On Wed, Feb 17, 2016 at 2:02 AM, M Farkas-Dyck
I quietly posted this library to Hackage nearly a year ago, but lately learned that some seeking such a package had difficulty finding it, so i announce it now ☺
https://hackage.haskell.org/package/filtrable
class Functor f => Filtrable f where mapMaybe :: (a -> Maybe b) -> f a -> f b catMaybes :: f (Maybe a) -> f a filter :: (a -> Bool) -> f a -> f a
For laws, see docs on Hackage.
This is something I’ve made for myself once or twice (I called it
“Siftable”).
One law I didn’t notice you mention is:
mapMaybe f . mapMaybe g = mapMaybe (f <=< g)
In other words, mapMaybe is a functor from the Kleisli category over Maybe
to Hask. (This may come free from parametricity and mapMaybe Just = id.)
--
Dave Menendez