有条件的IO操作 [英] Conditional IO action

问题描述

我正在编写一个程序，它应该能够直接从命令行读取输入或从文件读取输入，具体取决于传递的参数。例如。但是，我无法有条件地使IO事件。我首先想到的是：

  import System.Environment 
 $ b $ main :: IO（）
 main = do 
 cla<  -  getArgs 
 let flags = parseFlags（filter isFlag cla）[] 
 let args = filter（not.isFlag）cla 
 input< ;  - 如果（eleme标志）then（head args）else（readFile（head args））
  

<但是这不起作用，因为（head args）不是IO动作。我的下一个想法是：

  import System.Environment 
 $ b $ main :: IO（）
 main = do 
 cla<  -  getArgs 
 let flags = parseFlags（filter isFlag cla）[] 
 let args = filter（not.isFlag）cla 
 file< ;  - （readFile（head args））
 let input = if（elemeflags）then（head args）else（file）
  

但是，如果输入不是文件的名称，这将会报错。有没有一个明智的方法来做到这一点？我正在处理这个问题吗？

c $ c>好像它在IO monad中一样。由于它只是一个原始值，因此在将其用作IO操作之前，需要将它放入monad中。

只需将（head args）更改为（return（head args）） code>。

I am writing a program that should be able to read input either from the command line directly or from a file depending on what args are passed. For example. However I am having trouble making the IO event conditional. My first thought was to do:

import System.Environment

main :: IO ()
main = do
 cla <- getArgs
 let flags = parseFlags (filter isFlag cla) []
 let args  = filter (not.isFlag) cla
 input <- if (elem "e" flags) then (head args) else (readFile (head args))

However this doesn't work because (head args) is not a IO action. My next thought was to do:

import System.Environment

main :: IO ()
main = do
 cla <- getArgs
 let flags = parseFlags (filter isFlag cla) []
 let args  = filter (not.isFlag) cla
 file <- (readFile (head args))
 let input = if (elem "e" flags) then (head args) else (file)

But this will error if the input is not the name of a file. Is there a sensible way to do this? Am I approaching the problem incorrectly?

解决方案

You were trying to treat (head args) as though it was in the IO monad. Since it's just a raw value, you need to place it into the monad before it can be used as an IO action.

Just change (head args) to (return (head args)).

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