如何將嵌套列表中的每個列表分別寫入文件？

Question

我正在嘗試將嵌套列表中的每個列表寫入同一個文件。 但是我希望每個列表都以換行符開頭。

我做了這個並且它有效，但它非常低效並且看起來很糟糕：

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")

我嘗試了類似下面的操作，但我似乎無法正確使用映射 function：

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

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

txt 文件現在看起來像這樣，沒關系，我只是想知道如何改進我的代碼並學習如何使用映射 function。

Title,Subtitle,Date,Author
[["Een gezonde samenleving? \226\128\156Het belang van sporten wordt onderschat\226\128\157","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 Oekra\195\175ne","Statement van het CvB","07 maart 2022","HAN redactie"]]

Answer 1

要在類似地圖的循環中執行此操作，您通常會使用mapM_庫 function 。 function 名稱中的最后一個下划線 _ 表示操作結果被忽略，這很適合我們，因為我們想要IO ()的結果類型。

獲取文件句柄似乎是合適的，以避免要求操作系統重復重新打開同一個 output 文件，這是一種浪費。

可能的代碼：

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

測試：

$ 
$ 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"]
$