MonadPlus instance for ContT

[Edward Kmett]

On Mon, Jun 24, 2013 at 2:53 PM, Dan Burton <danburton.email at gmail.com>wrote:

> I also agree with Henning.
>
> If anything, the 1st instance could be a ContT instance for Monoid.
>
> instance (Monoid r, Monad m) => Monoid (ContT r m a) where
>   mempty = ContT $ const $ return mempty
>   m `mappend` n = ContT $ \ c -> liftM2 mappend (runContT m c) (runContT n
> c)
>
> That way both behaviors are easily accessible. Is there any other sensible
> ContT instance for Monoid?
>

Yes.

The third construction is the 'universal' lifting that works for every
Applicative, which would look like:

instance (Monad m, Monoid a) => Monoid (ContT r m a) where
  mempty = return mempty
  mappend = liftM2 mappend

-Edward
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