Allow for using Maybe in options types to specify optional arguments about optparse-applicative HOT 2 CLOSED

I found a fairly simple way to accomplish the same:

data Opts = Opts
  { inputDb :: String
  , optKeywords :: Maybe String  -- Maybe here means this is optional
  }

options :: Parser Opts
options = Opts
       <$> strOption (long "input-db"
                      & metavar "FILE"
                      & help "Input database file")
       <*> option (long "keywords"
                      & value Nothing
                      & reader Just
                      & help "Comma-delimited list of bug keywords")

I still maintain that it would be useful to have an example that demonstrates how to do this.

from optparse-applicative.

You can do this more directly using optional in Control.Applicative:

optional (option (long "keywords"))

But you're right that this should be stated more explicitly in the documentation. You're not the first one to ask this question. I'll leave this issue open as a reminder to improve the docs.

from optparse-applicative.