chat_server.ex

defmodule ChatServer do
  def accept(port) do
    {:ok, sock} = :gen_tcp.listen(port, [:binary, packet: :line, active: false, reuseaddr: true])
    loop_accept(sock, [])
  end
  
  defp loop_accept(sock, queue) do
    {:ok, client} = :gen_tcp.accept(sock)
    queue = queue ++ [client]
    case queue do
      [first | [second | []]] ->
        IO.puts "Found chat user!"
        spawn_link(fn -> serve(first, second) end)
        spawn_link(fn -> serve(second, first) end)
        queue = []
      _ ->
        IO.puts "waiting another connection..."
    end
    loop_accept(sock, queue)
  end

  defp serve(sock, sock2) do
    sock
    |> read_line()
    |> write_line(sock2)

    serve(sock, sock2)
  end

  defp read_line(sock) do
    {:ok, line} = :gen_tcp.recv(sock, 0)
    line
  end

  defp write_line(line, sock) do
    :gen_tcp.send(sock, line)
  end
end

ChatServer.accept(4040)