为什么在IO Monad中包含IO结果 [英] Why wrap an IO result in IO Monad

问题描述

在Haskell中，我们有一个函数 readFile :: FilePath - > IO字符串。理解monad时我的问题是为什么将它包装在 IO 中？我们不能只写这样的函数：

 （lines.readFile）path 
  

而不是

 （readFile> > =行）路径
  

IO包装提供了什么好处？

解决方案

Haskell表达式是引用透明。这意味着如果 readFile 真的有一个类型的 FilePath - >字符串，则表达式 readFilea.txt总会产生相同的结果。即使你读了文件，然后改变它，然后再读，你会得到它的第一个状态的内容。

因此，我们需要在值和动作，这就是 IO 的用途。它不会让您在其他表达式中使用结果 readFilea.exe，直到执行与其关联的操作。因此，在更改文件后，必须再次执行读取操作，才能获取文件内容，因此您将能够看到更改。

In Haskell we have a function readFile :: FilePath -> IO String. My question while understanding monad is why wrap it in IO? Couldn't we just have written function like these:

(lines.readFile) path

Rather than

(readFile >>= lines) path 

What benefit does the IO wrapper provide?

解决方案

Haskell expressions are referentially transparent. This means that if readFile would really have a type of FilePath -> String, then expression readFile "a.txt" would always yield the same result. Even if you read the file, then change it, and then read again, you will get the contents in its first state.

Thus, we need to distingush between values and actions, and this is what IO is for. It doesn't let you use the result readFile "a.exe" in other expressions until you perform an action associated with it. As a consequence, after changing your file you have to perform the reading action again, to get file contents, and because of that you will able to see the changes.

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