"application.get_key > modules" will return :undefined

I want to get a list of the modules in a certain namespace at the compile time, as a macro.

defmodule MyApp.MyModuleLoader do
  defmacro __using__(opts) do
    quote bind_quoted: [opts: opts] do

      a1 = :application.get_key(:my_app)
      modules = :application.get_key(:my_app, :modules)

      IO.puts("****:a1: #{Kernel.inspect(a1)}")
      IO.puts("****:modules: #{Kernel.inspect(modules)}")

      # ====> :undefined

      # filter out the ones I don't need...
      # ....
    end
  end
end

I'll then use MyModuleLoader in a schema-model module.
But it'll print out :undefined for both variables during compilation.

Why? How to fix this?

If we add Application.ensure_loaded(:my_app) like:

defmodule MyApp.MyModuleLoader do
  defmacro __using__(opts) do
    quote bind_quoted: [opts: opts] do
      Application.ensure_loaded(:my_app) # <-

      a1 = :application.get_key(:my_app)
      modules = :application.get_key(:my_app, :modules)

      IO.puts("****:a1: #{Kernel.inspect(a1)}")
      IO.puts("****:modules: #{Kernel.inspect(modules)}")

      # ====> :undefined

      # filter out the ones I don't need...
      # ....
    end
  end
end

Then the output (ran via iex -S mix) will be:

****:a1: :undefined
****:modules: {:ok, [<list of modules>]}

After changing IO.puts(...) to IO.inspect(...) we get the same output from mix compile command.