match.md

% Match

Often, a simple if/else isn’t enough, because you have more than two possible options. Also, conditions can get quite complex. Rust has a keyword, match, that allows you to replace complicated if/else groupings with something more powerful. Check it out:

let x = 5;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    4 => println!("four"),
    5 => println!("five"),
    _ => println!("something else"),
}

match takes an expression and then branches based on its value. Each ‘arm’ of the branch is of the form val => expression. When the value matches, that arm’s expression will be evaluated. It’s called match because of the term ‘pattern matching’, which match is an implementation of. There’s an entire section on patterns that covers all the patterns that are possible here.

So what’s the big advantage? Well, there are a few. First of all, match enforces ‘exhaustiveness checking’. Do you see that last arm, the one with the underscore (_)? If we remove that arm, Rust will give us an error:

error: non-exhaustive patterns: `_` not covered

In other words, Rust is trying to tell us we forgot a value. Because x is an integer, Rust knows that it can have a number of different values – for example, 6. Without the _, however, there is no arm that could match, and so Rust refuses to compile the code. _ acts like a ‘catch-all arm’. If none of the other arms match, the arm with _ will, and since we have this catch-all arm, we now have an arm for every possible value of x, and so our program will compile successfully.

match is also an expression, which means we can use it on the right-hand side of a let binding or directly where an expression is used:

let x = 5;

let number = match x {
    1 => "one",
    2 => "two",
    3 => "three",
    4 => "four",
    5 => "five",
    _ => "something else",
};

Sometimes it’s a nice way of converting something from one type to another.

Matching on enums

Another important use of the match keyword is to process the possible variants of an enum:

enum Message {
    Quit,
    ChangeColor(i32, i32, i32),
    Move { x: i32, y: i32 },
    Write(String),
}

fn quit() { /* ... */ }
fn change_color(r: i32, g: i32, b: i32) { /* ... */ }
fn move_cursor(x: i32, y: i32) { /* ... */ }

fn process_message(msg: Message) {
    match msg {
        Message::Quit => quit(),
        Message::ChangeColor(r, g, b) => change_color(r, g, b),
        Message::Move { x: x, y: y } => move_cursor(x, y),
        Message::Write(s) => println!("{}", s),
    };
}

Again, the Rust compiler checks exhaustiveness, so it demands that you have a match arm for every variant of the enum. If you leave one off, it will give you a compile-time error unless you use _.

Unlike the previous uses of match, you can’t use the normal if statement to do this. You can use the if let statement, which can be seen as an abbreviated form of match.