test_sup.ex

defmodule Foo 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

    # Define workers and child supervisors to be supervised
    children = [
      # Starts a worker by calling: Foo.Worker.start_link(arg1, arg2, arg3)
      worker(Foo.Service, []),
    ]

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


defmodule Foo.Service do
  use GenServer
 
  def start_link do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end
 
  def init(:ok) do
    {:ok, :ok}
  end
 
  def check do
    GenServer.call(__MODULE__, {:check})
  end

  def test_args({:abc}) do
    GenServer.call(__MODULE__, {:abc})
  end
 
  def handle_call({:abc}, _from, :ok) do
    {:reply, :ok, :ok}
  end
 
  def handle_call({:check}, _from, :ok) do
    {:reply, :ok, :ok}
  end
end

Interactive Elixir (1.3.4) - press Ctrl+C to exit (type h() ENTER for help)
iex> pid = Process.whereis(Foo.Service)
#PID<0.146.0>
iex> Foo.Service.check
:ok
iex> GenServer.call(pid, {:fds})
** (exit) exited in: GenServer.call(#PID<0.146.0>, {:fds}, 5000)
    ** (EXIT) an exception was raised:
        ** (FunctionClauseError) no function clause matching in Foo.Service.handle_call/3
            (foo) lib/foo_service.ex:20: Foo.Service.handle_call({:fds}, {#PID<0.147.0>, #Reference<0.0.4.491>}, :ok)
            (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
            (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
            (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3

08:06:01.326 [error] GenServer Foo.Service terminating
** (FunctionClauseError) no function clause matching in Foo.Service.handle_call/3
    (foo) lib/foo_service.ex:20: Foo.Service.handle_call({:fds}, {#PID<0.147.0>, #Reference<0.0.4.491>}, :ok)
    (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
    (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: {:fds}
State: :ok
    (elixir) lib/gen_server.ex:604: GenServer.call/3
iex> pid = Process.whereis(Foo.Service)
#PID<0.152.0>