module Data.Filtrable where
import Prelude hiding (filter)
import Control.Applicative
import Control.Monad
import Data.Proxy
import Data.Traversable
class Functor f => Filtrable f where
mapMaybe :: (a -> Maybe b) -> f a -> f b
mapMaybe f = catMaybes . fmap f
catMaybes :: f (Maybe a) -> f a
catMaybes = mapMaybe id
filter :: (a -> Bool) -> f a -> f a
filter f = mapMaybe (liftA2 (<$) id (guard . f))
instance Filtrable [] where
mapMaybe f = foldr (maybe id (:) . f) []
instance Filtrable Maybe where
mapMaybe = (=<<)
catMaybes = join
instance Filtrable Proxy where
mapMaybe _ Proxy = Proxy
instance Filtrable (Const a) where
mapMaybe _ (Const x) = Const x
mapMaybeA :: (Filtrable f, Traversable f, Applicative p) => (a -> p (Maybe b)) -> f a -> p (f b)
mapMaybeA f xs = catMaybes <$> traverse f xs
filterA :: (Filtrable f, Traversable f, Applicative p) => (a -> p Bool) -> f a -> p (f a)
filterA f = mapMaybeA (\ x -> (x <$) . guard <$> f x)