Mastering Macro Repetition in Rust

Introduction

In this lab, you will learn about using macros with the + and * symbols in the argument list to indicate repetition. Specifically, the example provided demonstrates how to use $(...),+ to match one or more expressions separated by commas. The macro, find_min!, calculates the minimum value among a variable number of arguments.

Note: If the lab does not specify a file name, you can use any file name you want. For example, you can use main.rs, compile and run it with rustc main.rs && ./main.

Skills Graph

Repeat

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.

In the following example, surrounding the matcher with $(...),+ will match one or more expression, separated by commas. Also note that the semicolon is optional on the last case.

// `find_min!` will calculate the minimum of any number of arguments.
macro_rules! find_min {
    // Base case:
    ($x:expr) => ($x);
    // `$x` followed by at least one `$y,`
    ($x:expr, $($y:expr),+) => (
        // Call `find_min!` on the tail `$y`
        std::cmp::min($x, find_min!($($y),+))
    )
}

fn main() {
    println!("{}", find_min!(1));
    println!("{}", find_min!(1 + 2, 2));
    println!("{}", find_min!(5, 2 * 3, 4));
}

Summary

Congratulations! You have completed the Repeat lab. You can practice more labs in LabEx to improve your skills.