Functionaly pure message passing

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Being idealistic, I think this could be an improvement, but on the other hand, being realistic I know there’s just no chance to change GenServer (and probably for a good reason).

Fortunately, what you propose is already implemented in gen_statem which accepts a list of “actions” in return from state functions:

defmodule Statem do
  @behaviour :gen_statem

  def callback_mode(), do: :state_functions
  
  def init(_args) do
    {:ok, :none, :no_data}
  end

  def none({:call, from1}, {:pair, pid1}, _data) do
    {:next_state, :waiting, {pid1, from1}}
  end

  def waiting({:call, from2}, {:pair, pid2}, {pid1, from1}) do
    {:next_state, :none, :no_data, [{:reply, from1, pid2}, {:reply, from2, pid1}]}
  end
end
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