Macro Rules

Macro Matcher

Named Syntax

A macro matcher can include literal tokens, which lets a call read like a tiny domain-specific syntax.

Program

Play the program to choose the retry count in a macro call with a named argument.

macro_named_argument.rs 
macro_rules! command_line {
    ($name:expr, retries = $count:expr) => {
        format!("{} retries={}", $name, $count)
    };
}

fn main() {
    let retries = ;
    let command = command_line!("deploy", retries = retries);
    println!("{command}");
}

macro_rules! command_line {
    ($name:expr, retries = $count:expr) => {
        format!("{} retries={}", $name, $count)
    };
}

fn main() {
    let retries = ;
    let command = command_line!("deploy", retries = retries);
    println!("{command}");
}

macro_rules! command_line {
    ($name:expr, retries = $count:expr) => {
        format!("{} retries={}", $name, $count)
    };
}

fn main() {
    let retries = ;
    let command = command_line!("deploy", retries = retries);
    println!("{command}");
}
literal tokens The matcher requires the literal `retries =` tokens at the call site.
fragment specifier `$count:expr` captures the expression after the named token.
DSL shape Literal tokens can make macro calls read like a small, checked syntax.