实例显示树数据结构

Question

我是haskell的新手，我希望用类show实例Tree a。

  data Tree a = Null
               |Node (Tree a) a (Tree a)

  instance Show (Tree a) where
    show Null = ""
    show Node ((Tree l) (v) (Tree r)) = "|"--I don´t know how i can do this step

感谢您的帮助。

Answer 1

递归申请show：

data Tree a = Null | Node (Tree a) a (Tree a)

instance Show a => Show (Tree a) where
  show Null = "Null"
  show (Node l v r) = "(" ++ show l ++ " " ++ show v ++ " " ++ show r ++ ")"