match as an Expression

Goal of This Episode

Learn to use match as an expression, so it returns a value.

Concept

Remember from Chapter 1 that if can be an expression?

fn main() {
    let condition = true;
    let x = if condition { 1 } else { 2 };
}

match can too! You can put a whole match on the right side of a let, with each arm returning a value:

enum Color {
    Red,
    Green,
    Blue,
}

fn main() {
    let c = Color::Red;
    let msg = match c {
        Color::Red => "red",
        Color::Green => "green",
        Color::Blue => "blue",
    };
}

Note the semicolon ; at the very end — the whole let msg = match ... { ... }; is one let statement.

The values returned by all the arms must have matching types. If the first arm returns &str, every other arm must return &str too.

Example Code

enum Season {
    Spring,
    Summer,
    Autumn,
    Winter,
}

fn main() {
    // match as an expression, returning &str
    let season = Season::Autumn;
    let name = match season {
        Season::Spring => "spring",
        Season::Summer => "summer",
        Season::Autumn => "autumn",
        Season::Winter => "winter",
    };
    println!("It's {} now", name);

    // Another example: match returning i32
    let weather = Season::Summer;
    let temp = match weather {
        Season::Spring => 22,
        Season::Summer => 35,
        Season::Autumn => 18,
        Season::Winter => 8,
    };
    println!("About {} degrees", temp);
}

Recap

• match can be an expression; the whole match returns a value.
• Usage: let x = match ... { ... }; (don’t forget the final semicolon).
• All arms must return values of matching types.
• Same idea as the if expression — lots of things in Rust are expressions.