NestedOptParseApplicative.hs

import Options.Applicative
import Control.Monad

parser :: Parser (IO ())
parser = subparser $
       command "add" (info (helper <*> addMode) mempty)
    <> command "test" (info (pure $ putStrLn "testing") mempty)
  where
    addMode :: Parser (IO ())
    addMode = subparser $
         command "add" (info (pure $ putStrLn "adding message") mempty)
      <> command "remove" (info (pure $ putStrLn "removing message") mempty)

main :: IO ()
main = join $ execParser $ info (helper <*> parser) mempty

from comments on https://www.reddit.com/r/haskell/comments/44zhdk/im_making_a_command_line_argument_parser_inspired/