Macro Rules

Macro Repetition

Capture a List

A macro repetition can accept a comma-separated list and expand that list into another expression.

Program

Play the program to choose the middle word in a repeated macro input.

macro_repetition_words.rs 
macro_rules! collect_words {
    ($($word:expr),+ $(,)?) => {{
        let values = vec![$($word),+];
        values.join(",")
    }};
}

fn main() {
    let extra = ;
    let joined = collect_words!("red", extra, "blue");
    println!("{joined}");
}

macro_rules! collect_words {
    ($($word:expr),+ $(,)?) => {{
        let values = vec![$($word),+];
        values.join(",")
    }};
}

fn main() {
    let extra = ;
    let joined = collect_words!("red", extra, "blue");
    println!("{joined}");
}

macro_rules! collect_words {
    ($($word:expr),+ $(,)?) => {{
        let values = vec![$($word),+];
        values.join(",")
    }};
}

fn main() {
    let extra = ;
    let joined = collect_words!("red", extra, "blue");
    println!("{joined}");
}
repetition `$($word:expr),+` captures one or more comma-separated expressions.
optional trailing comma `$(,)?` accepts a final comma without requiring one.
expansion list `$($word),+` repeats the captured expressions inside `vec!`.