Retrieving Mix configurations at compile-time

Hey guys, continuing from my other thread I’m still building and exploring some compile-time stuff that Elixir can provide.

Right now I’m trying to operate on the Mix config files at compile-time, but I keep getting nil values from the app keys. At first I thought that maybe the configs could only be read at runtime, but then I found this in the Mix.Config doc page:

Configuration set using Mix.Config will set the application env, so that Application.get_env/3 and other Application functions can be used at run or compile time to retrieve or change the configuration.

Is there a specific way to retrieve the config values at compile-time, then? I’ve been trying both:

inside a macro

defmodule A do
  defmacro test do
    IO.inspect Application.get_env(:app, :key)
  end
end

and just inside a module

defmodule A do
  IO.inspect Application.get_env(:app, :key)
end

but I always get nil . Any tips would be appreciated.