Repetitions

We use $(<metavariable>)[delimiter]<*|?|+> to specify metavariable repetition in both the pattern (matcher) and the expansion (transcriber).

• *: Zero or more times
• ?: Zero or one time
• +: One or more times

Optional Repetitions (?)

macro_rules! greet{
  ($($msg:literal)?) => {
    let msg = concat!("Hello, world! ", $($msg)?);
    println!("{}", msg);
  }
}

fn main(){
  greet!();
  greet!("Hello, Rustacean! 👋");
}

Zero or More Repetitions (*)

macro_rules! sum{
  ($($n:expr)*) => {
      let mut sum=0;
      $(sum += $n;)*
      println!("The sum is {}", sum);
  }
}
fn main(){
  sum!(1 2 3);
}

Repetitions with Delimiters

macro_rules! sum{
  ($($n:expr),*) => {
      let mut sum=0;
      $(sum += $n;)*
      println!("The sum is {}", sum);
  }
}
fn main(){
  sum!(1, 2, 3);
}

Custom Delimiter Tokens

macro_rules! sum{
  ($($n:literal)add*) => {
      let mut sum=0;
      $(sum += $n;)*
      println!("The sum is {}", sum);
  }
}
fn main(){
  sum!(1 add 2 add 3);
}

Tip

The delimiter token can be any token other than a delimiter or one of the repetition operators, but ; and , are the most common.