Validating whether a responder is configured

Next, we want to validate whether an HTTPServer module is properly configured to use a responder since it is a requirement to use the HTTPServer module. We can do this by checking for the presence of a responder configuration at the time of starting the server. So, in the start/1 function, let’s validate the presence of a responder before creating a listening socket:

lib/goldcrest_http_server.ex

defmodule Goldcrest.HTTPServer do
   # ..
   def start(port) do
    ensure_configured!()
    case :gen_tcp.listen(port, @server_options) do
      {:ok, sock} ->
        Logger.info("Started a webserver on port #{port}")
        listen(sock)
      {:error, error} ->
        ...