如何将嵌套列表中的每个列表分别写入文件？

Question

我正在尝试将嵌套列表中的每个列表写入同一个文件。但是我希望每个列表都以换行符开始。

我已经做了这个并且它可以工作，但是它非常低效并且看起来很糟糕：

appendFile "scraped.txt" (show (take 1 l))
appendFile "scraped.txt" ("\n")
let l3 = (drop 1 l)
appendFile "scraped.txt" (show (take 1 l3))
appendFile "scraped.txt" ("\n")
let l4 = (drop 1 l3)
appendFile "scraped.txt" (show (take 1 l4))
appendFile "scraped.txt" ("\n")
let l5 = (drop 1 l4)
appendFile "scraped.txt" (show (take 1 l5))
appendFile "scraped.txt" ("\n")
let l6 = (drop 1 l5)
appendFile "scraped.txt" (show (take 1 l6))
appendFile "scraped.txt" ("\n")

我试过类似下面的方法，但我似乎无法正确使用映射函数：

listValues :: [[String]] -> [String]
listValues :: (map . map)

appendValues :: [[String]] -> IO ()
appendValues = appendFile "scraped.txt" listValues

现在的txt文件是这样的，没关系，我只是想知道如何改进我的代码，学习如何使用映射功能。

Title,Subtitle,Date,Author
[["Een gezonde samenleving? 686Het belang van sporten wordt onderschat687","Teamsport","16 maart 2022","HAN redactie"]]
[["Zo vader, zo dochter","Carsten en Kirsten","10 maart 2022","HAN redactie"]]
[["Milieuvriendelijk vervoer met waterstof","Kennisclip","09 maart 2022","HAN redactie"]]
[["\"Ik heb zin in wat nog komen gaat\"","Master Mind","08 maart 2022","HAN redactie"]]
[["Oorlog in Oekra55ne","Statement van het CvB","07 maart 2022","HAN redactie"]]

Answer 1

要在 map-like 循环中执行此操作，您通常会使用 mapM_ library function。函数名称中的最后一个下划线_表示操作结果被忽略，这很适合我们，因为我们想要 IO ().

的结果类型

获取文件句柄似乎是合适的，以避免要求 OS 重复重新打开同一个输出文件，这是一种浪费。

可能的代码：

import System.IO

type FileName = String

writeNestedList :: Show a => FileName -> [[a]] -> IO ()
writeNestedList fileName xss =
    do
       fh <- openFile  fileName  WriteMode  -- get a file handle
       mapM_  ((hPutStrLn fh) . show)  xss
       hClose fh


main :: IO ()
main = do
   let  xss = [ ["ab","bc"] ,["de","ef"], ["gh","hi"] ]
   writeNestedList  "scraped.txt"  xss

测试：

$ 
$ ghc q71608762.hs  -o q71608762.x
[1 of 1] Compiling Main             ( q71608762.hs, q71608762.o )
Linking ./q71608762.x ...
$ 
$ q71608762.x
$ 
$ cat scraped.txt
["ab","bc"]
["de","ef"]
["gh","hi"]
$

如何将嵌套列表中的每个列表分别写入文件？

How can I write every list from a nested list separately to a file?

haskell

writefile

nested-lists

appendfile

测试：