import Options.Applicative

data Sample = Sample
  { hello :: String
  , quiet :: Bool }

sample :: Parser Sample
sample = Sample
  <$> strOption
      ( long "hello"
      <> metavar "TARGET"
      <> help "Target for the greeting" )
  <*> switch
      ( long "quiet"
      <> help "Whether to be quiet" )

greet :: Sample -> IO ()
greet (Sample h False) = putStrLn $ "Hello, " ++ h
greet _ = return ()

main :: IO ()
main = execParser opts >>= greet
  where
    opts = info (helper <*> sample)
      ( fullDesc
      <> progDesc "Print a greeting for TARGET"
      <> header "hello - a test for optparse-applicative" )

$ ./optest 
Usage: optest --hello TARGET [--quiet]

$ ./optest --hello Haskell
Hello, Haskell

$ ./optest --help
hello - a test for optparse-applicative

Usage: optest --hello TARGET [--quiet]
  Print a greeting for TARGET

Available options:
  -h,--help                Show this help text
  --hello TARGET           Target for the greeting
  --quiet                  Whether to be quiet