cc.hs

data Hoge a = Hoge a
data Piyo a = Piyo { left, right :: a }

class Container c where
    get  :: c a -> a
    getf :: (a -> b) -> c a -> b

instance Container Hoge where
    get    (Hoge x) = x
    getf f (Hoge x) = f x

instance Container Piyo where
    get    (Piyo l r) = l
    getf f (Piyo l r) = f l

main = do let h = Hoge 100
              p = Piyo "p1" "p2"
          print $ get h  -- 100
          print $ get p  -- "p1"
          print $ getf (*2) h           -- 200
          print $ getf (replicate 2) p  -- ["piyo", "piyo"]