Background

I have an application :myapp and I am trying to add some web functionality to it from zero using Plug. However, even though I have all the files and modules, present, I still can’t launch it because iex can’t find the modules that are outside the lib folder.

Error

When running iex -S mix I get the following error:

** (Mix) Could not start application myapp: exited in: MyApp.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (UndefinedFunctionError) function MyApp.Web.MetricsExporter.setup/0 is undefined (module MyApp.Web.MetricsExporter is not available)
            MyApp.Web.MetricsExporter.setup()
            (myapp) lib/application.ex:19: MyApp.start/2
            (kernel) application_master.erl:277: :application_master.start_it_old/4

Code

To fix the above error, I added the following lines to my mix.exs:

  defp elixirc_paths(:test), do: ["lib", "web", "test/support"]
  defp elixirc_paths(_),     do: ["lib", "web"]

However the error still persists. My folder structure is the following:

config
lib
  |_ application.ex
web
  |_MetricsExporter.ex

Question

Why can’t iex find and compile the MetricsExporter file?