Macro

In Rust, macros are commands that generate other Rust code. They are known by the trailing exclamation mark, like "println!" We can create our own, custom macros.

With the macro_rules macro, we create a macro. We use some complex syntax to specify an argument list of expressions and then can output code statements for each expression.

Example

This program specifies a macro similar to the "vec!" macro, but with a name of bird. So we can create a bird struct with various attributes by calling "bird!"

Part 1 We use macro_export and a repeated expr to generate some code statements for each call to the "bird!" macro.

Part 2 In a special syntax block, we can specify exactly which statements are duplicated for each expr (expression).

Part 3 For using the "bird!" macro, we have a Bird struct with a vector field. An impl block contains the add_category() function for the macro.

Part 4 We invoke the "bird!" macro with a list of expressions, and the Bird class is created, and the add_category() called for each expr.

// Part 1: specify a macro that generates some Rust statements.
#[macro_export]
macro_rules! bird {
    ( $( $x:expr ),* ) => {
        {
            let mut b = Bird { categories: Vec::new() };
            // Part 2: this code is generated for each expression encountered.
            $(
                b.add_category($x);
            )*
            b
        }
    };
}

// Part 3: the Bird class and an impl block that contains the method add_category.
#[derive(Debug)]
struct Bird {
    categories: Vec<usize>,
}

impl Bird {
    fn add_category(&mut self, c: usize) {
        self.categories.push(c);
    }
}

fn main() {
    // Part 4: use the new bird macro.
    let b = bird![10, 20, 100];
    println!("{:?}", b);

    let b = bird![990];
    println!("{:?}", b);
}Bird { categories: [10, 20, 100] }
Bird { categories: [990] }

Macro notes

It would be possible to create a special function that initializes and returns Bird structs, and avoid the macro. But the "bird!" macro is likely easier to specify and remember.

Summary

Macros have complex syntax that is unlike other Rust method syntax. They can lead to helpful features throughout programs, but can also be hard to develop and debug.