gistfile1.cpp

struct Widget {
  virtual ~Widget();
  
  virtual void foo() = 0;
};

class Label : public Widget {
  std::string _value;

public:
  Label(std::string const &v) : _value(v) {}
  ~Label() {}
  
  override void foo() {
    std::cout << _value << std::endl;
  }
};

gistfile2.hs

data Widget = Widget {
    foo :: IO ()
  }

data Label = Label String

toWidget :: Label -> Widget
toWidget (Label str) = Widget $ putStrLn str

gistfile3.hs

-- encore mieux, pour gérer plus de widgets par la suite

data Widget = Widget {
    foo :: IO ()
  }

class ToWidget a where
  toWidget :: a -> Widget

data Label = Label String

instance ToWidget Label where
  toWidget (Label str) = Widget $ putStrLn str