There were quite a few interesting comments on my previous entry about supporting unit tests directly inside a language, so I thought I’d address some of these observations.

Michael suggests to keep the unit tests near the method they test:

@Test(3, 4: 7),
@Test(-2, 0: -2)
public int add(int x, int y) { return x + y; }

My main hesitation about this approach is that the production code is now littered with test code, and even if you use an IDE that conveniently lets you fold these tests away, it’s still painful to read such code in non-IDE environments, such as web browsing, code reviews, terminal diff, etc… This is one of the reasons why I suggested to have the unit test section in one place.

There is also the problem that unit tests usually require more than just “parameters in, result out”: very often, you will need to initialize your objects or perform some arbitrary code to set things up.

Dan is worried about pulling test dependencies in your production code:

What about test-only dependencies? Your production code would depend on mock libraries, Hamcrest expressions, etc.

Yes, this is used to be my strongest argument when I was against the idea of mixing unit tests with code, but this objection disappears if you actually embed this functionality in the language. I argued in my post for a new keyword called unittest that would define a brand new section, so I don’t see why this section couldn’t include its own imports as well:

unittest {
import org.testng.Assert;
public void positiveNumbers() {
Sum sum = new Sum();
Assert.assertTrue(sum.add(3,4) == 7);

This code will only be compiled if the compiler is invoked with the option -unittest, and otherwise, it will be simply ignored.

Dan adds:

the main gotcha is standardizing on how to run unit tests

Exactly, and that’s my point: this aspect is very underestimated and it’s the reason why so much code is written every day without accompanying unit tests. If you move this functionality inside the language, you create a universal framework across all companies and all projects that make testing a very natural part of the coding process in that particular language.

When you receive a piece of code for review, you now expect the accompanying unit test to be part of that file.

A few people chimed in to advocate Maven, but regardless of my feelings toward this tool, Maven doesn’t solve the problem I’m trying to address here since it’s yet another tool that’s external to the language, which means that it won’t always be available wherever that language is used.

A reader called Mikew is using something similar to my suggestion in Java at his workplace. He and his teammates are using inner classes to put test code inside their production code, and it seems to be working for them, despite the downsides of such an approach. As I pointed out, a lot of these downsides would disappear if such a functionality was supported natively.

Python’s Doctest doesn’t seem very scalable to me, especially because it makes you write code in comments and that the scope of the tests that you can write is extremely limited, but I agree with the underlying intention.

Phil Goodwin brought Eiffel into this discussion:

This was an assertion framework, and so did not quite support the whole testing story. Nevertheless, it provides a substantial foundation for embedding test code into source.

Eiffel’s support for testing, and the whole idea of Design by Contract in general, has always struck me as a toy that only works in trivial cases. Eiffel supports preconditions, postconditions and invariants, but in my experience, these concepts only work for foundation and library classes (e.g. containers) but scale very poorly in the real world (good luck coming up with an invariant for a class that’s manipulating database records while managing transactions).

I wrote a lot of code in Eiffel a long time ago and I distinctly remember that writing either of these contracts became plain impossible very quickly.

Having said that, Eiffel is the only language (besides D) that I can think of that supports arbitrary blocks of code to be included in classes while having a special status attached to them (“this is test code”).

Overall, the concept of something similar to a unittest section in a programming language continues to be very appealing to me and I am betting that if a language supporting this feature emerges in the next few years, the overall quality of code written in that language will be significantly higher than what we are used to.