Provides support for processing command-line arguments. This is a simple wrapper around get-opt. Here is an example of a typical usage:

data Settings = Settings
  { verbose :: Bool
  , inPar   :: Int
  , files   :: [String]
  }

options :: OptSpec Settings
options = optSpec
  { progDescription = [ "A useful utility." ]

  , progOptions =
      [ Option ['v'] ["verbose"]
        "Display more information while working."
        $ NoArg $ \s -> Right s { verbose = True }

      , Option ['p'] ["par"]
        "Process that many files at once."
        $ ReqArg "NUM" $ \a s ->
          case readMaybe a of
            Just n | n > 0  -> Right s { inPar = n }
            _               -> Left "Invalid value for `par`"
      ]

  , progParamDocs =
      [ ("FILES",   "The files that need processing.") ]

  , progParams = \p s -> Right s { files = p : files s }

  , progArgOrder = Permute
  }

Here is what the usage information looks like:

*Main> dumpUsage options
A useful utility.

Parameters:
  FILES    The files that need processing.

Flags:
  -v      --verbose  Display more information while working.
  -p NUM  --par=NUM  Process that many files at once.