Coroutine-local storage for Lua

tls.lua

local true_storage = {}

tls = setmetatable({}, {
  __mode  = 'k',
  __index = function(self, k)
    local current = coroutine.running() or 'main'
    local storage = true_storage[current]

    if not storage then
      storage               = {}
      true_storage[current] = storage
    end 

    return storage[k]
  end,
  __newindex = function(self, k, v)
    local current = coroutine.running() or 'main'
    local storage = true_storage[current]

    if not storage then
      storage               = {}
      true_storage[current] = storage
    end 

    storage[k] = v 
  end,
})