Luvit module example (using local _M table) (my personal choice)

geometry.lua

local OO = require"oo"

local _M = {}

do _M.Shape = OO.class({
        
    })
    function _M.Shape:__init()
        error("attempt to instantiate abstract shape")
    end
end

do _M.Rectangle = OO.class(_M.Shape, {
        x = 0, y = 0,
        w = 0, h = 0,
    })
    function _M.Rectangle:__init(x, y, w, h)
        return setmetatable({
            x = tonumber(x), y = tonumber(y),
            w = tonumber(w), h = tonumber(h),
        }, self)
    end
    function _M.Rectangle:getArea()
        return math.sqrt(self.w^2+self.h^2)
    end
end

do _M.Circle = OO.class(_M.Shape, {
      --userdata = newproxy(),
        radius = 0,
    })
    function _M.Circle:__init(obj)
        assert(type(obj.userdata) == "userdata", "attempt to construct Circle with invalid userdata")
        -- the only reason setmetatable is preformed manually is that in the game engine I use it in I
        -- use classes on userdata (created with newproxy). For these, debug.setmetatable must be called.
        -- Hence, I never forced setmetatable to be used on the object internally.
        return setmetatable(obj, self)
    end
    function _M.Circle:getArea(obj)
        return math.pi*self.radius^2
    end
end

function _M.spawnCircle(radius) -- let's assume Circles are like processes
    return _M.Circle{
        userdata = newproxy(), -- UV.get_circle
        radius = radius
    }
end

return _M

main.lua

require"oo"
require"geometry"

local rect = geometry.Rectangle(0, 0, 3, 4)
print(rect:getArea(), OO.instanceof(rect, geometry.Shape)) -- 5    true

local circ = geometry.spawnCircle(2)
print(circ:getArea()) -- 12.566370614359

oo.lua

local setmetatable = setmetatable
local table, debug = table, debug

local _M = {}

do --class
    local function class_new(self, ...)
        return setmetatable(self.__init and self:__init(...) or {}, self)
    end
    _M.class = function(...)
        local supers = {...}
        local class = table.remove(supers, #supers)
        
        class.__index = (
                class.__index ~= nil and class.__index
            or  class.__index_def and class.__index_def(class)
            or  class
        )
        class[class] = true
        
        local supers_len = #supers -- hopefully this will help LuaJIT unroll the loop statically
        return setmetatable(
            class,
            {
                __supers = supers,
                __class = class,
                __call = class.__init or class_new,
                __index = (
                        supers_len == 0 and nil
                    or  supers_len == 1 and supers[1]
                    or  function(self, key)
                            for super_i = 1, supers_len do
                                local value = supers[super_i][key]
                                if value then return value end
                            end
                        end
                ),
            }
        )
    end
end
do _M.classof
    = debug.getmetatable -- hopefully this aliasing doesn't mess up the fast function optimisation
end
do _M.instanceof
    = function(instance, class)
        return debug.getmetatable(instance)[class]
    end
end
do _M.supersof
    = function(class)
        return debug.getmetatable(class).__supers
    end
end
do _M.supersofinstance -- == supersof(classof(instance))
    = function(instance)
        return debug.getmetatable(debug.getmetatable(instance)).__supers
    end
end

return _M