Inject variable into macro function body

My termininology with the question title is probably all over the place, but I’m really struggling on how to phrase it.

What I essentially want to do, is something like plug does in its “get”/“post”, etc. macros: It injects the “conn” variable without having to declare it anywhere.

I have this macro:

defmacro socket(matcher, do: expr) do
  quote do
    def websocket_handle({:json, unquote(matcher) = json}, state) do
      try do
        e -> Logger.error("Websocket Error - #{inspect(e)}")

and use it like this:

socket %{"action" => "join_chat", "id" => id} do
  my_fun(json) # :(
  my_fun(state) # :(
  {:reply, %{}, state}

Now, the “id” variable declared in the matcher is readily available, however “json” and “state” are not. It’s certainly related to scope, but I have tried all sorts of different things - to no avail.

The “var!” macro didn’t really help because that required me to declare the variable first, which is the exact opposite of what I want. I looked at the plug source code, but I had a hard time following it.

Is what I want to do feasable with a small macro? If yes, how - if no, what are my alternatives for giving access to the “json” and “state” variables?

Thank you for your time!

All variables have to be declared before use, this is what plug does as well. You should be able to just replace the variable names in the function definition inside the quote I’d imagine, to be something like this:

def websocket_handle({:json, unquote(matcher) = unquote(Macro.var(:json, nil))}, unquote(Macro.var(:state, nil))) do

Does that not work?


This seems to work really well! Thank you very much for this - you been a massive time saver.

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