duck_type.hs

class CanQuack a where
  quack :: a -> IO ()

class HasFeathers a where
  feathers :: a -> IO ()

data Duck = Duck
data Person = Person

instance CanQuack Duck where
  quack _ = putStrLn "Quaaaaack!"

instance HasFeathers Duck where
  feathers _ = putStrLn "The duck has white and gray feathers."

instance CanQuack Person where
  quack _ = putStrLn "The persom imitates a duck."

instance HasFeathers Person where
  feathers _ = putStrLn "The person takes a feather from the ground and shows it."

-- Optional type is:
-- inTheForst :: (CanQuack obj, HasFeathers obj) => obj -> IO ()
inTheForst duck = do
  quack duck
  feathers duck

game = do
  let donald = Duck
  let john = Person
  inTheForst donald
  inTheForst john