In Phoenix apps, the web module provides this “re-dispatch into methods” technique for use (see __using__):

defmodule ExampleWeb.Router do
  use ExampleWeb, :router
end

defmodule ExampleWeb.TodoController do
  use ExampleWeb, :controller
end

defmodule ExampleWeb.Layouts do
  use ExampleWeb, :html
end

defmodule ExampleWeb do
  @doc """
  When used, dispatch to the appropriate controller/live_view/etc.
  """
  defmacro __using__(which) when is_atom(which) do
    apply(__MODULE__, which, [])
  end

  def router do
    #
  end

  def controller do
    quote do
      # ...
    end
  end

  def html do
    quote do
      # ...
    end
  end
end

What was the goal/benefit behind doing it that way? I’m guessing it’s for cutting on boilerplate, as otherwise it would’ve been:

defmodule ExampleWeb.TodoController do
  use ExampleWeb.Controller
end

defmodule ExampleWeb.Layouts do
  use ExampleWeb.HTML
end

defmodule ExampleWeb.Router do
  # hmm, probably inlined because same module name
end

defmodule ExampleWeb.Controller do
  defmacro __using__ do
    # ...
  end
end


defmodule ExampleWeb.HTML do
  defmacro __using__ do
    # ...
  end
end

Do we see more examples of this technique being used in libraries and projects besides Phoenix?

I’m trying to grasp the philosophy behind every decision