Create behaviour from a module

make_behaviour.ex

defmodule MakeBehaviour do
  defmacro __using__(opts) do
    module = opts[:module]
    quote do
      if not is_nil(unquote(module)) do
        import unquote(module)
      end
      @on_definition {MakeBehaviour, :on_def}
    end
  end

  def on_def(%{module: module}, :def, name, args, _guards, _body) do
    Module.spec_to_callback(module, {name, length(args)})
  end

  def on_def(_, _, _, _, _, _), do: nil
end

module_to_become_behaviour.ex

defmodule ExampleModule do
  use MakeBehaviour

  @spec example() :: String.t()
  def example, do: "this will become a callback"
end