Help with implementing a macro

Questions & Help Questions
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1

I’m trying to implement a conceptual test with the following macro. I want to be able to pass an action argument which will call a function of the same name in another module with the msg macro argument as the callback function’s argument. This is the same design as the instrument/3 macro in Phoenix.Endpoint.Instrument. But I’m getting an error when I try to compile it.

Can anyone help me understand why?

Error:
warning: variable "action" does not exist and is being expanded to "action()", please use parentheses to remove the ambiguity or change the variable name
  macro.ex:11


== Compilation error on file macro.ex ==
** (CompileError) macro.ex:11: undefined function action/0
    (elixir) expanding macro: Kernel.def/2
    macro.ex:10: MacroTest (module)
    (elixir) lib/kernel/parallel_compiler.ex:117: anonymous fn/4 in Kernel.ParallelCompiler.spawn_compilers/1

** (exit) shutdown: 1
    (elixir) lib/kernel/parallel_compiler.ex:291: Kernel.ParallelCompiler.handle_failure/3
    (elixir) lib/kernel/parallel_compiler.ex:247: Kernel.ParallelCompiler.wait_for_messages/1
    (elixir) lib/kernel/parallel_compiler.ex:62: Kernel.ParallelCompiler.spawn_compilers/3
       (iex) lib/iex/helpers.ex:170: IEx.Helpers.c/2

The code:

defmodule MacroTest do

  defmacro my_macro(action, msg) do
    sender = " from #{__MODULE__}"
    quote do
      unquote(__MODULE__).my_fun(unquote(action), unquote(msg), unquote(sender))
    end
  end

  def my_fun(action, var!(msg), var!(sender)) do
    unquote(MacroTest.run_action(action))
  end

  def run_action(action) do
    quote do
      MyOtherModule.unquote(action)(var!(msg), var!(sender))
    end
  end

end

defmodule MyOtherModule do

  def sender(msg, sender) do
    send self(), msg <> sender
  end

  def printer(msg, sender) do
    IO.puts msg <> sender
  end

end