Is there a reason we don't have a Supervisor.child_spec/1?

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2

The only supported versions in the list of children are module(), {module, arg} or :supervisor.child_spec(). There’s no tuple-3 option at all. I blogged about that system in more detail yesterday: Child Specs in Elixir

So you need to send everything in one arg to be sent to child_spec/1. You could do something like this:

defmodule MySupervisor do
  def child_spec(init_arg) do
    {children, opts} = extract_from_init_arg(init_arg)
    %{
      id: __MODULE__
      start: {Supervisor, :start_link, [children, opts]}
      type: :supervisor
    }
  end
end

Which would allow for {MySupervisor, some_children_and_opts_format}. If that’s actually cleaner than a private function building the child_spec manually I’m not so sure about, given that the list of children is unlikely just some small argument to place in the tuple format.