Question

我在创建打印树内容的函数时遇到了一些麻烦。我对数据类型Tree的定义如下：

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show)

打印树的代码是：

printTree :: Tree a -> String
printTree (Node a left right) = printTreeAux (Node a left right) 0

printTreeAux :: Tree a -> Int -> String
printTreeAux EmptyTree _ = ""
printTreeAux (Node a left right) depth = (replicate (6 * depth) ' ') ++ show a ++ "\n" ++ (printTreeAux left (depth + 1)) ++ "\n" ++ (printTreeAux right (depth + 1))

将文件加载到Hugs时，我收到以下错误：

ERROR file:.\Tree.hs:30 - Cannot justify constraints in explicitly typed binding
*** Expression    : printTreeAux
*** Type          : Tree a -> Int -> String
*** Given context : ()
*** Constraints   : Show a

我已经搜索了一些澄清但发现没有什么可以真正帮助... 提前谢谢。

Answer 1

变化：

printTree :: Tree a -> String
printTree (Node a left right) = printTreeAux (Node a left right) 0

为：

printTree :: Tree a -> String
printTree x = printTreeAux x 0

甚至更好：

printTree :: Tree a -> String
printTree = (flip printTreeAux) 0

当您将参数转发给printTree时，无需在printTreeAux上进行模式匹配。实际上，通过模式匹配，EmptyTree永远不会匹配printTree，因此会产生错误。

您还应该在a上添加约束以请求它Show能够，否则show a将无法编译：

printTree :: (Show a) => Tree a -> String

和

printTreeAux :: (Show a) => Tree a -> Int -> String

作为you can see，有了这些修正，程序将编译，run，就好了。

无法证明明确键入的绑定错误中的约束 - 树打印

1 个答案: