Inheritance

The extends keyword can be used in a class declaration to inherit the properties and methods from another class.

MoonScript:

class BackPack extends Inventory
  size: 10
  add_item: (name) =>
    if #@items > size then error "backpack is full"
    super name

Lua:

local BackPack
do
  local _parent_0 = Inventory
  local _base_0 = {
    size = 10,
    add_item = function(self, name)
      if #self.items > size then
        error("backpack is full")
      end
      return _parent_0.add_item(self, name)
    end
  }
  _base_0.__index = _base_0
  setmetatable(_base_0, _parent_0.__base)
  local _class_0 = setmetatable({
    __init = function(self, ...)
      return _parent_0.__init(self, ...)
    end,
    __base = _base_0,
    __name = "BackPack",
    __parent = _parent_0
  }, {
    __index = function(cls, name)
      local val = rawget(_base_0, name)
      if val == nil then
        return _parent_0[name]
      else
        return val
      end
    end,
    __call = function(cls, ...)
      local _self_0 = setmetatable({}, _base_0)
      cls.__init(_self_0, ...)
      return _self_0
    end
  })
  _base_0.__class = _class_0
  if _parent_0.__inherited then
    _parent_0.__inherited(_parent_0, _class_0)
  end
  BackPack = _class_0
end

Here we extend our Inventory class, and limit the amount of items it can carry.

In this example, we don’t define a constructor on the subclass, so the parent class' constructor is called when we make a new instance. If we did define a constructor then we can use the super method to call the parent constructor.

Whenever a class inherits from another, it sends a message to the parent class by calling the method __inherited on the parent class if it exists. The function receives two arguments, the class that is being inherited and the child class.

MoonScript:

class Shelf
  @__inherited: (child) =>
    print @__name, "was inherited by", child.__name

-- will print: Shelf was inherited by Cupboard
class Cupboard extends Shelf