Roman Cheplyaka

optparse-applicative quick start

Published on ; updated on

When I need to write a command-line program in Haskell, I invariably pick Paolo Capriotti’s optparse-applicative library.

Unfortunately, the minimal working example is complicated enough that I cannot reproduce it from memory, and the example in the README is very different from the style I prefer.

So I decided to put up a template here for a program using optparse-applicative. I am going to copy it into all of my future projects, and you are welcome to do so, too.

import Options.Applicative
import Control.Monad (join)

main :: IO ()
main = join . customExecParser (prefs showHelpOnError) $
  info (helper <*> parser)
  (  fullDesc
  <> header "General program title/description"
  <> progDesc "What does this thing do?"
  )
  where
    parser :: Parser (IO ())
    parser =
      work
        <$> strOption
            (  long "string_param"
            <> short 's'
            <> metavar "STRING"
            <> help "string parameter"
            )
        <*> option auto
            (  long "number_param"
            <> short 'n'
            <> metavar "NUMBER"
            <> help "number parameter"
            <> value 1
            <> showDefault
            )

work :: String -> Int -> IO ()
work _ _ = return ()