# 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