Spec to Callback

foo.ex

defmodule MyApp.Foo do
  @on_definition MyApp.SpecToCallback
  
  @spec bar(String.t()) :: String.t()
  def bar(foobar) do
    impl().bar(foobar)
  end
  
  defp impl, do: Application.get_env(:my_app, :my_app_foo_impl, __MODULE__.DefaultImpl)
end

foo_default_impl.ex

defmodule MyApp.Foo.DefaultImpl do
  @behaviour MyApp.Foo

  @impl true
  @spec bar(String.t()) :: String.t()
  def bar(foobar) do
    foobar
  end
end

spec_to_callback.ex

defmodule MyApp.SpecToCallback do
  def __on_definition__(env, :def, name, args, _guards, _body) do
    Module.spec_to_callback(env.module, {name, length(args)})
  end

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