This is an old thread, but I think I can close the loop by mentioning that Elixir 1.7 removed the need to use anonymous functions with Logger.

See Logger compile-time purging in Elixir 1.7 release notes.

Also, there is a credo check specifically for this, which is disabled for Elixir 1.7+.

Since it’s a macro that returns a quoted expression, the argument will be evaluated later.

This is not correct. The only thing that being a macro does is give Logger the opportunity to purge at compile time. If you do not do that then it leaves the AST largely alone, and then at runtime it’s evaluated just as you would expect.

Logger COULD always wrap the argument in an anonymous function so that it was always lazy, but this could produce quite a bit of unexpected behaviour for people, so it is not what is done

Plenty of people forego compile_time_purge_level because they want to be able to alter the log level dynamically. If the debug values are compile time purged then even if you change the log level while it’s running the debug calls won’t happen. If you wrap expensive debug calls in an anonymous function, then you can switch to debug logging and get the values dynamically

What you have is exactly what happens with compile time purge on, but is not what happens when compile time purge is off. Your check of the log level happens at compile time which makes it impossible to change dynamically at runtime.

Hi @benwilson512, thank you for the answer.

Logger COULD always wrap the argument in an anonymous function so that it was always lazy, but this could produce quite a bit of unexpected behaviour for people, so it is not what is done

Plenty of people forego compile_time_purge_level because they want to be able to alter the log level dynamically. If the debug values are compile time purged then even if you change the log level while it’s running the debug calls won’t happen. If you wrap expensive debug calls in an anonymous function, then you can switch to debug logging and get the values dynamically

I see, this makes sense.

Since it’s a macro that returns a quoted expression, the argument will be evaluated later.

This is not correct. The only thing that being a macro does is give Logger the opportunity to purge at compile time. If you do not do that then it leaves the AST largely alone, and then at runtime it’s evaluated just as you would expect.

I guess this is the bit I’m still not sure about.
For example, with this simple test I can see that the evaluation is deferred and only executed when necessary:

defmodule NaiveLogger do
  def do_log(level, message) do
    IO.puts "log[#{level}] : #{message}"
  end

  defmacro log(level, message) do
    if level > 2 do
      quote do
        NaiveLogger.do_log(unquote(level), unquote(message))
      end
    else
      :ok
    end
  end
end

give_me_the_message = fn() ->
  IO.puts "I'm being executed!"
  "something is happening"
end


require NaiveLogger

NaiveLogger.log(3, "message: #{give_me_the_message.()}")
# I'm being executed!
# log[3] : message: something is happening
# :ok

NaiveLogger.log(1, "message: #{give_me_the_message.()}")
# :ok

Were you referring to something different, or have I missed a step?