Elixir: defmacro and benchmark of pipline

Question

I have macro which should help me to meaaure latency:

  defmacro instrument_duration(endpoint_name, block) do
    quote do
      {time, value} = :timer.tc(fn -> unquote(block) end)
      Histogram.observe(
        [name: :endpoint_duration_seconds, labels: [endpoint_name]],
        time
      )
      value
    end
  end

And following code that is using it:

    response =
      Instrumenter.instrument_duration(id,
        do_handle(params, context)
        |> prepare_response())

But I get Reason:undef\n', options: [] error. What I did wrong here? Is it correct way at all?

Answer 1

I am to put this as an answer here.

TL;DR: labels: [endpoint_name] → labels: [unquote(endpoint_name)]

---

Your code does not unquote endpoint_name inside the quoted block, resulting in the unsuccessful compiler’s attempt to resolve endpoint_name within the context of the quoted block.

Luckily enough, elixir provides warnings during the compilation stage, and there must have been somewhat like

warning: variable "endpoint_name" is unused (if the variable
         is not meant to be used, prefix it with an underscore)

Compiler warnings are not to be ignored if one wants to have a robust code, they are provided on purpose and are to be taken into consideration.