How to get a warning on undefined module use?


I want to match an exception in a rescue block, matching it to several exception modules.
One of the modules is not defined (because I forgot an alias) but neither the compiler, dialyzer, nor credo emit a warning about it.

defmodule ErrorHandler do
  alias Errors.A
  alias Errors.B

  def handle_errors(fun) do
    error in A ->
      {:hard_failure, error}

    error in B ->
      {:outage, error}

    # undefined/unaliased module C but no warning emitted
    error in C ->
      {:transient_error, error}

Is it possible to have a warning emitted in such cases somehow?
If not, what would be the best place to introduce a warning like this, if this makes sense at all?

Thanks in advance!

1 Like

Well, there could be a C dynamically loaded at runtime, so there is nothing the compiler could complain about.

Yeah, I found this behavior shocking when I learned about it. The bottom line seems to be that an original BEAM core design is to support a highly dynamic runtime environment; where code can be swapped in and out (“dynamically loaded”). And the issue is, we’re using this system for a whole new set of kinds of apps which are static in nature. E.g., a CRUD Phoenix web app probably is designed to work only with the modules packaged with it at deploy time. And a static code update is easy in the modern era when we can e.g. re-deploy instantly to Heroku without any downtime.

Personally, I’d appreciate a way to flag my app as “non-dynamic”, and then get linking errors at compilation time.

I bumped into your problem in a different context: I want compile errors from router.ex, but everything compiles fine. (?)


I have posted this on another thread but, for completeness, if you don’t want your code to compile in those cases, you can enable warnings as errors:

[elixirc_options: [warnings_as_errors: true]]

Or if you want to enable it only for xref:

[aliases: ["compile.xref": "compile.xref --warnings-as-errors"]]

PS: @dogweather sorry that you are getting duplicate answers but since people may arrive here from Google, etc, I wanted to post an alternative to get an error on undefined modules.