在字典中调整了插入实现

Question

在Haskell中实现以下字典：

module TreeDict (Dict, empty, insert, lookup) where       

import Prelude hiding(lookup)                                               

data Dict k v = Empty | Node { key :: k, val :: v, left :: Dict k v, right :: Dict k v }
    deriving (Eq, Show)                                                     

empty :: Dict k v                                                           
empty = Empty                                                               

insert :: (Ord k) => k -> v -> Dict k v -> Dict k v                         
insert k v Empty = Node k v empty empty                                     
insert k v d                                                                
        | k < key d = Node (key d) (val d) (insert k v (left d)) (right d)  
        | k > key d = Node (key d) (val d) (left d) (insert k v (right d))  
        | otherwise = Node k v (left d) (right d)                           

lookup :: (Ord k) => k -> Dict k v -> Maybe v                               
lookup k Empty = Nothing                                                    
lookup k d                                                                  
        | k < key d = lookup k (left d)                                     
        | k > key d = lookup k (right d)                                    
        | otherwise = Just (val d)        

我想编写一个类似于insert的函数，它允许我将给定键插入树的次数保持为一个值（但仅限Ord k类）。

例如：对于列表[aa，bb，aa，aa]，我的字典会保留：{aa - ＆gt; 3，bb - ＆gt; 1}。

知道如何调整我的代码吗？