Ok, now I can write about it.

To be able to listen to the data for given process, then you need to instantiate your own handler. Simplest approach to that would be:

defmodule CaptureStructuredLogs do
  def capture_log(callback) do
    this = self()
    handler_id = :"handler_#{inspect(this)}"

    :ok = :logger.add_handler(handler_id, __MODULE__, %{config: %{pid: this}})
    try do
      callback.()
    after
      :logger.remove_handler(handler_id)
    end

    fetch_all_messages([])
  end

  defp fetch_all_messages(messages) do
    receive do
      {{__MODULE__, :log}, log_event} ->
        fetch_all_messages([log_event | messages])
    after
      0 -> Enum.reverse(messages)
    end
  end

  def adding_handler(%{config: %{pid: pid}} = config) when is_pid(pid), do: {:ok, config}
  def adding_handler(_), do: {:error, :required_pid}

  def log(log_event, %{config: %{pid: pid}}) do
    send(pid, {{__MODULE__, :log}, log_event)
  end
end

Now you can use it like “old” function:

test "capture log" do
  import CaptureStructuredLogs
  require Logger
  assert [%{msg: {:report, %{key: "value"}}}] = capture_log(fn -> Logger.error(%{key: "value"}) end)
end

I would just slap logging in place. While this mean that there is a little of “impurity” in “core” that doesn’t really matter, as that “impurity” do not affect the overall system. So unless there are other considerations that you do not listed in your post, then I would just do not care about it.

However if you want to have it just in case when you need debugging, then you probably would prefer setup dynamic tracing on that function.

If you just want to test logging, can use CaptureLog:

  test "capture error log" do
    import ExUnit.CaptureLog
    require Logger
    assert capture_log(fn -> Logger.error("?") end) =~ "?"
  end

If I understand correctly, you want to log the file/line where the error is produced? I typically don’t obsess about that, but instead aim to make different branches return different errors, which means that I can deduce the source of the error from its content.

However, if I really wanted to preserve the stack trace I’d probably log in place, as @hauleth said.

But, if you want to stick to pure functional for fun & sport, you could use __ENV__ to include file & line in the returned error, and then include that info in the logged message.

Because of the backward compatibility. I can check out if that could be “fixed” by adding new module to ExUnit though. I will try to provide PR tomorrow (CET) unless you will do that first. You can ping me in PR if you want.