Hey,

I’m currently trying to DRY up some tests which have a lot of boilerplate code in the form of:

test "user is updated", %{user_id: user_id} do
  pid = spawn(fn ->
    EventStore.subscribe(UserUpdated)
    assert_receive event = %UserUpdated{aggregate_id: ^user_id}
    EventStore.acknowledge(event)
  end)

  ref = Process.monitor(pid)

  # the actual test

   assert_receive {:DOWN, ^ref, _, _, _}
end

Basically I’m checking if the event UserUpdated is fired in our custom event_store. I tried to implement a custom macro like this:

defmacro assert_event(event, do: block) do
  quote do
    pid =
      spawn(fn ->
        EventStore.subscribe(unquote(event).__struct__)
        assert_receive event = unquote(event)
        EventStore.acknowledge(event)
      end)

    ref = Process.monitor(pid)

    unquote(block)

    assert_receive {:DOWN, ^ref, _, _, _}
  end
end

Which can then be used like this:

test "user is updated", %{user_id: user_id} do
  assert_event %UserUpdated{aggregate_id: user_id} do
    # the actual test
  end
end

But this results in the following warning:

warning: variable “user_id” is unused (there is a variable with the same name in the context, use the pin operator (^) to match on it or prefix this variable with underscore if it is not meant to be used)

But pinning user_id results in the following error:

cannot use ^user_id outside of match clauses

So, my question is (after reading the complicated source code of assert_receive), is there a way to mark some code as a pattern and just forward it to the assert_receive macro inside my custom assert_event macro?