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:
- https://elixir-lang.org/getting-started/meta/macros.html
- Metaprogramming · Elixir School
- Understanding Elixir Macros | HackerNoon
- https://medium.com/@Mike_Andr/understanding-elixirs-macros-by-phoenix-example-e99827a60987
But still no closer to understanding the why/when/how we would use __using__/1
macro …
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: GitHub - dwyl/phoenix-chat-example: 💬 The Step-by-Step Beginners Tutorial for Building, Testing & Deploying a Chat app in Phoenix 1.7 [Latest] 🚀
We are tracking test coverage as a learning exercise …
There is only one line of code that is not being covered by Tests:
Codecov
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!