haskell - 具有树数据结构的实例展示

Question

我是 haskell 的新手，我想在课堂表演中实例化 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

谢谢你的帮助。

Answer 1

show递归应用：

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 ++ ")"