How to parse an optional flag as a Maybe value?

Question

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 

Answer 1

See the following passage of the optparse-applicative README:

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" ) 

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}