Overload

Macros can be overloaded to accept different combinations of arguments.
In that regard, macro_rules! can work similarly to a match block:

  1. // `test!` will compare `$left` and `$right`
  2. // in different ways depending on how you invoke it:
  3. macro_rules! test {
  4.     // Arguments don't need to be separated by a comma.
  5.     // Any template can be used!
  6.     ($left:expr; and $right:expr) => (
  7.         println!("{:?} and {:?} is {:?}",
  8.                  stringify!($left),
  9.                  stringify!($right),
  10.                  $left && $right)
  11.     );
  12.     // ^ each arm must end with a semicolon.
  13.     ($left:expr; or $right:expr) => (
  14.         println!("{:?} or {:?} is {:?}",
  15.                  stringify!($left),
  16.                  stringify!($right),
  17.                  $left || $right)
  18.     );
  19. }
  21. fn main() {
  22.     test!(1i32 + 1 == 2i32; and 2i32 * 2 == 4i32);
  23.     test!(true; or false);
  24. }