How can I create enums with constant values in Rust?

I can do this:

enum MyEnum {
    A(i32),
    B(i32),
}

but not this:

enum MyEnum {
    A(123), // 123 is a constant
    B(456), // 456 is a constant
}

I can create the structures for A and B with a single field and then implement that field, but I think there might be an easier way. Is there any?

The best way to answer this is working out why you want constants in an enum: are you associating a value with each variant, or do you want each variant to be that value (like an enum in C or C++)?

For the first case, it probably makes more sense to just leave the enum variants with no data, and make a function:

enum MyEnum {
    A,
    B,
}

impl MyEnum {
    fn value(&self) -> i32 {
        match *self {
            MyEnum::A => 123,
            MyEnum::B => 456,
        }
    }
}
// call like some_myenum_value.value()

This approach can be applied many times, to associate many separate pieces of information with each variant, e.g. maybe you want a .name() -> &'static str method too. In the future, these functions can even be marked as const functions.

For the second case, you can assign explicit integer tag values, just like C/C++:

enum MyEnum {
    A = 123,
    B = 456,
}

This can be matched on in all the same ways, but can also be cast to an integer MyEnum::A as i32. (Note that computations like MyEnum::A | MyEnum::B are not automatically legal in Rust: enums have specific values, they're not bit-flags.)

Stack Overflow Q#36928569, score: 185