在 Haskell 中条件处理 IO 的惯用方法

发布于 2024-11-02 16:28:46 字数 871 浏览 3 评论 0原文

我正在 Haskell 中编写一个小 shell 脚本，它可以接受可选参数。但是，如果参数不存在，我想从 stdin 获取一行来请求值。

在 Haskell 中执行此操作的惯用方法是什么？

#!/usr/bin/env runhaskell

import Control.Applicative ((<$>))
import Data.Char (toLower)
import IO (hFlush, stdout)
import System.Environment (getArgs)

main :: IO ()
main = do args <- getArgs
          -- here should be some sort of branching logic that reads
          -- the prompt unless `length args == 1`
          name <- lowerCase <$> readPrompt "Gimme arg: "
          putStrLn name

lowerCase = map toLower

flushString :: String -> IO ()
flushString s = putStr s >> hFlush stdout

readPrompt :: String -> IO String
readPrompt prompt = flushString prompt >> getLine

哦，如果有办法使用 Control.Applicative 或 Control.Arrow 中的某些内容来实现这一点，我想知道。我对这两个模块非常感兴趣。

谢谢！

原文

I'm writing a little shell script in Haskell which can take an optional argument. However, if the argument is not present, I'd like to get a line from stdin in which to ask for a value.

What would be the idiomatic way to do this in Haskell?

#!/usr/bin/env runhaskell

import Control.Applicative ((<gt;))
import Data.Char (toLower)
import IO (hFlush, stdout)
import System.Environment (getArgs)

main :: IO ()
main = do args <- getArgs
          -- here should be some sort of branching logic that reads
          -- the prompt unless `length args == 1`
          name <- lowerCase <gt; readPrompt "Gimme arg: "
          putStrLn name

lowerCase = map toLower

flushString :: String -> IO ()
flushString s = putStr s >> hFlush stdout

readPrompt :: String -> IO String
readPrompt prompt = flushString prompt >> getLine

Oh, and if there's a way to do it with something from Control.Applicative or Control.Arrow I'd like to know. I've become quite keen on these two modules.

Thanks!

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鹊巢 2024-11-09 16:28:46

main :: IO ()
main = do args <- getArgs
          name <- lowerCase <gt; case args of
            [arg] -> return arg
            _     -> readPrompt "Gimme arg: "
          putStrLn name

main :: IO ()
main = do args <- getArgs
          name <- lowerCase <gt; case args of
            [arg] -> return arg
            _     -> readPrompt "Gimme arg: "
          putStrLn name

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被你宠の有点坏 2024-11-09 16:28:46

这不适合您的特定用例，但问题标题让我立即想到 Control.Monad 中的 when 。直接来自文档：

当 :: Monad m =>布尔-> m()->;米()
一元表达式的条件执行。

示例：

main = do args <- getArgs
          -- arg <- something like what FUZxxl did..
          when (length args == 1) (putStrLn $ "Using command line arg: " ++ arg)
          -- continue using arg...

您还可以以类似的方式使用 when 的表兄弟 unless。

This doesn't fit your specific use case, but the question title made me think immediately of when from Control.Monad. Straight from the docs:

when :: Monad m => Bool -> m () -> m ()
Conditional execution of monadic expressions.

Example:

main = do args <- getArgs
          -- arg <- something like what FUZxxl did..
          when (length args == 1) (putStrLn $ "Using command line arg: " ++ arg)
          -- continue using arg...

You can also use when's cousin unless in similar fashion.

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