Dialyzer says function will never be called, even though it is

Question

I am using the elixir_talk library. After connecting I want to call a private function once connected to beanstalkd. I just added typespecs and ran Dialyzer (via dialyxir). I get the errors:

my_module.ex:3: The specification for 'Elixir.MyModule':f/0 states that the function might also return 'ok' | {'error',_} but the inferred return is none()
my_module.ex:4: Function f/0 has no local return
my_module.ex:14: Function g/1 will never be called

The minimal example I could find that produces this is

defmodule MyModule do
  @spec f() :: :ok | {:error, term}
  def f() do
    case ElixirTalk.connect('127.0.0.1', 11300) do
      {:ok, conn} ->
        g(conn)
      {:error, err} ->
        {:error, err}
    end
  end

  @spec g(pid) :: :ok
  defp g(pid) do
    :ok
  end
end

If I replace the call to ElixirTalk.connect with a call to spawn instead, Dialyzer no longer reports any problems.

defmodule MyModule do
  @spec f() :: :ok
  def f() do
    x = spawn fn -> :done end
    g(x)
  end

  @spec g(pid) :: :ok
  defp g(pid) do
    :ok
  end
end

Does anyone know why Dialyzer is getting confused here?

Bergess

Do you get the same result if you pass a timeout value as the third argument to ElixirTalk.connect? – Spongin

@Spongin that resolved it! Why did that cause this issue? – Bergess

Looking at the source, the type spec says that the third argument is always an integer, even though the default value is the atom infinity. Thus, a call to ElixirTalk.connect with infinite timeout would be against the type spec. In Erlang you'd fix this by specifying the type as timeout() instead, which allows both integers and infinity; not sure how that translates to Elixir. – Spongin

Nice! This should also be manifesting as a warning for ElixirTalk.connect/2 itself, but since it is probably in the PLT it is not shown? – Rebuff

Hi @legoscia--maybe you should extract your comment as an answer so it's obviously visible to everyone. :) – Chlamys

What are the return values of ElixirTalk.connect? – Tour

Answer 1

Looking at the source, the type spec says that the third argument is always an integer, even though the default value is the atom infinity. Thus, a call to ElixirTalk.connect with infinite timeout would be against the type spec. In Erlang you'd fix this by specifying the type as timeout() instead, which allows both integers and infinity; not sure how that translates to Elixir. – legoscia May 16 '16 at 15:56

Schoolfellow

type timeout :: integer | :infinity – Skilful