Declarative Macros | Macros

What are Declarative Macros?

Declarative macros in Rust allow you to write code that generates more code through a pattern matching system. Unlike procedural macros, which operate on the syntax tree directly, declarative macros use a simpler syntax and work by matching against specific patterns.

Basic Syntax

Declarative macros are defined using the macro_rules! keyword. The basic structure looks like this:

macro_rules! my_macro {
($x:expr) => {
println!("Value: {}", $x);
};
}

In this example, my_macro! takes a single expression and prints its value.

Using Declarative Macros

To use a declarative macro, you simply call it with the ! suffix, passing in the required arguments:

fn main() {
my_macro!(10);
}

This will output: Value: 10.

Pattern Matching

Declarative macros can match multiple patterns. Here’s an example that demonstrates matching different types of inputs:

macro_rules! print_value {
($x:expr) => { println!("Value: {}", $x); };
($x:expr, $y:expr) => { println!("Values: {}, {}", $x, $y); };
}

Using this macro:

fn main() {
print_value!(42);
print_value!(10, 20);
}

It will output:

Value: 42
Values: 10, 20

Using Repetition

Declarative macros also support repetitions, allowing you to write more concise and flexible macros. Here’s an example:

macro_rules! create_array {
($($x:expr),*) => {
vec![$($x),*];
};
}

Usage of this macro would look like this:

fn main() {
let my_vec = create_array!(1, 2, 3, 4);
}

This generates a vector containing the values 1, 2, 3, 4.

Best Practices

When using declarative macros, consider the following best practices:

Keep macros simple and focused on a single task.
Avoid complex logic within macros; prefer functions for clarity and maintainability.
Document your macros well to ensure they are understandable by others.

Conclusion

Declarative macros are a powerful feature in Rust that enables code generation in a clean and structured way. By leveraging pattern matching and repetition, you can create reusable code snippets that enhance your productivity. Remember to follow best practices to ensure your macros remain clear and maintainable.

Declarative Macros in Rust