具有键值的Haskell二叉树

Question

我想用带有元组(k,v)的键值叶子构建二叉树.

我的代码:

data Tree k v = EmptyTree 
                | Node (k, v) (Tree k v) (Tree k v)
                deriving (Show, Eq, Ord, Read)

emptyTree :: (k,v) -> Tree k v  
emptyTree (k,v) = Node (k, v) EmptyTree EmptyTree

treeInsert :: (Ord k) => (k,v) -> Tree k v -> Tree k v
treeInsert (k,v) EmptyTree = emptyTree (k, v)
treeInsert (a, b) (Node (k,v) left right) 
        | a == k = (Node (a,b) left right)
        | a < k = (Node (a, b) (treeInsert (a, b) left) right)   
        | a > k = (Node (a, b) left (treeInsert (a, b) right))

现在我正在尝试填充这棵树:

fillTree :: Int -> Tree k v -> Tree k v
fillTree x tree = treeInsert (x, x) tree

但我得到这个错误:

Couldn't match type `v' with `Int'
      `v' is a rigid type variable bound by
          the type signature for fillTree :: Int -> Tree k v -> Tree k v

原因是什么,我该如何解决？

Answer 1

您的类型太笼统或太具体.它应该是

fillTree :: Int -> Tree Int Int -> Tree Int Int

要么

fillTree :: (Ord a) => a -> Tree a a -> Tree a a

您原来的宣言试图插入(Int, Int)到Tree k v 任何k,v.它说无论你有什么样的树,我们都可以插入一对Ints.这显然是无稽之谈,并且作为您的签名treeInsert表示,只有一对类型(k, v)可以插入到Tree k v.

treeInsert :: (Ord k) => (k, v) -> Tree k v -> Tree k v