While everything suggested above is absolutely correct, I’m to answer your original question.

One does not technically need module attributes, nor hooks here. The issue is scopes and we have unquote fragments for that.

rule_ids_ is defines in the outer scope for rule_ids/0 call, therefore one needs to unquote it there. THe code below would work (I also fixed the issue with condition/3 head mistakenly falling under a match during the parse stage.)

defmodule Rules do

  # Collect the rule ids at compile time
  {_ast, rule_ids_} = File.read!(__ENV__.file)
    |> Code.string_to_quoted()
    |> Macro.traverse([],
      fn
        {:def, _, [{:condition, _, [rule_id, {:v, _, nil}, {:site, _, nil}]} | _]} = x, acc 
            when not is_tuple(rule_id) ->
          {x, [rule_id | acc]}

        other, acc ->
         {other, acc}
      end,
      fn other, acc -> {other, acc} end
    )
  # print it out and see how it goes, this line is to be removed
  |> tap(& &1 |> Macro.to_string() |> IO.puts())

  # unquoting from the outer scope
  # HERE              ⇓⇓⇓⇓⇓⇓⇓                 
  def rule_ids(), do: unquote(rule_ids_)

  def condition(rule_id, v, site)

  def condition(1018, v, site) do
    v[site][7018] < 50 && v[site][7019] < 50 && v[site][7020] < 50
  end

  def condition(1017, v, site) do
    v[site][7027] > 42
  end
  # ...
end

Yeah, that’s what I was hinting at. You can remove the hardcoded Rules in the Hooks module by replacing it with env.module. Often all the boilerplate you have in Rules is also contained in a __using__ macro of Hooks, so a use Hooks does everything you need within Rules.

These is what module attributes are for: Module — Elixir v1.18.3

I’ve never checked it, but this should work:

  Module.register_attribute(__MODULE__, :rule_ids)

 # Collect the rule ids at compile time
  {_ast, rule_ids_} = File.read!(__ENV__.file)
    |> Code.string_to_quoted()
    |> Macro.traverse([],
      fn
        {:def, _, [{:condition, _, [rule_id, {:v, _, nil}, {:site, _, nil}]} | _]} = x, acc -> {x, [rule_id | acc]}
        other, acc -> {other, acc}
      end,
      fn other, acc -> {other, acc} end
    )
  
  Module.put_attribute(__MODULE__, :rule_ids, rule_ids_)

  # How could I use rule_ids_ here?
  def rule_ids(), do: @rule_ids

  def condition(rule_id, v, site)

  def condition(1018, v, site) do
    v[site][7018] < 50 && v[site][7019] < 50 && v[site][7020] < 50
  end

  def condition(1017, v, site) do
    v[site][7027] > 42
  end

  # ...

end

It worked with:

Module.register_attribute(__MODULE__, :rule_ids, accumulate: false, persist: false)