macro_rules!Rust provides a powerful macro system that allows metaprogramming. As you've
seen in previous chapters, macros look like functions, except that their name
ends with a bang !, but instead of generating a function call, macros are
expanded into source code that gets compiled with the rest of the program.
Macros are created using the macro_rules! macro.
// simple.rs
// macros are behind a feature gate
#![feature(macro_rules)]
// This is the simplest macro, `say_hello` is the name of the macro
macro_rules! say_hello {
// `()` indicates that the macro takes no argument
() => {
// the macro will expand into the contents of this block
println!("Hello!");
}
}
fn main() {
// this call will expand into `println!("Hello");`
say_hello!()
}
{simple.out}
The arguments of a macro are prefixed by a dollar sign $ and type annotated
with a designator.
// designators.rs
#![feature(macro_rules)]
macro_rules! create_function {
// this macro takes an argument of "type" `ident`
// the `ident` designator is used for variable/function names
($func_name:ident) => {
// this macro creates a function with name `$func_name`
fn $func_name() {
// the stringify! macro converts an `ident` into a string
println!("You called {}()",
stringify!($func_name))
}
}
}
create_function!(foo);
create_function!(bar);
macro_rules! print_result {
// the `expr` designator is used for expressions
($expression:expr) => {
// stringify! will convert the expression *as it is* into a string
println!("{} = {}",
stringify!($expression),
$expression)
}
}
fn main() {
foo();
bar();
print_result!(1u + 1);
// remember that blocks are expressions
print_result!({
let x = 1u;
x * x + 2 * x - 1
});
}
{designators.out}
Macros can be overloaded to accept different combinations of arguments.
// overload.rs
#![feature(macro_rules)]
// macro_rules! is similar to a match block
macro_rules! test {
// the arguments don't need to be separated by a comma
// any template can be used
($left:expr and $right:expr) => {
println!("{} and {} is {}",
stringify!($left),
stringify!($right),
$left && $right)
};
// ^ each arm must be ended with a semicolon
($left:expr or $right:expr) => {
println!("{} or {} is {}",
stringify!($left),
stringify!($right),
$left || $right)
};
}
fn main() {
test!(1i + 1 == 2i and 2i * 2 == 4i);
test!(true or false);
}
{overload.out}
Macros can use + in the argument list, to indicate that an argument may
repeat at least once, or *, to indicate that the argument may repeat zero or
more times.
// repeat.rs
#![feature(macro_rules)]
// min! will calculate the minimum of any number of arguments
macro_rules! min {
// base case
($x:expr) => {
$x
};
// `$x` followed by at least one `$y,`
($x:expr, $($y:expr),+) => {
// call min! on the tail `$y`
std::cmp::min($x, min!($($y),+))
}
}
fn main() {
println!("{}", min!(1u));
println!("{}", min!(1u + 2 , 2u));
println!("{}", min!(5u, 2u * 3, 4u));
}
{repeat.out}
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>.
// dry.rs
#![feature(macro_rules)]
use std::iter;
macro_rules! assert_equal_len {
($a:ident, $b: ident, $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, 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.$method(*y);
}
}
}
}
// implement add_assign, mul_assign, and sub_assign functions
op!(add_assign, Add, +=, add);
op!(mul_assign, Mul, *=, mul);
op!(sub_assign, Sub, -=, sub);
fn main() {
let mut xs = iter::repeat(0f64).take(5).collect();
let ys = iter::repeat(1f64).take(6).collect();
// this operation will fail at runtime
add_assign(&mut xs, &ys);
}
mod test {
macro_rules! test {
($func: ident, $x:expr, $y:expr, $z:expr) => {
#[test]
fn $func() {
for size in range(0u, 10) {
let mut x = Vec::from_elem(size, $x);
let y = Vec::from_elem(size, $y);
let z = Vec::from_elem(size, $z);
super::$func(&mut x, &y);
assert_eq!(x, z);
}
}
}
}
// test add_assign, mul_assign and sub_assign
test!(add_assign, 1u, 2u, 3u);
test!(mul_assign, 2u, 3u, 6u);
test!(sub_assign, 3u, 2u, 1u);
}
{dry.out}
$ 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