Expanded inventory example using Semigroups.

gistfile1.scala

scala> import scalaz._
import scalaz._

scala> import Scalaz._
import Scalaz._

scala> :paste
// Entering paste mode (ctrl-D to finish)

sealed abstract class Inventory
case object UserInventory extends Inventory
case object BankInventory extends Inventory

// Exiting paste mode, now interpreting.

defined class Inventory
defined module UserInventory
defined module BankInventory

scala> val currentInventories: Map[Inventory, Map[String, Int]] = Map(BankInventory -> Map("monkeywrench" -> 3, "toothbrush" -> 1), UserInventory -> Map("map" -> 1))
currentInventories: Map[Inventory,Map[String,Int]] = Map(BankInventory -> Map(monkeywrench -> 3, toothbrush -> 1), UserInventory -> Map(map -> 1))

scala> val inventoryChange: Map[Inventory, Map[String, Int]] = Map(BankInventory -> Map("monkeywrench" -> -1), UserInventory -> Map("monkeywrench" -> 1))
inventoryChange: Map[Inventory,Map[String,Int]] = Map(BankInventory -> Map(monkeywrench -> -1), UserInventory -> Map(monkeywrench -> 1))

scala> val updatedInventories = currentInventories |+| inventoryChange
updatedInventories: Map[Inventory,Map[String,Int]] = Map(BankInventory -> Map(monkeywrench -> 2, toothbrush -> 1), UserInventory -> Map(monkeywrench -> 1, map -> 1))