Lazy Delegate implementation for a mutable, but lazily initialized variable

MutableLazy.kt

import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty

/**
 * Property delegate to allow lazy initialization of a mutable variable.
 */
class MutableLazy<T>(val init: () -> T) : ReadWriteProperty<Any?, T> {

    private var value: Optional<T> = Optional.None()

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        if(value is Optional.None) {
            value = Optional.Some(init())
        }
        return value.get()
    }

    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        this.value = Optional.Some(value)
    }
}

sealed class Optional<out T> {

    abstract fun get(): T

    class Some<out T>(val value: T): Optional<T>() {
        override fun get() = value
    }
    class None<out T>(): Optional<T>() {
        override fun get(): T {
            throw NoSuchElementException("Can't get object from Optional.None")
        }
    }
}