Example of object composition in Lua

composition.lua

-- Generic function declarations
local Barker = {
  barks = function(self)
    print(self.name .. ' barks')
  end
}

local Killer = {
  kills = function(self, target)
    print(self.name .. ' kills '.. target .. ' with its '.. self.weapon)
  end
}

local Eater = {
  eats = function(self)
    print(self.name .. ' eats')
  end
}

local Meower = {
  meows = function(self)
    print(self.name .. ' meows')
  end
}

local Dog = {name = nil}
Dog.__index = Dog -- functions are looked up in Dog's metatable
Dog.eats = Eater.eats
Dog.barks = Barker.barks

-- could have an 'Animal' class that initializes according to species, but
-- that would defeat the purpose of this exercise
function Dog.new(name)
  local pupper = {name = name}
  setmetatable(pupper, Dog)
  return pupper
end

local Cat = {name = nil}
Cat.__index = Cat
Cat.meows = Meower.meows
Cat.eats = Eater.eats

function Cat.new(name)
  local kitty = {name = name}
  setmetatable(kitty, Cat)
  return kitty
end

local Robot = {name = nil, weapon = nil}
Robot.__index = Robot
Robot.kills = Killer.kills

function Robot.new(name, weapon)
  local robo = {['name'] = name, ['weapon'] = weapon}
  setmetatable(robo, Robot)
  return robo
end


local blue = Dog.new('Blue')
blue:eats()
blue:barks()

local roxi = Cat.new('Roxi')
roxi:eats()
roxi:meows()

local ralph = Robot.new('Ralph', 'robo arms')
ralph:kills('a bird')