Data Structure: Stack

local Stack = {
  new = function(...)
    local data = {...}

    local obj = setmetatable({}, {
      __newindex = function(self, key, value)
        if type(value) == "function" then
          local func = setmetatable({}, {
            __call = function(_, ...)
              if rawequal(self, ({...})[1]) then
                return value(...)
              end
              return value(self, ...)
            end;
          })
          rawset(self, key, func)
        else
          rawset(self, key, value)
        end
      end;
    })

    function obj.push(self, value)
      data[#data+1] = value
      return self
    end

    function obj.pop(self)
      assert(not self.isEmpty(), "stack underflow")
      return table.remove(data)
    end

    function obj.size(self)
      return #data
    end

    function obj.isEmpty(self)
      return self.size() == 0
    end

    function obj.top(self)
      assert(not self.isEmpty(), "stack underflow")
      return data[#data]
    end

    return obj
  end
}