Compactable, Filterable, Witherable

[andywhite37]

It would be cool 😎 to add a few more typeclasses to the mix: Compactable, Filterable, and Witherable. These are all related to compacting/filtering/partitioning data structures. We already have some of these functions for certain types, so this would just be to formalize the abstractions, even though abstractions are not always super easy to use in Reason.

https://github.com/LiamGoodacre/purescript-filterable

class Compactable f where
  compact :: forall a.
    f (Maybe a) -> f a

  separate :: forall l r.
    f (Either l r) -> { left :: f l, right :: f r }

class (Compactable f, Functor f) <= Filterable f where
  partitionMap :: forall a l r.
    (a -> Either l r) -> f a -> { left :: f l, right :: f r }

  partition :: forall a.
    (a -> Boolean) -> f a -> { no :: f a, yes :: f a }

  filterMap :: forall a b.
    (a -> Maybe b) -> f a -> f b

  filter :: forall a.
    (a -> Boolean) -> f a -> f a

class (Filterable t, Traversable t) <= Witherable t where
  wilt :: forall m a l r. Applicative m =>
    (a -> m (Either l r)) -> t a -> m { left :: t l, right :: t r }

  wither :: forall m a b. Applicative m =>
    (a -> m (Maybe b)) -> t a -> m (t b)