Keep D unittests separated from production code

D has built-in unit tests, and unittest is even a keyword. This has been fantastically successful for the language, since there is no need to use an external framework to write tests, it comes with the compiler. Just as importantly, a unittest after a function can be used as documentation, with the test(s) showing up as “examples”. This is the opposite approach of running code in documentation as tests in Python – generate documentation from the tests instead.

As such, in D (similarly to Rust), it’s usual, idiomatic even, to have the tests written next to the code they’re testing. It’s easy to know where to see examples of the code in action: scroll down a bit and there are the unit tests.

I’m going to argue that this is an anti-pattern.

Let me start by saying that some tests should go along with the production code. Exactly the kind of “examply” tests that only exercise the happy path. Have them be executable documentation, but only have one of those per function and keep them short. The others? Hide them away as you would in C++. Here’s why I think that’s the case:

They increase build times.

If you edit a test, and that test lives next to production code, then every module that imports that module has to be rebuilt, because there’s currently no good way to figure out whether or not any of the API/ABI of that module has changed. Essentially, every D module is like a C++ header, and you go and recompile the world. D compiles a lot faster than C++, but when you’re doing TDD (in my case, pretty much always), every millisecond in build times count.

If the tests are in their own files, then editing a test means that usually only one file needs to be recompiled. Since test code is code, recompiling production code and its tests takes longer than just compiling production code alone.

I’m currently toying with the idea of trying to compile per package for production code but per module for test code – the test code shouldn’t have any dependencies other than the production code itself. I’ll have to time it to make sure it’s actually faster.

version(unittest) will cause you problems if you write libraries.

Let’s say that you’re writing a library. Let’s also say that to test that library you want to have a dependency on a testing library from, like unit-threaded. So you add this to your dub.sdl:

configuration "default" {
configuration "unittest" {
     dependency "unit-threaded" version="~>0.7.0"

Normal build? No dependency. Test build? Link to unit-threaded, but your clients never have the extra dependency. Great, right? So you want to use unit-threaded in your tests, which means an import:

module production_code;
version(unittest) import unit_threaded;

Now someone goes and adds your library as a dependency in their dub.sdl, but they’re not using unit-threaded because they don’t want to. And now they get a compiler error because when they compile their code with -unittest, the compiler will try and import a module/package that doesn’t exist.

So instead, the library has to do this in their dub.sdl;

configuration "unittest" {
    # ...
    versions "TestingMyLibrary"

And then:

version(TestingMyLibrary) import unit_threaded;

It might even be worse – your library might have code that should exist for version(unittest) but not version(TestingMyLibrary) – it’s happend to me. Even in the standard library, this happened.

Keep calm and keep your tests separated.

You’ll be happier that way. I am.

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2 thoughts on “Keep D unittests separated from production code

  1. Sjoerd Nijboer says:

    There was an interesting question on reddit about your blogpost. (
    Were you able to verify your assertion that your compile times increased by including unittests? If so, how bad is it? The results could be a major motivator if they make a significant impact on compile time.

    • atilaneves says:

      I always knew compile times were impacted – compiling just one file is always going to be faster than compiling several. I don’t have properly benchmarked times for you though. I also replied on the reddit thread just now.

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