Full description
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.
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