Why avoid mocks

Firstly, the “verb/noun” distinction has a root in OOP and I believe it is not relevant in Elixir, mocking or not.

That’s pointing to the wrong direction. First of all, OOP and FP are not mutually exclusive domains. They are properties shared by both paradigms. I will come back to this later on. The “verb/noun” guidance was not meant to be at the abstraction level but a guidance on how you are expressing your intent in the code.

The latter uses an implementation that is in fact the same as something I define in a mocking library, only adding my own potential bugs.

That’s one of the points of the linked article. You can use mock libraries as long as they are helping you build mocks rather than dynamically replacing existing components. If your mock library is leading you down the former path, then please go ahead! The article is far from a call to “forget mocks because they are bad”.

The article talks about dependency injection - which is based on “interfaces” in other languages and is re-defined as “protocols” in Elixir. Both are OO concepts.

Elixir protocols are not interfaces. Behaviours would be the closest thing to interfaces. However, typed contracts are not exclusive to OO languages. Protocols build on top of behaviours to add data-type polymorphism (a form of ad-hoc polymorphism). Polymorphism is not a concept exclusive to OO as well. Protocols in Elixir provide the same kind of polymorphism as Haskell type classes.

That brings me to the first point: why does it even matter if it is an OO concept? Being an OO concept doesn’t invalidate it nor implies it is a bad concept. In fact, the paper that introduced type classes references polymorphism in OO languages multiple times and does a good job in explaining how type classes address issues commonly found in OO implementations. Similar concept with different implementations.

That’s why I said this discussion is pointing to the wrong direction. Maybe there is a valid argument against those constructs in the context of testing but “because it is OO” is certainly not the reason (nor it is true).

I never liked dependency injection because it changes the design of a program to accomodate testing. In the functional paradigm we should aim at modules/functions that are free of side effects and dependency injection is a side effect.

If by side-effect you mean the functional definition of side-effects, dependency injection is not a side-effect. It would only be a side-effect if the code you are invoking contain side-effects. In languages like Haskell you can do dependency injection via higher order functions or type classes without involving a monad.

On the other hand, I completely understand how “dependency injection” can be a source of confusion. The page for dependency injection in Wikipedia is quite convoluted and I find it personally hard to see how that would map to “passing a function or a module as argument” in Elixir.

Finally the article also answers why I don’t personally agree with the “dependency injection changes the design of a program to accomodate testing”. A test is a consumer of your code like any other API. If your code is hard to test, it will very likely be hard to extend. Sometimes that’s fine, we don’t want all code to be extensible, that would be a nightmare to reason about. But if you feel like you need to make the code more extensible to properly test it, it is very likely the extension will be useful beyond testing. The point of the blog post is to help you reason about those trade-offs rather than find one size fits all solution.

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.

Hard to argue with that.