haskell haskell@gentoo.org ReadArgs provides the @readArgs@ IO action, which lets you tell the compiler to parse the command line arguments to fit the type signature you give. For example @(a :: Int, b :: String, c :: Float) <- readArgs@ would parse the first runtime argument as an @Int@, the second as a @String@ (no quotes required) and the third as a @Float@. If the runtime arguments are incompatible with the type signature, then a simple usage statement is given of the types needed. Continuing the previous example, if it was used in a program named @Example@, the error message for the above action would be: @ usage: Example Int String Float @ Any type that has both @Typeable@ and @Read@ instances can be used. @Char@, @String@, and @Text@ are handled specially so that command line arguments for both do not require quotes (as their @Read@ instances do). A special instance is provided for @FilePath@ so that no constructor or quotes are required. @readArgs@ also supports optional arguments and variadic arguments. Optional arguments are specified using @Maybe@, and variadic arguments using a list. @(a :: Int, b :: Maybe String, c :: [Float]) <- readArgs@ would successfully parse any of the following sets of command line arguments: @ Example 1 Example 1 2 3 4 Example 1 foo Example 1 foo 2 3 4 @ But not @ Example Example foo Example 1.0 @ Usage statements for optional and variadic arguments use command-line parlance: @ usage: Example Int [String] [Float..] @ Note that both optional and variadic parsers are greedy by default (so @Example 1 2 3 4@ was parsed as @(1, "2", [3.0,4.0])@. They may both be made non-greedy through use of the @NonGreedy@ constructor: @ ( a :: Int , NonGreedy b :: NonGreedy Maybe String , NonGreedy c :: NonGreedy [] Float ) <- readArgs @