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 2 3 4
Example 1 foo
Example 1 foo 2 3 4
Usage statements for optional and variadic arguments use command-line
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