Dialyzer (dialyxir) unmatched_return warning

So, I’m writing my home project with Elixir, everything’s going fine, decided to plug dialyxir to scan for some potential issues and it gives me this one error:

lib/http_server.ex:60:unmatched_return
The expression produces a value of type:

:ok | pid()

but this value is unmatched.

Code in question:

  defp accept_loop(listen_socket, %State{} = state) do
    state = %{state | total_calls: state.total_calls + 1}

    case :gen_tcp.accept(listen_socket) do # LINE 60, issue is here
      {:ok, client_socket} ->
        spawn(fn -> serve(client_socket, state) end)

      {:error, data} ->
        Logger.notice("Error accepting client socket - #{inspect(data)}")
    end

    accept_loop(listen_socket, state)
  end

Error about unmatched value from gen_tcp.accept. Everything’s working fine and according to documentation I’m matching values just fine - Erlang -- gen_tcp :

accept(ListenSocket) -> {ok, Socket} | {error, Reason}

I am matching both {:ok, socket} and {:error, reason} so I am at a loss now. What is going on?

Tried adding guard to :ok match - {:ok, client_socket} when is_port(client_socket) -> but mix dialyzer still returns same error.

Tried capturing first variable and checking whether it’s :ok or a pid - {test, client_socket} when test == :ok or is_pid(test) - still same error.

Sorry, can’t give you a solution. But maybe this might help. If not, you still learned some valuable lessons.

I once stumbled upon a blog post
Help Dialyzer Help You!. …or Why you should use specs if you use… | by Brujo Benavides | Erlang Battleground | Medium.

The blog post mentions the next video which helps me ‘think like dialyzer’ and so find solutions.

But before anything, remove your dialyzer cache. Lesson learned the hard way. See:

1 Like

Ah, I see.

It’s the case statement itself that returns either pid from first match - spawn is called - or :ok from second match - Logger.notice call - and THAT is unmatched. So, with this insight, this is an easy “”“fix”"", as I don’t give a damn about what case returns.

  defp accept_loop(listen_socket, %State{} = state) do
    state = %{state | total_calls: state.total_calls + 1}

    _ = case :gen_tcp.accept(listen_socket) do
      {:ok, client_socket} ->
        spawn(fn -> serve(client_socket, state) end)

      {:error, data} ->
        Logger.notice("Error accepting client socket - #{inspect(data)}")
    end

    accept_loop(listen_socket, state)
  end