I have a library that uses option parser to format the output. The input is a string coming from the code itself, not a shell.

I am using this code to merge the switches given from the user with some predefined switches. The predefined switches are:

• The default hard coded in the module
• If are there any coming from config.exs merge them with the hard coded

If the user provides own switches, they will be merged to the predefined switches

defmodule Clouseau.Switches do
  #...
  @predefined_switches [
    file: true,
    full_path: false,
    module: true,
    line: true,
    text: true,
    border: false,
    colors: false
  ]

  #...
  @default_switches Keyword.merge(
                      @predefined_switches,
                      Application.get_env(
                        :clouseau, 
                        :default_switches, 
                        @predefined_switches
                        )
                    )

 #...
  defp apply(switches) do
    Keyword.merge(@default_switches, switches)
  end
end

The @default_switches module attribute is set during compile and Application.put_env(:clouseau, :default_switches, file: false) has no effect.

How can I test that changing the config options changes the behavior of my libray?

So far I came up with this test file.

defmodule SwitchesTest do
  use ClouseauCase

  setup_all  do
    # Save current config if exists
    env_switches = Application.get_env(:clousau, :default_switches)
    # Put new config 
    Application.put_env(:clouseau, :default_switches, file: false)
    # Recompile the file to refresh the @default_switches module attribute
    Kernel.ParallelCompiler.compile(["lib/switches.ex"])

    # On exit revert :default_switches config
    on_exit(fn ->
      # If it existed before put back the old one
      if env_switches do
        Application.put_env(:clouseau, :default_switches, env_switches)
      # if didn't exist before just delete the current one
      else
        Application.delete_env(:clouseau, :default_switches)
      end
      
      # Regenerate the @default_switches module attribute
      Kernel.ParallelCompiler.compile(["lib/switches.ex"])
    end)
  end
end

In the setup_all block I save the current config, recompile the switches module, and on_exit I revert back whatever config was there and recompile once again the switches module.

This works so far but it shows warnings during the test

warning: redefining module Clouseau.Switches (current version loaded from _build/test/lib/clouseau/ebin/Elixir.Clouseau.Switches.beam)
  lib/switches.ex:1

.warning: redefining module Clouseau.Switches (current version defined in memory)
  lib/switches.ex:1

Is there any better way to test the config options? Or maybe a better way to design the switches module but avoiding merging the hard coded switches with the configured every time in runtime?