Plain Lua Inventory Class Example from MoonScript reference (http://moonscript.org/reference/#object_oriented_programming)

InventoryClassExample.lua

local Inventory = {}
Inventory.__index = Inventory

function Inventory:new()
  local new_inventory = {}
  new_inventory.items = {}
  setmetatable(new_inventory, Inventory)
  return new_inventory
end

function Inventory:add_item(name)
  if self.items[name] then
    self.items[name] = self.items[name] + 1
  else
    self.items[name] = 1
  end
end