my_app.exs

defmodule MyApp do
  use Application

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    children = [
      worker(Todo, [[]])
    ]

    # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

** (Mix) Could not start application my_app: MyApp.start(:normal, []) returned an error: shutdown: failed to start child: Todo
** (EXIT) an exception was raised:
** (UndefinedFunctionError) undefined function Todo.start_link/1
(my_app) Todo.start_link([])
(stdlib) supervisor.erl:343: :supervisor.do_start_child/2
(stdlib) supervisor.erl:326: :supervisor.start_children/3
(stdlib) supervisor.erl:292: :supervisor.init_children/2
(stdlib) gen_server.erl:328: :gen_server.init_it/6
(stdlib) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

@jmsevold can you post your Todo GenServer also?

@jmsevold Could it be that you start your GenServer with "start" and not "start_link"?