Enums and match

// Enums and match: every case covered, checked by the compiler.
//
// A variant may carry one value. Leaving a case out is an error that names the case, rather than a
// run-time surprise on the one afternoon the light is amber.

enum Light {
    Red
    Amber
    Green
}

enum Shape {
    Dot
    Circle(f32)
}

fn go(light: Light) -> bool {
    return match light {
        Red => false
        Amber => false
        Green => true
    }
}

// `_` covers the rest, which is a decision rather than an oversight: it says the remaining cases
// are handled the same way, and it keeps saying that when a new variant is added.
fn describe(light: Light) -> String {
    return match light {
        Green => "go"
        _ => "wait"
    }
}

fn area(shape: Shape) -> f32 {
    return match shape {
        Dot => 0
        Circle(radius) => radius * radius
    }
}

// Deliberately refused.
//
// A `match` must cover every variant, and the error names the ones it missed rather than saying
// "not exhaustive", which is the answer you were about to go and look up. `Amber` is missing here,
// and the alternative to this error is a run-time surprise on the one afternoon the light is amber.

enum Light {
    Red
    Amber
    Green
}

fn go(light: Light) -> bool {
    return match light {
        Red => false
        Green => true
    }
}