Oh, I agree with you in that global config for a library must be avoided.
I should have been more clear on this part 
I wanted to give hints to move away from using System.get_env/1 first…
For starter - and this kind of library, this is very likely to have one instance - which is “acceptable” (although I don’t prefer) to have (global) application config. That’s why I refer to the library guides lines, which has a section for Avoid application configuration 
@timpile for the configuration - it’s better to have a library to provide building blocks and let library users load config as needed.
For example:
# in config
config :my_app, :handwrite, url: "https://example.com", api_key: "..."
# in your helper module
defmodule MyApp.Handwrite do
def client do
opts = Application.get_env(:my_app, :handwrite)
Handwrite.client(Keyword.fetch!(opts, :url), Keyword.fetch!(opts, :api_key)
end
end
# then you use it like this
MyApp.Handwrite.client()
|> Handwrite.create_whatever(my_opts)
By doing this, your library does not care how to store and retrieve the configuration at all - and that is in the library user code.
I did make a mistake when I wrote the first library - I had default config value and make authentication happens when not passed to make it easy to use.. but I found it’s not good. See my commit to change from httpoison to tesla with 1) dropping default config and 2) making all functions to take a tesla struct, which holds the all information such as url and auth token - see Replace HTTPoison with Tesla · chulkilee/ex_force@8d32d9a · GitHub
