local imported = {}

local function package_stub(name)
  local stub = {}
  local stub_meta = {
    __index = function(_, index)
      error(string.format("member `%s' is accessed before package `%s' is fully imported", index, name))
    end,
    __newindex = function(_, index, _)
      error(string.format("member `%s' is assigned a value before package `%s' is fully imported", index, name))
    end,
  }
  setmetatable(stub, stub_meta)
  return stub
end

local function lock_stub(stub)
  local meta = getmetatable(stub)
  meta.__metatable = false
  return meta
end

local function unlock_stub(stub, meta)
  meta.__metatable = nil
  setmetatable(stub, nil)
end

local function locate(name)
  local path = LUA_PATH
  if type(path) ~= "string" then
    path = os.getenv "LUA_PATH" or "?.lua"
  end
  for path in string.gfind(path, "[^;]+") do
    path = string.gsub(path, "%?", name)
    local chunk = loadfile(path)
    if chunk then return chunk, path end
  end
  return nil, path
end

local function _import(name)
  local package = imported[name]
  if package then return package end
  local chunk, path = locate(name)
  if not chunk then
    error(string.format("could not locate package `%s' in `%s'", name, path))
  end
  package = package_stub(name)
  imported[name] = package
  setglobals(chunk, getglobals(2))
  local key = lock_stub(package)
  chunk = chunk()
  unlock_stub(package, key)
  if type(chunk) == "function" then
    chunk(package, name, path)
  end
  return package
end

function import(name)
  return _import(name)
end

setglobals(import, {__globals = false})