Question

data Node = Blue | Green | Red
data Tree = Leaf Node | Node [Tree]

main = do
  let a = Blue[Leaf Red]
  print 1

它给了我以下错误：

main.hs:5:15: error:
• Couldn't match expected type ‘[Tree] -> t’
              with actual type ‘Node’
• The function ‘Blue’ is applied to one argument,
  but its type ‘Node’ has none
  In the expression: Blue [Leaf Red]
  In an equation for ‘a’: a = Blue [Leaf Red]
• Relevant bindings include a :: t (bound at main.hs:5:11)

而且我不明白为什么它说它得到了Node而不是[Tree]

Answer 1

您可以使用其中一种颜色创建Tree作为单个叶子，例如Leaf Red或作为具有其他树列表的节点，例如Node [Leaf Red]。您的main函数应如下所示：

main = do
  let a = Node [Leaf Red]
  print 1

或者：

main = do
  let a = Node [Leaf Blue, Leaf Red]
  print 1

如果您还想为节点添加颜色，可以将Tree数据类型重新定义为：

data Tree = Leaf Node | Node Node [Tree]
...
let a = Node Blue [Leaf Red]

阅读此内容可能会造成混淆，因此您应该考虑重命名Node类型或Node数据构造函数。

树数据不会在haskell

1 个答案: