Code based on Matthias Käppler's post about Monoids and Sum Types, check https://medium.com/@mttkay/refactoring-with-monoids-and-sum-types-part-1-bddc9ea61f5f for the complete explanation of this code. Thanks to @mttkay for the excellent blogpost!

game.kt

import java.util.Arrays

val inventory = Array<Elem>(size = 3) { Empty }

fun main(args: Array<String>) {
  val sword1 = Sword("Sword 1")
  val sword2 = Sword("Sword 2")
  val pickaxe = Pickaxe("Pickaxe 1")
  val item1 = Item(sword1)
  val item2 = Item(sword2)
  val item3 = Item(pickaxe)
  val stack = item1 + item2 // creates an ItemStack
  val stillItem1 = Empty + item1 // this compiles and works!

  inventory[0] = item1
  inventory[1] = item2
  inventory[2] = item3
  move(inventory, 0, 1)
  move(inventory, 2, 1)
  println(Arrays.deepToString(inventory))
}

fun move(inventory: Array<Elem>, from: Int, to: Int) {
  inventory[to] = inventory[to] + inventory[from]
  inventory[from] = Empty
}

sealed class GameObject
data class Pickaxe(val name: String) : GameObject()
data class Sword(val name: String) : GameObject()

sealed class Elem {
  abstract operator fun plus(el: Elem): Elem
  inline fun yieldFirst(f: (Item) -> Unit) =
      when (this) {
        is Empty -> Unit
        is Item -> f(this)
        is ItemStack -> f(this.items.first())
      }
}

object Empty : Elem() {
  override fun plus(e: Elem): Elem = e
}

data class Item(val obj: GameObject) : Elem() {
  override fun plus(el: Elem): Elem =
      when (el) {
        is Empty -> this
        is Item -> ItemStack(setOf(this, el))
        is ItemStack -> ItemStack(el.items + this)
      }
}

data class ItemStack(val items: Set<Item>) : Elem() {
  init {
    require(items.size > 1) { "Stacks must contain > 1 items" }
  }

  override fun plus(el: Elem): Elem =
      when (el) {
        is Empty -> this
        is Item -> ItemStack(this.items + el)
        is ItemStack -> ItemStack(this.items + el.items)
      }
}