[Rust] Guards & Binding

// A function `age` which returns a `u32`.
fn age() -> u32 {
    15
}

fn main() {
    println!("Tell me what type of person you are");

    match age() {
        0             => println!("I haven't celebrated my first birthday yet"),
        // Could `match` 1 ..= 12 directly but then what age
        // would the child be? Instead, bind to `n` for the
        // sequence of 1 ..= 12. Now the age can be reported.
        n @ 1  ..= 12 => println!("I'm a child of age {:?}", n),
        n @ 13 ..= 19 => println!("I'm a teen of age {:?}", n),
        // Nothing bound. Return the result.
        n             => println!("I'm an old person of age {:?}", n),
    }
}

@는 if로 바꿔 쓸 수 있는데

n @ 1  ..= 12 => println!("I'm a child of age {:?}", n)

위 문장을 아래처럼 바꿔 쓸 수 있다.

n if 1 < n && n <= 12 => println!("I'm a child of age {:?}", n),

fn some_number() -> Option<u32> {
    Some(42)
}

fn main() {
    match some_number() {
        // Got `Some` variant, match if its value, bound to `n`,
        // is equal to 42.
        Some(n @ 42) => println!("The Answer: {}!", n),
        // Match any other number.
        Some(n)      => println!("Not interesting... {}", n),
        // Match anything else (`None` variant).
        _            => (),
    }
}

Option에서도 guard를 쓸 수 있는데 위 예제처럼 Some(n @ 42) 형태로 사용하면 된다.