicative, and the
> pair-with-monoid Functor instance in Control.Monad.Instances, and the (->)
> e and Either e monad instances also in Control.Monad.Instances.
>
> I'm looking for something like
>
>     instance Monoid o => Monad ((,) o) where
>       return a    = (mempty,a)
>       (o,a) >>= f = (o `mappend` o', b)
>         where (o',b) = f a
>
> Where is it hiding from me?
>

http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Control-Monad-Trans-Writer-Lazy.html#t:WriterT?

Regards,
Sean
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