ex21.1.lua

local Stack = { data = {} }
Stack.__index = Stack

function Stack:new(o)
  o = o or {}
  setmetatable(o, Stack)
  return o
end

function Stack:is_empty()
  return #self.data == 0
end

function Stack:push(e)
  table.insert(self.data, e)
end

function Stack:pop(e)
  assert(not self:is_empty())
  table.remove(self.data, e)
end

function Stack:top()
  assert(not self:is_empty())
  return self.data[#self.data]
end

local my_stack = Stack:new()

assert(my_stack:is_empty())
my_stack:push(1)
assert(not my_stack:is_empty())
assert(1 == my_stack:top())
my_stack:pop()
assert(my_stack:is_empty())