similar to `Delegates.vetoable` but with exception...

# language-proposals

[cy]

similar to Delegates.vetoable but with exception

something like that

class AssignOnceDelegate<T>() : ReadWriteProperty<Any?, T> {
    private var value: T? = null
    private var initialized = false

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        if (initialized) {
            return value as T
        }

        throw IllegalStateException("${property.name} not yet initialized")
    }

    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        if (initialized) {
            throw IllegalStateException("${property.name} is already initialized")
        }

        this.value = value
        initialized = true
    }
}

class Example {
    var p by AssignOnceDelegate<String>()
}