Why/How/When to use the __using__(which) macro in Phoenix Controller/View/etc?

This is a bit of a “strange” question but please bare with me; I expect it will be useful/insightful to others learning Elixir/Phoenix wanting to understand the __using__/1 macro.

Context

Inside every Phoenix (Elixir Web Framework) App, at the bottom of the /lib/{yourapp}_web.ex file
e.g: /lib/chat_web.ex
there is a __using__/1 macro defined as:

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

Question: what is the __using__/1 macro used for?

If you can, share (or link to) a usage example which will help demonstrate it in a real world context.

Where does the apply function come from given that it’s not “imported” in the /lib/{yourapp}_web.ex file and what is the effect of the “apply” ?

We have tried googling and reading several docs, tutorials, blog posts etc. on Macros. e.g:

But still no closer to understanding the why/when/how we would use __using__/1 macro … :frowning:

If we attempt to comment out or delete it from lib/chat_web.ex the app does not compile even though it is not invoked from with chat_web.ex … and excoveralls (test coverage reporting) reports that it’s not being executed.

I find this confusing / non-beginner-friendly and searching the Phoenix guide (docs) is not particularly insightful e.g:


Further context:

In the Phoenix Chat Example/Tutorial: https://github.com/dwyl/phoenix-chat-example

We are tracking test coverage as a learning exercise …

There is only one line of code that is not being covered by Tests:
https://codecov.io/gh/dwyl/phoenix-chat-example/src/b57cc174d7f1c9aac22947f23170b29d4c303776/lib/chat_web.ex#L65

How is it that the line is not being executed (“covered”) when we run the tests, but if we comment out the line the tests fail?

Is this macro “magic” in that it is being “used” without actually being called?
Any shoshin insight much appreciated!

1 Like

@nelsonic: Here are some helpful resources:

  1. https://elixir-lang.org/getting-started/alias-require-and-import.html#use

  2. https://hexdocs.pm/elixir/Kernel.html#use/2

  3. http://www.zohaib.me/use-in-elixir-explained/

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I have a demo app “rlp” from this topic. Looking at rlp_web.ex I notice this:

This can be used in your application as:

      use RlpWeb, :controller
      use RlpWeb, :view

Shortly thereafter this:

  def controller do
    quote do
      use Phoenix.Controller, namespace: RlpWeb
      import Plug.Conn
      import RlpWeb.Router.Helpers
      import RlpWeb.Gettext
    end
  end

  def view do
    quote do
      use Phoenix.View, root: "lib/rlp_web/templates",
                        namespace: RlpWeb

      # Import convenience functions from controllers
      import Phoenix.Controller, only: [get_flash: 2, view_module: 1]

      # Use all HTML functionality (forms, tags, etc)
      use Phoenix.HTML

      import RlpWeb.Router.Helpers
      import RlpWeb.ErrorHelpers
      import RlpWeb.Gettext
    end
  end

  def router do
    quote do
      use Phoenix.Router
      import Plug.Conn
      import Phoenix.Controller
    end
  end

  def channel do
    quote do
      use Phoenix.Channel
      import RlpWeb.Gettext
    end
  end

One thing you should notice is that each of these functions wrap:

quote do
...
end

That should tell you that the controller, view, router, channel functions generate code. This happens at compile-time, not run-time. So these functions are intended for compile-time use, not run-time use.

  defmacro __using__(which) when is_atom(which) do
    apply(__MODULE__, which, [])
  end

apply is simply Kernel.apply/3. And again defmacro tells you that you are dealing with a compile-time construct.

So

use RlpWeb, :controller

would become at compile time

Kernel.apply(RlpWeb, :controller, [])

which is equivalent to

RlpWeb.controller()

which refers to that function we’ve just come across. So in page_controller.ex:

defmodule RlpWeb.PageController do
  use RlpWeb, :controller

  def index(conn, _params) do
    render conn, "index.html"
  end
end

The use RlpWeb, :controller causes RlpWeb.controller() to be run at compile time which results in the code wrapped in quote do ... end being dumped unceremoniously into RlpWeb.PageController resulting in

defmodule RlpWeb.PageController do
  use Phoenix.Controller, namespace: RlpWeb
  import Plug.Conn
  import RlpWeb.Router.Helpers
  import RlpWeb.Gettext

  def index(conn, _params) do
    render conn, "index.html"
  end
end

Notice how there is now another use there that needs exactly the same treatment during compile time.

Understanding exactly what code becomes available when during compilation can be a bit confusing. For that How does Elixir compile/execute code? may help.

8 Likes