| Expand multi-rule and recursive macro_rules d459281 nandithebull 9h ago | 1 | // Rules are tried top to bottom and the first whose matcher matches wins, so |
| 2 | // their order is semantics rather than style. That is also what makes a |
| 3 | // recursive macro terminate: a base-case rule sits above the recursive one. |
| 4 | |
| 5 | macro_rules! describe { |
| 6 | () => { 0 }; |
| 7 | ($x:expr) => { $x }; |
| 8 | ($x:expr, $y:expr) => { $x * 100 + $y }; |
| 9 | } |
| 10 | |
| 11 | // Recursion, shedding one argument per step. |
| 12 | macro_rules! total { |
| 13 | () => { 0 }; |
| 14 | ($x:expr) => { $x }; |
| 15 | ($x:expr, $($rest:expr),+) => { $x + total!($($rest),+) }; |
| 16 | } |
| 17 | |
| 18 | macro_rules! depth { |
| 19 | ($x:expr) => { 1 }; |
| 20 | ($x:expr, $($rest:expr),+) => { 1 + depth!($($rest),+) }; |
| 21 | } |
| 22 | |
| 23 | // Different shapes, not just different arities. |
| 24 | macro_rules! pick { |
| 25 | (first $a:expr, $b:expr) => { $a }; |
| 26 | (second $a:expr, $b:expr) => { $b }; |
| 27 | (sum $a:expr, $b:expr) => { $a + $b }; |
| 28 | } |
| 29 | |
| 30 | fn main() { |
| 31 | println!("{} {} {}", describe!(), describe!(7), describe!(3, 4)); |
| 32 | |
| 33 | println!("{}", total!()); |
| 34 | println!("{}", total!(5)); |
| 35 | println!("{}", total!(1, 2, 3, 4, 5)); |
| 36 | println!("{}", total!(2 * 3, 4 + 1)); |
| 37 | |
| 38 | println!("{} {}", depth!(9), depth!(9, 9, 9, 9)); |
| 39 | |
| 40 | println!("{} {} {}", pick!(first 10, 20), pick!(second 10, 20), pick!(sum 10, 20)); |
| 41 | |
| 42 | let a: i64 = 1000; |
| 43 | println!("{}", total!(a, a, a)); |
| 44 | } |