loops - 从列表中删除的项目回来

Question

我想从列表中删除一个项目，但它又回来了。

main = do
  let y = ["aa","bb","cc","dd","ee","ff"]
  let n = length y
  replicateM_ (n-1) (deleteWord y)

deleteWord y = do
  putStrLn "Write a word: "
  word <- getLine
  let new_y = delete word y
  print new_y

输出:

*Main> main
Write a word: 
aa
["bb","cc","dd","ee","ff"]
Write a word: 
bb
["aa","cc","dd","ee","ff"]
Write a word: 
cc
["aa","bb","dd","ee","ff"]

我希望 "aa" 和 "bb" 保持删除状态，不再返回到列表中。

Answer 1

正如评论中所解释的那样，y 值一旦定义，就像 Haskell 中的每个值一样是不可变的。

但是有一个monadic库函数，nest :: Monad m => Int -> (a -> m a) -> a -> m a ，这允许您将某个操作的结果重新注入(inject)到同一个操作中，多次。

为了使用它，您的基本操作需要返回一个结果:

$ ghci
GHCi, version 8.8.4: https://www.haskell.org/ghc/  :? for help
...
 λ>
 λ> import Data.List(delete)
 λ> import Control.Monad.HT(nest)
 λ> 
 λ> 
 λ> :{
|λ> deleteWord y = do
|λ>   putStrLn "Write a word: "
|λ>   word <- getLine
|λ>   let new_y = delete word y
|λ>   print new_y
|λ>   return new_y  -- HERE !!!
|λ> :}
 λ> 
 λ> :type deleteWord
 deleteWord :: [String] -> IO [String]
 λ> 
 λ> action3 = nest 3 deleteWord
 λ> 
 λ> :type action3
 action3 :: [String] -> IO [String]
 λ> 

那么让我们尝试运行那个嵌套操作:

 λ> 
 λ> res3 <- action3 ["aa","bb","cc","dd","ee","ff"]
Write a word: 
ff
["aa","bb","cc","dd","ee"]
Write a word: 
aa
["bb","cc","dd","ee"]
Write a word: 
dd
["bb","cc","ee"]
 λ> 
 λ>
 λ> res3
["bb","cc","ee"]
 λ> 

附录:

source code for nest对于初学者 Haskell 程序员来说，这可能不是很有启发性。但库源代码的主要目标是最大限度地提高运行时效率。

可以写一个更简单的递归明确的版本:

myNest :: Monad m => Int -> (a -> m a) -> a -> m a
myNest n fn a0 =
    if (n <= 0) then  (return a0)
                else  do
                          a1 <- fn a0
                          myNest (n-1) fn a1