如何将可选标志解析为Maybe值？ [英] How to parse an optional flag as a Maybe value?

问题描述

我正在尝试使用 optparse-applicative 来解析也许是字符串，但是我在任何地方都找不到如何处理也许的方法。我发现的唯一一件事就是添加一个默认值，但是如果用户没有提供选项而不是 ，我真的需要 Nothing code>。有什么方法可以实现？

I'm trying to use optparse-applicative to parse a Maybe String but I can't find anywhere how to deal with Maybe. The only thing I found is to add a default value but I really need a Nothing if user didn't supply an option instead of "". Is there any way to achieve this ?

以下是工作代码示例：

import Options.Applicative

data Config = Config
    { cIn :: String
    , cOut :: String
    } deriving Show

configParser :: Parser Config
configParser = Config
    <$> strOption (long "in" <> short 'i')
    <*> strOption (long "out" <> short 'o')


main :: IO ()
main = do
    conf <- execParser (info configParser fullDesc)
    print conf

但是，我希望参数是可选的，并且在 Config 中使用也许是字符串代替 String ：

However, I would like the parameters to be optional and use Maybe String instead of String in Config :

data Config = Config
    { cIn :: Maybe String
    , cOut :: Maybe String
    } deriving Show

推荐答案

请参见以下 optparse-applicative 自述文件：

See the following passage of the optparse-applicative README:




解析器是 Applicative 和替代，并与任何通用组合器（例如很多和 some >。例如，要使
a选项返回 Nothing 而不是在未提供时失败，则
可以使用可选 Control.Applicative 中的code>组合器：

Parsers are instances of both Applicative and Alternative, and work with any generic combinator, like many and some. For example, to make a option return Nothing instead of failing when it's not supplied, you can use the optional combinator in Control.Applicative:

optional $ strOption
   ( long "output"
  <> metavar "DIRECTORY" )

相应地，您要做的就是将可选组合器应用于 strOption ：

Accordingly, all you have to do is apply the optional combinator to the result of strOption:

import Options.Applicative

data Config = Config
    { cIn  :: Maybe String
    , cOut :: Maybe String
    } deriving Show

configParser :: Parser Config
configParser = Config
    <$> (optional $ strOption $ long "in" <> short 'i')
    <*> (optional $ strOption $ long "out" <> short 'o')

main :: IO ()
main = do
    conf <- execParser (info configParser fullDesc)
    print conf

在命令行中进行测试：

$ main --in foo -o bar
Config {cIn = Just "foo", cOut = Just "bar"}
$ main -i foo
Config {cIn = Just "foo", cOut = Nothing}

这篇关于如何将可选标志解析为Maybe值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！