string - Haskell 从文件中删除尾随和前导空格

Question

假设我有一个文件

mary had a little lamb
         It's fleece was white as snow    
Everywhere   
    the child went   
   The lamb, the lamb was sure to go, yeah    

我如何将文件作为字符串读取，并删除尾随和前导空格？它可以是空格或制表符。删除空格后会这样打印:

mary had a little lamb
It's fleece was white as snow
Everywhere
the child went
The lamb, the lamb was sure to go, yeah

这是我目前拥有的:

import Data.Text as T

readTheFile = do
    handle <- openFile "mary.txt" ReadMode
    contents <- hGetContents handle
    putStrLn contents
    hClose handle
    return(contents)

main :: IO ()
main = do
    file <- readTheFile
    file2 <- (T.strip file)
    return()

Answer 1

您的代码暗示了一些对 Haskell 的误解，所以在找到解决方案之前让我们仔细检查一下您的代码。

import Data.Text as T

您正在使用 Text，太棒了!我建议您还使用读写 Text 类型的 IO 操作，而不是使用在 String(字符链接列表)上工作的 prelude 提供的操作。即，将 Data.Text.IO 导入为 T

readTheFile = do
    handle <- openFile "mary.txt" ReadMode
    contents <- hGetContents handle
    putStrLn contents
    hClose handle
    return(contents)

哦，嘿，使用 hGetContents 和手动打开和关闭文件可能容易出错。考虑 readTheFile = T.readFile "mary.txt"。

main :: IO ()
main = do
    file <- readTheFile
    file2 <- (T.strip file)
    return()

这里有两个问题。

问题一 请注意，您在这里使用了 strip 就好像它是一个 IO 操作...但它不是。我建议您了解更多关于 IO 和绑定(bind)(do 表示法)与 let 绑定(bind)变量的信息。 strip 计算一个类型为 Text 的新值，并且您可能想对该值做一些有用的事情，例如写入它。

问题二 剥离整个文件不同于一次剥离每一行。我建议你阅读 mathk 的回答。

所以最后我想你想要:

-- Qualified imports are accessed via `T.someSymbol`
import qualified Data.Text.IO as T
import qualified Data.Text as T

-- Not really need as a separate function unless you want to also
-- put the stripping here too.
readTheFile :: IO T.Text
readTheFile = T.readFile "mary.txt"

-- First read, then strip each line, then write a new file.
main :: IO ()
main =
    do file <- readTheFile
       let strippedFile = T.unlines $ map T.strip $ T.lines file
       T.writeFile "newfile.txt" (T.strip strippedFile)