witherable

宿りせし花橘も枯れなくになど郭公声絶えぬらむ - 大江千里

code:haskell

class Functor f => Filterable f where

mapMaybe :: (a -> Maybe b) -> f a -> f b

catMaybes :: f (Maybe a) -> f a

filter :: (a -> Bool) -> f a -> f a

class (Traversable t, Filterable t) => Witherable t where

wither :: Applicative f => (a -> f (Maybe b)) -> t a -> f (t b)