Question

I’m having a bit problem with reading argument value in haskell:

I’m having options declared like this:

options :: [OptDescr ArgFlag]
options = [Option [] ["help"] (NoArg Help) "",
           Option [] ["version"] (NoArg Version) "",
           Option ['a'] [] (NoArg FSAll) "",
           Option ['L'] [] (ReqArg FSLevel "NUM") ""]

And I read the arguments like this (I can read which argument is active):

main = do
         args <- getArgs
         case getOpt RequireOrder options args of
             ([Version], [], []) -> do ...

             ([Help], [], []) -> do ...

             (flags, files, []) -> do ........ *

             otherwise -> do ...

Where there is ‘*’ character, I need to read value of possible ‘-l 3’ flag -> I searched through google but I haven’t found nothing (maybe wrong keyword though). They just detect whether argument is active everywhere, not its value.

Answer 1

I presume you have defined

data ArgFlag = Help
             | Version
             | FSAll
             | FSLevel String

With a helper function

unFSLevel :: ArgFlag -> Maybe String
unFSLevel (FSLevel value) = Just value
unFSLevel _               = Nothing

you can then get the value like this:

case listToMaybe $ mapMaybe unFSLevel of
     Just value -> -- code to handle the -L flag having this value
     Nothing    -> -- code to handle there not being an -L flag

(You will need to import mapMaybe and listToMaybe from Data.Maybe.)