operator_overloading.kt

data class Topping(
    val id: String,
    val name: String,
)

data class Article(
    val id: String,
    val name: String,
    val toppings: MutableList<Topping> = mutableListOf(),
) {
    operator fun plusAssign(topping: Topping) {
        toppings += topping
    }
}

data class Cart(
    val id: String,
    val articles: MutableList<Article> = mutableListOf(),
) {
    operator fun plusAssign(article: Article) {
        articles += article
    }
}

fun main() {
    val currentState = CardState.Collapsed
    val newState = !currentState

    val topping = Topping("id", "Topping")
    val article = Article("id", "Article")
    val cart = Cart("id")

    article += topping
    cart += article
}

enum class CardState { Collapsed, Expanded, }

operator fun CardState.not() =
    if (this == CardState.Collapsed) CardState.Expanded
    else CardState.Collapsed