DRY (Don't Repeat Yourself)
Macros allow writing DRY code by factoring out the common parts of functions
and/or test suites. Here is an example that implements and tests the +=
, *=
and -=
operators on Vec<T>
:
use std::ops::{Add, Mul, Sub}; macro_rules! assert_equal_len { // The `tt` (token tree) designator is used for // operators and tokens. ($a:expr, $b:expr, $func:ident, $op:tt) => { assert!($a.len() == $b.len(), "{:?}: dimension mismatch: {:?} {:?} {:?}", stringify!($func), ($a.len(),), stringify!($op), ($b.len(),)); }; } macro_rules! op { ($func:ident, $bound:ident, $op:tt, $method:ident) => { fn $func<T: $bound<T, Output=T> + Copy>(xs: &mut Vec<T>, ys: &Vec<T>) { assert_equal_len!(xs, ys, $func, $op); for (x, y) in xs.iter_mut().zip(ys.iter()) { *x = $bound::$method(*x, *y); // *x = x.$method(*y); } } }; } // Implement `add_assign`, `mul_assign`, and `sub_assign` functions. op!(add_assign, Add, +=, add); op!(mul_assign, Mul, *=, mul); op!(sub_assign, Sub, -=, sub); mod test { use std::iter; macro_rules! test { ($func:ident, $x:expr, $y:expr, $z:expr) => { #[test] fn $func() { for size in 0usize..10 { let mut x: Vec<_> = iter::repeat($x).take(size).collect(); let y: Vec<_> = iter::repeat($y).take(size).collect(); let z: Vec<_> = iter::repeat($z).take(size).collect(); super::$func(&mut x, &y); assert_eq!(x, z); } } }; } // Test `add_assign`, `mul_assign`, and `sub_assign`. test!(add_assign, 1u32, 2u32, 3u32); test!(mul_assign, 2u32, 3u32, 6u32); test!(sub_assign, 3u32, 2u32, 1u32); }
$ rustc --test dry.rs && ./dry
running 3 tests
test test::mul_assign ... ok
test test::add_assign ... ok
test test::sub_assign ... ok
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured