为什么此字典函数不起作用

Question

我正在尝试用here中的代码创建和使用字典：

import Data.List (lookup)

insert :: Eq a => (a,b) -> [(a,b)] -> [(a,b)]
insert (a,b)  []           = [(a,b)]
insert (a,b) ((c,d):rest) = if a == c
    then (a,b) : rest
    else (c,d) : insert (a,b) rest

dict :: [(String, String)]
dict = [("", "")]

main = do 
    insert ("onekey", "onevalue") dict
    print dict
    print $ lookup "onekey" dict

但是我得到了以下错误：

$ runghc rndict.hs

rndict.hs:22:1: error:
    • Couldn't match expected type ‘IO t0’ with actual type ‘[()]’
    • In the expression: main
      When checking the type of the IO action ‘main’

rndict.hs:24:9: error:
    • Couldn't match type ‘IO’ with ‘[]’
      Expected type: [()]
        Actual type: IO ()
    • In a stmt of a 'do' block: print dict
      In the expression:
        do { insert ("onekey", "onevalue") dict;
             print dict;
             print $ lookup "onekey" dict }
      In an equation for ‘main’:
          main
            = do { insert ("onekey", "onevalue") dict;
                   print dict;
                   print $ lookup "onekey" dict }

rndict.hs:25:9: error:
    • Couldn't match type ‘IO’ with ‘[]’
      Expected type: [()]
        Actual type: IO ()
    • In a stmt of a 'do' block: print $ lookup "onekey" dict
      In the expression:
        do { insert ("onekey", "onevalue") dict;
             print dict;
             print $ lookup "onekey" dict }
      In an equation for ‘main’:
          main
            = do { insert ("onekey", "onevalue") dict;
                   print dict;
                   print $ lookup "onekey" dict }

问题是什么?在Haskell中使用字典的正确方法是什么？

Answer 1

您需要使用let将新字典绑定到一个名称：

import Data.List (lookup)

insert :: Eq a => (a,b) -> [(a,b)] -> [(a,b)]
insert (a,b)  []           = [(a,b)]
insert (a,b) ((c,d):rest) = if a == c
    then (a,b) : rest
    else (c,d) : insert (a,b) rest

dict :: [(String, String)]
dict = [("", "")]

main = do
    let d = insert ("onekey", "onevalue") dict
    print d
    print $ lookup "onekey" d

您询问有关插入多个元素的问题。为此，您可以使用一个文件夹来编写一个名为insertMany的函数。您可能应该使用foldl'，但我将把它作为练习来找出原因。

import Data.List (lookup)

insert :: Eq a => (a,b) -> [(a,b)] -> [(a,b)]
insert (a,b)  []           = [(a,b)]
insert (a,b) ((c,d):rest) = if a == c
    then (a,b) : rest
    else (c,d) : insert (a,b) rest

insertMany :: Eq a => [(a,b)] -> [(a,b)] -> [(a,b)]
insertMany elements dict =
  foldl (flip insert) dict elements

dict :: [(String, String)]
dict = [("", "")]

main = do
    let d = insert ("onekey", "onevalue") dict
    print d
    print $ lookup "onekey" d
    print $ insertMany [("onekey", "newvalue"), ("anotherkey", "anothervalue")]
      dict