troll.ex

defmodule SteveTrollsElixir.Private do
  defmacro __using__(_) do
    quote do
      import unquote(__MODULE__)
      @on_definition {unquote(__MODULE__), :on_priv_def}
    end
  end

  defmacro private() do
    quote do
      Module.put_attribute(__MODULE__, :private, true)
    end
  end

  def on_priv_def(%Macro.Env{module: mod}, kind, name, args, _guards, _body) do
    private_context? = Module.get_attribute(mod, :private)

    if private_context? && kind != :defp do
      raise "Defined a public function in a private section. Define #{name}/#{length(args)} above the private definition or mark it defp"
    end
  end
end