Deriving Functor and Foldable, use existensial wrapper

{-# LANGUAGE DeriveTraversable, ExistentialQuantification #-}

data Tree a
    = L a        -- Leaf holds data
    | N [Tree a] -- Node holds list of trees
    deriving (Functor, Foldable, Traversable, Show)

aTree :: Tree Int
aTree
    = N
        [ L 5
        , N
            [ L 42
            , L 41
            ]
        , L 7
        ]

data Showable = forall a. Show a => Sh a

instance Show Showable where
  show (Sh a) = show a

-- deadly want to get rid of `Sh .`-boilerplate
actions :: [Tree Int -> Showable]
actions = [Sh . sum, Sh . fmap (+ 1)] -- and many more...

main :: IO ()
main = print $ map ($ aTree) actions  -- => [95,N [L 6,N [L 43,L 42],L 8]]