How to get application name at compile time?

I need to access the application name under which a module was defined, in other words, I need the name defined in the module’s parent mix.exs.

  def __on_definition__(env, _kind, func_name, args, _guards, _body) do
    app_name = Application.get_application(env.module)

Application.get_application/1 doesn’t always work, as the application may or may not be available when recompiling.

Is there a non-hacky way of getting this info? I’m tempted to look at the file directory looking for a mix.exs and parsing it.

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I don’t have an answer, but I’d like to double-down on your question about looking at files during compilation as I am currently tempted to do the same. Is this a truly terrible idea? And if so, it is wrong because it’s too hacky/surprising/confusing, or is it wrong because there are gotchas?

You could put it in a file called APPLICATION and read that into the mix file as the application name and wherever else. This is sometimes done for defining the version. Could be considered hacky tho :melting_face:

Yup, I’m gonna just do this, and change it later if a better solution arises.

The problem is that this is library code, that is supposed to look into your project, but also, other libraries, so it’s a no-no, to need to adhere to that standard.

For now goes like this:

app_name = find_app_name(env.file)


  def find_app_name(module_file) when is_binary(module_file) do
    module_file |> Path.dirname() |> find_app_name_in_dir()

  defp find_app_name_in_dir(dir) do
    {:ok, files} =

    if "mix.exs" in files do
      mix_file = Path.join(dir, "mix.exs")
      {:ok, content} =

      case Regex.named_captures(~r/app: :(?<app_name>[a-z_]+),*\n/, content) do
        %{"app_name" => app_name} -> String.to_existing_atom(app_name)
        _ -> raise("Failed to read an app name from #{mix_file}")
      dir |> Path.dirname() |> find_app_name_in_dir()

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Mix.Project.config(), but only invoke it at compile time, as Mix isn’t available inside releases. Or ask the user to pass it as parameter (as Phoenix/Ecto do).


I just have to ask, but wouldnt this enable most macros to avoid requiring the app option? Is it «safe» to fallback to the app property defined in mix config inside a library macro? Which obviously messes with our heads because its hard to understand without knowing how it works, why it works… its a «compile runtime» call actually. In the application using the macro.?

No. Because those macros are not necessarily used just by the top level mix application (if there even is one).


So there was a waaaay less hacky way of doing it :smile: