Using a custom Elixir Library that uses Env variables

I created a test elixir lib that uses a file config/config.exs to store env variables for example :

config.exs :

import Config

url_dev = "DEV"
url_prod = "PROD"

config :test_lib,
  dev: [
    url: url_dev,
  production: [
    url: url_prod,

and in the lib/test_lib_elixir_package.exs, I called tha variable :

defmodule TestLibElixirPackage do
  def testFunction do
    dev_url = Application.fetch_env!(:test_lib, :dev)

When I test the lib with IEX i can obtain the result of the function testFunction

But When I installed the LIB in an elixir app and tried to call the function I got this error :

“** (ArgumentError) could not fetch application environment :dev for application :test_lib because the application was not loaded nor configured
(elixir 1.13.1) lib/application.ex:683: Application.fetch_env!/2
(test_lib_elixir_package 0.1.2) lib/test_lib_elixir_package.ex:8: TestLibElixirPackage.testFunction/0”

Here is my mix.exs from the library (I included the config directory) :

defp package do
      files: ["lib", "config", "mix.exs", "README*"],
      description: "Testing build and publish elixir lib",
      maintainers: ["test"],
      licenses: ["MIT"],
      links: %{}

config/config.exs in library codebase is not loaded when used as a dependency.


Hi, But I can find it in the installed lib files :


I am missing something?

1 Like

It doesn’t matter if it’s there – config files of your dependencies are not used.


Hi LostKobrakai, So how can I make it work ?

1 Like

You can either move the values into your code or provide app env values (for your app only) via the mix.exs (see vintage_net/mix.exs at main · nerves-networking/vintage_net · GitHub)

1 Like

The idea is to provide one LIB, but the LIB provide 3 envs (dev, staging and prod) depending on the ENV of the app. How do you think it should be implemented on the LIB side ?

@LostKobrakai how this approche " or provide app env values (for your app only) via the mix.exs "

Can help me get the LIB vars ? you mean I should define the enviromnent in each app that installes the LIB ?

Ideally you should not use the application environment in libraries at all. But answering your question - yes, you should define configuration each time you use dependency.


To add to that, if you decide to rely on the application environment and would still like to rely on some defaults you can do something like the following:

Application.get_env(:test_lib, :dev, your: "default config")

Or a tad more readable:

@default [your: "default config"]

def some_function do
  config = Application.get_env(:test_lib, :dev, @default_config)
  # ...

I do something similar in my etag_plug package.

1 Like

@wolf4earth : Sorry to ask, I’m still beginner but this line will be on the LIB side ?

Application.get_env(:test_lib, :dev, your: "default config")

And this code also ?

@default [your: "default config"]

def some_function do

When we send the config from the APP to the LIB ?

Yes, that code would go into the library and then you don’t HAVE to add config :test_lib, ... into your application config.

Does that answer your question?

@wolf4earth , not really , I didn’t get the difference between :

@default [your: “default config”]



Espacially the “your”

This is just an example using a module attribute (used as constant - think of JavaScript’s const). There’s no special meaning behind it.

The important part is that you can use Application.get_env/3 to set defaults and don’t NEED to set the configuration values in config/ - assuming you can provide reasonable defaults.


Here :
Application.get_env(:test_lib, :dev, @default_config)

the Library will get the env variables from where exactly (:test_lib and :dev) ? How it will fetch it besides putting values in config/ ?

I’d suggest to go through the official docs.

And here is the documentation for Application.get_env/3:

1 Like