为什么/递归IO如何工作？ [英] Why/how does recursive IO work?

问题描述

Haskell IO通常被解释为整个程序是一个纯函数（ main ），它返回一个IO值（通常被描述为一个命令式IO程序），它然后由运行时执行。

这个心智模型对于简单的例子来说很好，但是当我看到一个递归的 main 在学习你一个Haskell 。例如：

  main = do 
 line<  -  getLine 
 putStrLn line 
 main 
  

或者，如果您愿意：

  main = getLine>> = putStrLn>>主要
  

由于主要永不终止，实际上返回一个IO值，然而程序无休止地读取和回显行就好了 - 所以上面的简单解释并不是很有效。我是否错过了一些简单的东西或者是否有更完整的解释（或者它只是'编译器的魔力'）？ 情况下， main 是类型 IO（）的值，而不是函数。你可以把它看作一系列 IO a 值：

  main = getLine>> = putStrLn>> main 
  

这使得它成为递归值，与无限列表不同：

  foo = 1：2：foo 
  

<我们可以返回这样的值，而不需要评估整个事情。事实上，这是一个相当常见的习惯用语。

尝试使用整个事情。但是对于 main 也是如此：除非使用某种外部方法来突破它，否则它将永远不会停止循环！但是你可以开始从 foo 中获取元素，或者执行 main 的部分，而不对它进行全部评估。 p>

Haskell IO is often explained in terms of the entire program being a pure function (main) that returns an IO value (often described as an imperative IO program), which is then executed by the runtime.

This mental model works fine for simple examples, but fell over for me as soon as I saw a recursive main in Learn You A Haskell. For example:

main = do
  line <- getLine
  putStrLn line
  main 

Or, if you prefer:

main = getLine >>= putStrLn >> main

Since main never terminates, it never actually returns an IO value, yet the program endlessly reads and echoes back lines just fine - so the simple explanation above doesn't quite work. Am I missing something simple or is there a more complete explanation (or is it 'simply' compiler magic) ?

解决方案

In this case, main is a value of type IO () rather than a function. You can think of it as a sequence of IO a values:

main = getLine >>= putStrLn >> main

This makes it a recursive value, not unlike infinite lists:

foo = 1 : 2 : foo

We can return a value like this without needing to evaluate the whole thing. In fact, it's a reasonably common idiom.

foo will loop forever if you try to use the whole thing. But that's true of main too: unless you use some external method to break out of it, it will never stop looping! But you can start getting elements out of foo, or executing parts of main, without evaluating all of it.

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