Macro Rules

Macro Expression

Expand to a Value

macro_rules! can expand a call into an expression that is checked after expansion.

Program

Play the program to choose an id and build a label through a small expression macro.

macro_expr_label.rs 
macro_rules! make_label {
    ($prefix:expr, $id:expr) => {
        format!("{}-{}", $prefix, $id)
    };
}

fn main() {
    let id = ;
    let label = make_label!("task", id);
    println!("{label}");
}

macro_rules! make_label {
    ($prefix:expr, $id:expr) => {
        format!("{}-{}", $prefix, $id)
    };
}

fn main() {
    let id = ;
    let label = make_label!("task", id);
    println!("{label}");
}

macro_rules! make_label {
    ($prefix:expr, $id:expr) => {
        format!("{}-{}", $prefix, $id)
    };
}

fn main() {
    let id = ;
    let label = make_label!("task", id);
    println!("{label}");
}
macro_rules! `macro_rules!` defines a declarative macro that rewrites syntax before type checking.
matcher `$prefix:expr` and `$id:expr` capture expression fragments from the call site.
expansion The macro expands to a `format!` expression that produces the final `String`.