When working on my recent project, I started getting the following error:

module Q is not loaded and could not be found

The error was shown when I run : mix test


  1. I confirmed that the module was listed in the mix.exs and was installed via mix deps.get
  2. I tried iex -S mix but no error. In fact Q.q(msg) worked.

I turned to Local Elixir Telegram group. Someone suggested that it might be a typo since the name of the module on hex (and in mix.exs) was qq, while I was getting an error for the module Q (But that was not the case, I have explained in the README of the qq package, about this discrepency)

Someone else shared this link.

It lead me to a possible problem/solution.


Q.q is a “quick and dirty debugging tool”. Needless to say it is needed (and should be used) only during the development. So I had mentioned the dependency as only: :dev

That is why it worked with iex -S mix (I think dev is default. But this is just a hunch) and failed only for mix test

Changing it to only: [:dev, :test] resolved the issue.

I also learnt that even if some dependency is dev-only, it might be better to list it as only: [:dev] - It could have been easier to identify the problem (or may be not)


Thanks to people on my local Elixir Telegram group for helping me solve this. Special thanks to Aditya aka BrainBuzzer for the link (mentioned above)