optparse-applicative

Utilities and combinators for parsing command line options

Here is a simple example of an applicative option parser: @ 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\" ) @ 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   where   opts = info (helper \<*\> sample)   ( fullDesc   \<\> progDesc \"Print a greeting for TARGET\"   \<\> 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.