如何用optparse-appative来解析 [英] How to parse Maybe with optparse-applicative

问题描述

我正尝试使用 optparse-applicative 来解析可能是String 但我无法找到任何地方如何处理也许。我唯一发现的是添加一个默认值，但如果用户没有提供一个选项而不是 code>。有什么办法可以达到这个目的？

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 ?

这是一个工作代码的例子：

Here is an example of working code:

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

然而，我希望参数是可选的，在 Config中使用 Maybe String 而不是字符串> p>



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

请参阅 optparse-applicative README a>：

See the following passage of the optparse-applicative README:




解析器是 Applicative 和 Alternative ，并使用任何通用组合器工作
，如许多和某些。例如，要使
a选项返回 Nothing ，而不是在未提供时失败，那么
可以使用可选的 code> Combinator in Control.Applicative ：



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" )

因此，您只需将可选的组合子应用于 strOption $ b

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

命令行测试：

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

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