filter.hs

import Data.Bool

filter' ::
  (a -> Either b c)
  -> [a]
  -> ([b], [c])
filter' _ [] =
  ([], [])
filter' f (a:as) = 
  let (bs, cs) = filter' f as
  in  case f a of
        Left b ->
          (b:bs, cs)
        Right c ->
          (bs, c:cs)

filter'' ::
  (a -> Maybe b)
  -> [a]
  -> [b]
filter'' f as =
  snd (filter' (maybe (Left ()) Right . f) as)

-- You think you want this.
-- But you don't. You want something like the above.
filter''' ::
  (a -> Bool)
  -> [a]
  -> [a]
filter''' f =
  filter'' (\a -> bool Nothing (Just a) (f a))