`catMaybes :: [Maybe a] -> [a]` の一般化

toList.hs

import Data.Foldable        (Foldable, toList)
import Data.Functor.Compose (Compose(Compose))
import Data.Tree            (Tree(Node))

flatten :: (Foldable f, Foldable g) => f (g a) -> [a]
flatten = toList . Compose

ex1 = [Just 1, Nothing, Just 2]
ex2 = Just [1,2]
ex3 = [[1],[2]]
ex4 = Node Nothing [Node (Just 1) [], Node Nothing [Node (Just 2) []]]

-- >>> flatten ex1
-- [1,2]
-- >>> flatten ex2
-- [1,2]
-- >>> flatten ex3
-- [1,2]
-- >>> flatten ex4
-- [1,2]

>>> import Control.Lens
>>> toListOf (folded . folded) ex1
[1,2]
>>> :t toListOf (folded . folded)
toListOf (folded . folded)
  :: (Foldable f1, Foldable f2) => f1 (f2 a) -> [a]