从haskell中的stdin读取输入并转换为整数列表 [英] Reading input from stdin in haskell and converting to list of integers

问题描述

以下代码有什么问题？我只想在文件中以下列格式转换输入：
n - 测试用例计数// n个数字
n1
n2
（通过stdin读取）到整数列表中并显示它？

What is wrong with the following code? I just wanted to connvert input in the following format in a file: n - count of test cases // n numbers n1 n2 (read via stdin) into list of integers and display it?

socks :: Int -> Int
socks x = x + 1
strToInt = read :: String -> Int
strLToIntL :: [String] -> [Int]
strLToIntL xs = map (strToInt) xs
main = do
    n <- readLn :: IO Int
    mapM_ putStrLn $ map show $ strLToIntL $ fmap (take n . lines) getContents

我运行它时遇到了编译错误：

I am getting the compile error when i run it:

Couldn't match expected type `Char' with actual type `[Char]'
Expected type: String -> [Char]
  Actual type: String -> [String]
In the second argument of `(.)', namely `lines'
In the first argument of `fmap', namely `(take n . lines)'

推荐答案

The problem is that

getContents :: IO String

so

fmap (take n . lines) getContents :: IO [String]

这不能反馈给期望 [String] 的东西。要解决这个问题，你需要绑定 IO 操作。使用 do 表示法，您可以将其写为

This can't be fed to something expecting a [String]. To fix this you need to "bind" the IO action. Using do notation you could write this as

main = do
  n <- readLine :: IO Int
  input <- fmap (take n . lines) getContents
  mapM_ putStrLn . map show . strLToIntL $ input

您可以将最后一行更改为

You could change that last line to just

 mapM print . strLToIntL $ input

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