Haskell文件阅读

问题描述

我刚刚开始学习Haskell，并且在尝试弄清文件阅读是如何工作的时候遇到了很多麻烦。

例如，我有一段文字文件test.txt
它包含一些数字，如：

  32 4 
 2 30 
 300 5 
  

我想要阅读每行，然后评估每个单词并添加它们。
因此，我试图做到这样的事情：

  import System.IO 
 import Control .Monad 
 
 main = do 
 let list = [] 
句柄<  -  openFiletest.txtReadMode 
 contents<  -  hGetContents句柄
单字<  - （单词内容）
列表< -f单字
打印列表
 h关闭句柄
 
f :: [字符串]  - > [Int] 
f =地图阅读
  

我知道这是完全错误的，不知道如何正确使用语法。
任何帮助将不胜感激。
除了这个之外，还有一些很好的教程链接，其中包含示例和代码解释：
我已经读完了

解决方案

不错的开始！唯一需要记住的是纯函数应用程序应该使用 let 来代替绑定< - 。

  import System.IO 
 import Control.Monad 
 
 main = do 
 let list = [] 
句柄<  -  openFiletest.txtReadMode 
 contents<  -  hGetContents句柄
 let singlewords =单词内容
 list = f单词
 print列表
 h关闭句柄
 
f :: [String]  - > [Int] 
f =地图读取
  

这是需要做的最小变化编译并运行。在风格上，我有几条评论：

1. 绑定 list 两次看起来有点阴暗。请注意，这并没有改变 list 这个值 - 它反映了旧的定义。


2. 内联纯函数更多！


3. 如果可能的话，使用 readFile 比手动打开，阅读和关闭文件更可取。

执行这些更改会产生如下所示的结果：

  main = do 
内容<  -  readFiletest.txt
 print。地图readInt。单词$ contents 
  - 或者，main = print。地图readInt。 words =<< readFiletest.txt
 
 readInt :: String  - > Int 
 readInt = read 
  

I have just recently started learning Haskell and I am having a lot of trouble trying to figure out how the file reading works.

For example, I have a text file "test.txt"And it contains lines of numbers eg:

32 4
2 30
300 5

I want to read each line and then evaluate each word and add them.Thus I am trying to do something like this so far:

import System.IO
import Control.Monad

main = do
        let list = []
        handle <- openFile "test.txt" ReadMode
        contents <- hGetContents handle
        singlewords <- (words contents)
        list <- f singlewords
        print list
        hClose handle

f :: [String] -> [Int]
f = map read

I know this is completely wrong, but I don't know how to use the syntax correctly at all.Any help will be greatly appreciated.As well as links to good tutorials that have examples and explanation of code except this one:http://learnyouahaskell.com/input-and-output I have read it fully

解决方案

Not a bad start! The only thing to remember is that pure function application should use let instead of the binding <-.

import System.IO
import Control.Monad

main = do
        let list = []
        handle <- openFile "test.txt" ReadMode
        contents <- hGetContents handle
        let singlewords = words contents
            list = f singlewords
        print list
        hClose handle

f :: [String] -> [Int]
f = map read

This is the minimal change needed to get the thing to compile and run. Stylistically, I have a few comments:

1. Binding list twice looks a bit shady. Note that this isn't mutating the value list -- it's instead shadowing the old definition.
2. Inline pure functions a lot more!
3. When possible, using readFile is preferable to manually opening, reading, and closing a file.

Implementing these changes gives something like this:

main = do
        contents <- readFile "test.txt"
        print . map readInt . words $ contents
-- alternately, main = print . map readInt . words =<< readFile "test.txt"

readInt :: String -> Int
readInt = read

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