How to return extra information when starting a GenServer

Questions / Help
,
1

Looking through the documentation on supervisors it looks like the support process that return more than their pid when starting. The second value of the on_start type is {:ok, child, term()}. However I cannot see how to return that third term when using a GenServer.

I wan’t something like the following

defmodule MyServer do
  use GenServer

  def init() do
    {:ok, :state, :term}
  end
end

{:ok, pid(), :term} = DynamicSupervisor.start_child(DynamicSupervisor, MyServer)

I have made an implementation of MyServer that works by using :proc_lib but not with GenServer. It seams strange that a feature available in supervisors can not be used with Gen* modules.

For reference what I want to return as the last term is a via tuple so I can experiment with persistent/virtual actor implementations

2

I wanted to do this too, but it seems discouraging, since the erlang documentation for supervisor says:

The start function must create and link to the child process, and must return {ok,Child} or {ok,Child,Info}, where Child is the pid of the child process and Info any term that is ignored by the supervisor.

3

It might be ignored by the supervisor. However that doesn’t matter because it is still returned by the supervisor to the calling process

4

It’s supported by the supervisor as input, but the standard behaviours (like GenServer) never return such a value from start_link.

1 Like
5

Ok. so that is what I concluded. would there be resistance to making it so GenServer could return an info term? I certainly would find it very useful

6

That’s probably a question to the OTP team, but I expect they would need concrete use-cases.

You could always start the process manually with proc_lib and call gen_server:enter_loop at the end to make it into a GenServer.

1 Like
7

So it’s blocked at the erlang level and not something that the Elixir wrapper has decided not to implement?

8

Judging by the documentation OTP proper settled on {ok,Pid} while {ok,Pid,Info} was an accommodation for special processes which needed to use proc_lib:start(_link) which returns an arbitrary term upon success.

9

It would most probably require dropping the Erlang implementation and reimplementing at least the initialization part before gen_server:enter_loop in Elixir, so I would say it’s unlikely.

10

That makes sense. I guess I will have to stick with proc_lib.