Optional fields and union types

Optional fields

? after a template field type permits its omission from initialization data.

data Reward {
    var amount: int = 10
    var note: string?
}

func main() {
    var reward = Reward()
    print(reward.amount)
}

The example supplies amount and omits note. Establish that a field is initialized before reading it. For external NBT paths, use NBTRef<T>.tryRead to handle presence.

Union types

(int | string) permits either listed type:

data DisplayValue {
    var value: (int | string) = 0
}

func main() {
    var numeric = DisplayValue()
    numeric.value = 10
    var named = DisplayValue()
    named.value = "Ready"
    print(numeric.value as int)
    print(named.value as string)
}

The compiler tracks the actual type of known fields. as selects the type view used at that point. The value must match that type. Check the layout when data comes from external sources.

Use typealias int as Score for another name for an existing type. Type aliases name types. Unions accept several value types.