[Haskell-cafe] Re: idioms ... for using Control.Applicative.WrapMonad or Control.Arrow.Kleisli?

Heinrich Apfelmus apfelmus at quantentunnel.de
Thu Mar 4 04:45:28 EST 2010

Nicolas Frisby wrote:
> Each time I find myself needing to use the wrapping functions
> necessary for this embeddings, I grumble. Does anyone have a favorite
> use-pattern for ameliorating these quickly ubiquitous conversions?
> For runKleisli, I was considering something like
> onKleisli ::
>   (Control.Arrow.Kleisli m a b -> Control.Arrow.Kleisli m' a' b')
>   -> (a -> m b) -> (a' -> m' b')
> onKleisli f = Control.Arrow.runKleisli . f . Control.Arrow.Kleisli
> but haven't really tested its usefulness. My most frequent use cases
> usually include Arrow.first, Arrow.second, &&&, ***, or +++. E.g.
> somefun :: (Monad m, Num a) => (a, d) -> m (a, d)
> somefun = onKleisli Control.Arrow.first (\ a -> return (a + 1))
> Perhaps a Control.Arrow.Kleisli, which would export (same-named)
> Kleisli specializations of all the Control.Arrow methods? And an
> analogous Control.Applicative.Monad? (Data.Traversable does this a
> little bit to specialize its interface for monads, such as
> Data.Traversable.sequence.)
> While writing this, I realized that you can't leave the Arrow-ness of
> Kleisli arrows implicit, since (->) a (m b) is two kinds of arrows
> depending on context -- which is precisely what the Kleisli newtype
> resolves. So I'm not seeing a reason to bring up the 'class
> Applicative m => Monad m where' dispute.

Yep, I don't think you can avoid wrapping and unwrapping a newtype.

While not directly related, I wonder whether Conor McBride's bag of tricks


might be of help.

Heinrich Apfelmus


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