I would strongly disagree that dependency injection is an OO concept. In fact I will even say that dependency injection is bread and butter of functional programming. How? Higher order functions. What is a higher order function if not a function where we “inject” the dependency - the code that executes some algorithm or performs some computations.
Injecting whole modules is no different, not really. A module is just a set of functions. Passing a module is a convenient way of passing multiple named functions. Having said that, passing the dependency through application environment is indeed a side-effect and may be not considered a good style, but it’s very simple and convenient.
When it comes to testing and it’s presence or not - I think this depends hugely on the company. I’m fortunate to work in one that requires thorough testing of all the code - code that is not tested is simply not shipping to production. In fact, working on Phoenix APIs, I rarely start the application - most of the time I work just on the tests.
