[Haskell] How to define gunfold on List?

Hi, Does some one know how to define gunfold on recursive data struction like List? The ghc doc only give the example for non-recursive data like below. data T a b = C1 a b | C2 deriving (Typeable, Data) GHC will generate an instance that is equivalent to instance (Data a, Data b) => Data (T a b) where gfoldl k z (C1 a b) = z C1 `k` a `k` b gfoldl k z C2 = z C2 gunfold k z c = case constrIndex c of 1 -> k (k (z C1)) 2 -> z C2 toConstr (C1 _ _) = con_C1 toConstr C2 = con_C2 dataTypeOf _ = ty_T con_C1 = mkConstr ty_T "C1" [] Prefix con_C2 = mkConstr ty_T "C2" [] Prefix ty_T = mkDataType "Module.T" [con_C1, con_C2]