Using `~p` dynamically inside a macro

Phoenix Forum Questions / Help
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1

I’m having a bit of trouble wrapping my brain around how I would pass a dynamic fragment to ~p inside a macro:

defmacro __using__(opts) do
  @path opts.path

  quote do
    def handle_event("event", _, socket) do
        path = ~p"/#{@path}"

        # ...
        {:noreply, socket}
    end
  end
end

I’ve tried a fair number of functions from the Macro module both inside, namely escape and expand/expand_once which are the ones I though would do it as well as unquote (and even quote for good measure). I understand that passing an argument to the ~p macro is going to quote it and when it’s called already inside a quote block, well, uh… oh boy :face_with_spiral_eyes: As I have it does produce the correct route but of course gives a console warning. I feel like this question must have been answered before but I can’t find it and my brain hurts. At the very least I’m treating this forum as a rubber duck but any help would be much appreciated.

1 Like
2

There is some macro confusion here.

This changed version compiles:

defmodule A do
  defmacro __using__(opts) do
    path = opts.path
  
    quote do
      def handle_event("event", _, socket) do
          path = ~p"/#{unquote(path)}"
  
          # ...
          {:noreply, socket}
      end
    end
  end
end

But there is one more problem.
Since you are using dot syntax, I assume you’re trying to pass a map. Instead use a keyword. IIRC, keywords are not escaped:

defmodule A do
  defmacro __using__(opts) do
    path = opts[:path]
  
    quote do
      def handle_event("event", _, socket) do
          path = ~p"/#{unquote(path)}"
  
          # ...
          {:noreply, socket}
      end
    end
  end
end

defmodule B do
  use A, path: "my-path"
end
3

Hey hey, thanks for the response and welcome to the forum!

I’ve tried your suggestions and I’m still getting the warning. Are you seeing it? Maybe I messed up still. My problem was never that it didn’t compile—it compiled and produced the correct path, but it failed to be verified. The warning is no route path for MyAppWeb.Router matches "/#{"some-path"}" so I need to be able to pass the ~p macro and already expanded variable… which is what I’m having a hard time wrapping my head around.

1 Like
4

That’s likely the clue here. The compile time validation doesn’t see ~p"/some-path", but ~p"/#{"some-path"}". Evaluate both and it’ll be the same, but their AST/the code isn’t. The latter is not using a static string, even if the dynamic input takes a static value.

You likely want this: path = sigil_p(unquote("/#{path}")) – build the static path before unquoting the result into the sigil macro call.

4 Likes
5

Hmm, I think sigil_p/2 pattern matches against ast and unquoting that expression results in fully evaluated binary.

I bet there would be a more intuitive solution but this works:

defmodule A do
  defmacro __using__(opts) do
    path = opts[:path]

    quote do
      use Phoenix.Component

      def handle_event("event", _, socket) do
        path = sigil_p(unquote({:<<>>, _meta = [], ["/#{path}"]}), [])

        # ...
        {:noreply, socket}
      end
    end
  end
end

defmodule B do
  use A, path: "my-path"
end

warning: no route path for InformedCMSWeb.Router matches "/my-path"
  iex:112: B.handle_event/3

So I passed the simplified version of desired ast:

iex> quote(do: "/#{path}")
{:<<>>, [],
 [
   "/",
   {:"::", [],
    [
      {{:., [], [Kernel, :to_string]}, [], [{:path, [], Elixir}]},
      {:binary, [], Elixir}
    ]}
 ]}
2 Likes
6

Thanks for your help, @0xG and @LostKobrakai. I’m ultimately going with a slightly more verbose but more flexible (and less brain-bendy for me) solution. I though I was only going to be dealing with URLs in the form of: ~p/resource/#{resource} but it turns out that is not the case and don’t want to get too crazy! Thanks again!