Max value of an enum

[gruszczy]

I have the following code:

#[derive(Debug)]
enum SettlementSize {
    Village,
    Town,
    City,
}

impl Distribution<SettlementSize> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> SettlementSize {
        let res = rng.gen_range(0, 3);
        match res {
            0 => SettlementSize::Village,
            1 => SettlementSize::Town,
            2 => SettlementSize::City,
            _ => panic!("Unknown value!"),
        }
    }
}

If I add a new entry to the SettlementSize, e.g. MegaCity, I wouldn't like to forget to increase the value in rng.gen_range(0, 3); to 4. Is there some way to detect the maximum value in the enum (as cast to a number)? Do I need to maintain a watcher/guard at the end of the enum for that purpose?

[fosskers]

Haskell has an Enum trait for just this purpose, and I've been looking for it in Rust, the same as you. Are we both just missing something trivial?

[gruszczy]

@fosskers I hoped for something similar to Java or Python.

I found the strum crate, which allows me to do: UniqueFeature::iter().count(). I am not sure how to remove the call to iter and just do UniqueFeature::count(). This is a on an enum with struct variants.

[elsuizo]

maybe you could do:

#[derive(Debug)]
enum SettlementSize {
    Village = 0,
    Town,
    City,
    Other,
    Last,
}

fn get_range() -> usize {
    SettlementSize::Last as usize
}

fn main() {
    let l = get_range();
    println!("l: {:}", l);
}

[kornel]

There's a crate for that!

enum-unitary

Adds max_value() to enums.

[gruszczy]

I tried that, but it only worked for non-struct variants.

[gruszczy]

Thanks, let me try it out!