How to test config options?

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(

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

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

    # 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
        Application.delete_env(:clouseau, :default_switches)
      # Regenerate the @default_switches module attribute

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)

.warning: redefining module Clouseau.Switches (current version defined in memory)

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?

You might want to read this one:


I have read that post and I read it again now. Unfortunately it looks like it is no easy way to put custom config at runtime while avoiding merges during runtime.

1 Like

Yeah that’s my issue with the general config style. I still think we need a staged configuration system like I’ve used in other languages. Essentially you have global configuration, which only affects code-gen and acts as defaults for the later stages. Then there is the load-time configs, which should only affect things that happen at load time, in addition to acting as defaults for later stages. Then run-time configuration, which when changed then updates the configuration (users of this should either access it every time they need it, or for something like a database link it should receive an event when it changes so it can update as necessary), which of course acts as defaults for later stages. And finally there is the scoped stage which is where you pass in the configuration overrides to a specific call or instance of something.

This is similar to how I treat my TaskAfter library. I keep meaning to make a StagedConfiguration library for Elixir sometime, it would simplify a lot of my own work…


I can see why keeping config in compile time can be desired, but if you can build your library to be configurable at runtime you can test it without being dependent on compile time. If you really want to stay at compile time you might want to look at ex_cldr, which does a bunch of compile time optimazations as well.