“'节点'应用于太少的参数”，即使有确切的数字

Posted 2021-03-28

s和a是类型变量。在构造函数中，前两个参数是数据，然后是它的父项，它在图中的级别，然后是它的子项列表。

data Node s a = Root | Node s a (Node s a) Int [Node s a]
createRoot :: (ProblemState s a) => s-> a -> Node s a
createRoot state act= Node (state act Root 0 [])

我已经向Node构造函数传递了完全相同数量的5个参数，但是我遇到了错误。

• Couldn't match expected type ‘Node s a’
              with actual type ‘a1
                                -> Node s1 a1 -> Int -> [Node s1 a1] -> Node s1 a1’
• Probable cause: ‘Node’ is applied to too few arguments
  In the expression: Node (state act Root 0 [])
  In an equation for ‘createRoot’:
      createRoot state act = Node (state act Root 0 [])
• Relevant bindings include
    act :: a (bound at Search.hs:43:24)
    state :: s (bound at Search.hs:43:18)
    createRoot :: s -> a -> Node s a (bound at Search.hs:43:1)

答案

括号用于分组表达式。 (length "hello" + 2)是一个单一的值，而不是4。

同样，Node (...)将Node应用于单个参数：(state act Root 0 [])。显然这是错的（并且需要state成为一个带有四个参数的函数）。

解决方案是删除括号：

Node state act Root 0 []

现在Node适用于五个论点。