Data.Witherable

This module generalizes filterable containers.

Documentation

class Traversable t => Witherable t where

Like traverse, but you can remove elements instead of updating them.

traverse f ≡ wither (fmap Just . f)

A definition of wither must satisfy the following laws:

identity

wither (pure . Just) ≡ pure

composition

Compose . fmap (wither f) . wither g ≡ wither (Compose . fmap (wither f) . g)

Parametricity implies the naturality law:

t . wither f = wither (t . f)

Minimal complete definition: wither or catMaybes. The default definitions can be overriden for efficiency.

Minimal complete definition

Nothing

Methods

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

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

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

filterA :: Applicative f => (a -> f Bool) -> t a -> f (t a)

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

Instances

Witherable []

Witherable Maybe

Witherable IntMap

Witherable Seq

Witherable Vector

Monoid e => Witherable (Either e)

Witherable (Const r)

Witherable (Proxy *)

Ord k => Witherable (Map k)

(Eq k, Hashable k) => Witherable (HashMap k)

witherM :: (Witherable t, Monad m) => (a -> MaybeT m b) -> t a -> m (t b)

blightM :: (Monad m, Witherable t) => t a -> (a -> MaybeT m b) -> m (t b)

blightM is witherM with its arguments flipped.