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 ?
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
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
See the following passage of the optparse-applicative README:
Parsers are instances of both
ApplicativeandAlternative, and work with any generic combinator, likemanyandsome. For example, to make a option returnNothinginstead of failing when it's not supplied, you can use theoptionalcombinator inControl.Applicative:optional $ strOption ( long "output" <> metavar "DIRECTORY" )
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
Tests at the command line:
$ 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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