Libraries are always compiled with Mix.env == :prod, so I think toggling based on that isn’t going to work. Libraries should not change themselves implicitly, the user of the library should always pass in configuration. What changes are you trying to do?

If you need base_url to be set when you’re developing your library locally, then you already figured out how to do it:

# my lib config.exs
config :mylib, base_url: "https://someapp.com"

But if you need applications using the library to pass configuration to it, they will have to do so in their own config files. You should also consider whether it’s a good idea for your library to avoid using the application configuration altogether.

I wouldn’t do it for at least 2 reasons:

• libraries can also run outside of Mix, if your application is running in a release for example then Mix isn’t even available so the notion of an environment at runtime doesn’t even make sense
• environment names are completely defined by the application using your lib. Most people use standard names like dev, test, prod etc. but they could use whatever. For all you know, they could use development instead of dev

If your library needs to consume a configuration, then have your library’s user pass that in. Only that should matter, not the name of the environment

The convention in this case would be that your install instructions would tell users to do this in their runtime.exs

config :your_lib, endpoint: "127.0.0.1:8288"

if config_env() == :prod do
  config :your_lib, endpoint: "[127.0.0.1:8288](https://inn.gs)"
end

Using the mix.env as a global variable to change logic in. your library both won’t work for the reasons I mentioned earlier, and is not a best practice.