Guards

A match guard can be added to filter the arm.

enum Temperature {
    Celsius(i32),
    Farenheit(i32),
}

fn main() {
    let temperature = Temperature::Celsius(35);
    // ^ TODO try different values for `temperature`

    match temperature {
        Temperature::Celsius(t) if t > 30 => println!("{}C is above 30 Celsius", t),
        // The `if condition` part ^ is a guard
        Temperature::Celsius(t) => println!("{}C is below 30 Celsius", t),

        Temperature::Farenheit(t) if t > 86 => println!("{}F is above 86 Farenheit", t),
        Temperature::Farenheit(t) => println!("{}F is below 86 Farenheit", t),
    }
}

Note that the compiler won't take guard conditions into account when checking if all patterns are covered by the match expression.

fn main() {
    let number: u8 = 4;

    match number {
        i if i == 0 => println!("Zero"),
        i if i > 0 => println!("Greater than zero"),
        // _ => unreachable!("Should never happen."),
        // TODO ^ uncomment to fix compilation
    }
}

See also:

Tuples Enums

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Last update: 2022-07-27