Defining a Macro
Using a Macro
Repetition In Macros

In Rust macros declarations start from macro_rules like macro_rules! some_macro{() => ()}. Here some_macro is name of the macro.

Example

macro_rules! add{
  ($x:expr,$y:expr) => ($x + $y);
}

To use a Macro in rust we write it's name with an exclamation point like add! .
Let say we define a macro that takes two expressions (say x and y) and evaluates to x + y on compiling.

Example

macro_rules! add{
  ($x:expr,$y:expr) => ($x + $y);
}

fn main(){
  println!("{}",add!(3,5));
  println!("{}",add!(3,add!(4,5)));
  println!("{}",add!({3-5},{5*3}));
}

Output for the above example will be

8
12
13

So println!("{}",add!(3,add!(4,5))); will be converted to println("{}",3 + 4 + 5); on compiling

vec! macro can take different no of parameters like let x = vec![1,2,3,4];
We can also create macros that can work with repeated values.

Example

macro_rules! double_vec{
    ($($x:expr),*) => ({
        let mut temp = Vec::new();
        $(
            temp.push($x*2);
        )*
        temp
    });
}

fn main(){
    let new_vec = double_vec![2,3,5,6];
    println!("{:?}",new_vec);
}

Breaking Down the code, $(...)* means zero or more parameters $(...)+ means one or more parameters. Here macro matches the pattern of parameters seperated by ',' and push twice the value of parameters in a vector and returns it.

Output for the above example will be

[4, 6, 10, 12]

For Full Documentation Go To
Macros - Rust Documentation