ghc-optparse-applicative

Utilities and combinators for parsing command line options

Here is a simple example of an applicative option parser: ' data Sample = Sample &#x20; &#x7b; hello :: String &#x20; , quiet :: Bool &#x7d; sample :: Parser Sample sample = Sample &#x20; <$> strOption &#x20; ( long "hello" &#x20; <> metavar "TARGET" &#x20; <> help "Target for the greeting" ) &#x20; <*> switch &#x20; ( long "quiet" &#x20; <> help "Whether to be quiet" ) ' The parser is built using applicative style starting from a set of basic combinators. In this example, 'hello' is defined as an 'option' with a 'String' argument, while 'quiet' is a boolean 'flag' (called 'switch'). A parser can be used like this: ' greet :: Sample -> IO () greet (Sample h False) = putStrLn $ "Hello, " ++ h greet _ = return () main :: IO () main = execParser opts >>= greet &#x20; where &#x20; opts = info (helper <*> sample) &#x20; ( fullDesc &#x20; <> progDesc "Print a greeting for TARGET" &#x20; <> header "hello - a test for optparse-applicative" ) ' The 'greet' function is the entry point of the program, while 'opts' is a complete description of the program, used when generating a help text. The 'helper' combinator takes any parser, and adds a 'help' option to it (which always fails). The 'hello' option in this example is mandatory (since it doesn't have a default value), so running the program without any argument will display a help text: >hello - a test for optparse-applicative > >Usage: hello --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 containing a short usage summary, and a detailed list of options with descriptions.