Am Dienstag, den 16.02.2016, 23:02 -0800 schrieb 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.

You might want to add laws in the style of 

    If this is also a Foldable, then 
       toList . mapMaybe f = mapMapybe f . toList
       toList . catMaybes = catMaybes . toList
       toList . filter f = filter f . toList

which would fix the behavior quite tightly.