Hello there,

I have the following scenario:

I have a library my_lib that is used in my_app. my_lib has a configuration file option that accepts a list of modules. Based on the list of modules provided, a Plug is generated.

The config in my_app is defined like this:

config :my_lib,
  modules: [MyApp.Plugs.Plug1, MyApp.Plugs.Plug2]

The code in my_lib looks like this:

modules = Application.get_env(:my_lib, :modules)
for module <- modules do
  quote do
    forward "/module/#{unquote module}", to: unquote(module)
  end
end

This compiles, but a request to /module/Elixir.MyApp.Plugs.Plug1 just yields a 404 error. Interestingly, I also get a warning in VS Code in the for line, that modules cannot be iterated since it is nil, which to my understanding cannot be the case since I defined a default value of [] for it the mix.exs file of my_lib (I can also certainly confirm that it is not nil, since logging it prints the expected value).

def application do
  [
    mod: {MyLib.Application, []},
    env: [modules: []]
  ]
end

At this point I assumed that I either made an error in my meta-programming or since my_lib is compiled before my_app, that its modules are simply not yet available.
According to the documentation Code.ensure_compiled/1 would be a good candidate to wait for the compilation of these modules. So I added a call to this function and printed out its results, which were quite baffling to me:

modules = Application.get_env(:my_lib, :modules)
for module <- modules do
  compiled = Code.ensure_compiled(module)
  IO.puts("ensure_compiled #{inspect module} -> #{inspect compiled}")
  # = ensure_compiled MyApp.Plugs.Plug1 -> {:error, :nofile}
  # = ensure_compiled MyApp.Plugs.Plug2 -> {:error, :nofile}
  quote do
    forward "/module/#{unquote module}", to: unquote(module)
  end
end

Apparently these modules do not exist at all. However, calling this function directly in IEx, the call resolves successfully:

iex(1)> Code.ensure_loaded MyApp.Plugs.Plug1
{:module, MyApp.Plugs.Plug1}

If I had to take another guess the module resolution fails since my_lib is looking for a source file in its own lib folder and does not take other applications (my_app) into account.

• Is there any way to use modules of the “parent” app during compilation? If so, how would I go about that?
• What did I do wrong that Plug did not throw an error or return a 500, but just silently ignore my module (well, or just works for that matter )?
• Did I set the default config value incorrectly? It seems to work when executing, but since VS Code (using the elixir-ls extension) complains about it, I guess there must be something not quite right with it.

Thank you for your time! I realize that this is quite the wall of text