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我尝试在考试前学习 Haskell，它对我来说仍然很神奇。今天我尝试编写一个程序，它读取一个文件，并将它的行以相反的顺序写入另一个文件。结果？很多错误。这是我的代码：

import IO

readLines :: Handle -> [String] -> [String]
readLines handler list = do
    eof <- hIsEOF handler
    if eof then list
        else do
            line <- hGetLine handler
            readLines handler (list ++ [line])

writeLines :: Handle -> [String] -> [String]
writeLines handler list = if length list == 0 then list
    else do
        line <- head list
        hPutStrLn handler line
        writeLines tail list

fileToList :: FilePath -> [String]
fileToList filename = do
    handler <- openFile filename ReadMode
    list <- (readLines handler [])
    hClose handler
    list

invLines :: FilePath -> FilePath -> IO ()
invLines input output = do
    handler <- openFile output WriteMode
    inverted <- reverse (fileToList input)
    writeLines handler inverted
    hClose handler

main = invLines "in.txt" "out.txt"

如果你能解释我的错误，我将不胜感激。

4

1 回答 1

6

好的，首先，

IO ==> System.IO

接下来让我们看看 readLines（PS readFile 在这里可以很好地工作）

readLines :: Handle -> [String] -> [String] --You have to return an IO [String] here.
    readLines handler list = do
    eof <- hIsEOF handler
    if eof then return list --Return an IO list
        else do
            line <- hGetLine handler
            readLines handler (list ++ [line])

接下来writeLines（PSwriteFile也会有帮助）

writeLines :: Handle -> [String] -> [String] -- You have to return IO of something, I'd recommend ().
writeLines handler list = if length list == 0
    then list --Change to return ()
    else do
        line <- head list --Should be let here. Like let line = head list
        hPutStrLn handler line
        writeLines tail list -- Should be writeLines handler $ tail list instead.

接下来是fileToList

fileToList :: FilePath -> [String] --Has to be IO [String]
fileToList filename = do
    handler <- openFile filename ReadMode
    list <- (readLines handler [])
    hClose handler
    list -- Have to use return list

哦，你似乎把线倒置了两次，所以实际上你最终得到了一个非常大的复制例程，在那里删除了反向，你得到了正确的结果。

最后的代码

import System.IO

readLines :: Handle -> [String] -> IO [String]
readLines handler list = do
    eof <- hIsEOF handler
    if eof 
    then return list
    else do
      line <- hGetLine handler
      readLines handler (line:list)

writeLines :: Handle -> [String] -> IO ()
writeLines handler list = if length list == 0 then return ()
    else do
        let line = head list
        hPutStrLn handler line
        writeLines handler $ tail list

fileToList :: FilePath -> IO [String]
fileToList filename = do
    handler <- openFile filename ReadMode
    list <- (readLines handler [])
    hClose handler
    return list

invLines :: FilePath -> FilePath -> IO ()
invLines input output = do
    handler <- openFile output WriteMode
    inputLines <- fileToList input
    writeLines handler inputLines
    hClose handler

main = invLines "in.txt" "out.txt"

并使用更多的库重写：

invLines input output = do
    inputLines <- readFile input
    writeFile output . unlines . reverse . lines $ inputLines

invLines2 input output = 
    readFile input >>= writeFile output . unlines . reverse . lines

要学习的重要课程您不能从 IOdo块内部返回正常功能。你必须返回一个 IO 东西

于 2013-01-12T05:28:15.130 回答