What is a declarative macro?
In Rust, declarative macros (aka “macros by example”) allow you to define flexible, reusable code patterns using macro_rules!. They work by matching specific patterns and generating corresponding code.
This is useful when you want to avoid repetitive code, like implementing traits or generating boilerplate.
I am still trying to wrap my brain around this, but there are some examples using these that I found helpful.
The first is a logging type macro to enable line and file, which would be super helpful.
#![allow(dead_code)]
macro_rules! log_println {
($fmt: expr) => (log_println!($fmt, ));
($fmt: expr, $($arg: tt)*) => ({
print!("[{}] {}: ln {} ", chrono::Local::now().format("%Y-%m-%d %H:%M:%S"), file!(), line!());
println!($fmt, $($arg)*);
})
}
#[derive(Debug, Clone)]
struct Person {
name: String,
age: u8,
}
fn main() {
let people : Vec<Person> = vec![
Person { name: String::from("Alice"), age: 30 },
];
println!("People: {:?}", people);
log_println!("People: {:?}", people);
log_println!("Error: {}", "This is an error message");
log_println!("Warning: {}", "This is a warning message");
log_println!("Info: {}", "This is an info message");
}
Running this would give an output like
[2025-04-18 14:05:11] src/main.rs: ln 44 People: [Person { name: "Alice", age: 30 }]
[2025-04-18 14:05:11] src/main.rs: ln 46 Error: This is an error message
[2025-04-18 14:05:11] src/main.rs: ln 47 Warning: This is a warning message
[2025-04-18 14:05:11] src/main.rs: ln 48 Info: This is an info message
Now what about something else using pattern matching?
macro_rules! speak {
// Match "hello" keyword with a name
(hello $name:expr) => {
println!("Hello, {}!", $name);
};
// Match "goodbye" keyword with a name
(goodbye $name:expr) => {
println!("Goodbye, {}!", $name);
};
// Default/fallback pattern
($other:tt $name:expr) => {
println!("I don't know how to respond to '{}'", stringify!($other));
};
}
fn main() {
// Using the speak! macro
speak!(hello "Alice");
speak!(goodbye "Bob");
// Using the speak! macro with a different keyword
speak!(hi "Alice");
}
This is pretty cool I thought as it lends itself to some different behavior depending on the type of keyword used.
I am just learning about these and find them a bit more advanced. Hopefully cover these more in depth in the future.
Until next time.