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between the abstractions we want and the abstractions we get.

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Your tests may belong elsewhere

It may seem obvious where unit tests should live, until we start looking at the alternatives - and there come the pleasant surprises.

For succinctness' sake, let's use "code" for "code under test".

separate code and test

Having spent a lot of time on C#, I used to take it for granted that tests (including unit tests), deserve their own separate project.

This can look completely natural but sooner or later there will be awkwardness. Typically,

These symptoms are all some form of duplication. As the size of the projects grows, so do the symptoms and the burden of maintenance.

test alongside code

When I first worked on a front-end (AngularJS) codebase, it was pleasant to find unit tests (specs) are placed alongside code under test. This greatly increases the visibility and significance of tests in a codebase. These questions are answered trivially,

Later I found in React, it's also conventional to have a __test__ folder within each folder to house tests. Such seemingly innocuous naming in effect pushes tests a step away from significance and towards obscurity. It's a form of separation and it can suffer from some of the symptoms as with separate projects.

P/S isn't it great that front-end conventions don't overdo the projects shenanigans?!


An honourary mention goes to the use of any REPL, as it's in my opinion one of the best techniques to get feedback about code.

The ability to try out a piece of code by copy-pasting into (if not sending directly to) the REPL, and to see immediately if it works or not, without completing the whole program, it's massively undervalued.

This lends naturally to compositional problem-solving, and it's been a game-changer for my process and productivity.

Blessed be any light-weight syntax that enables functions, expressions and composition, typically that of scripting or functional languages. Not so much for those object oriented - creating a class in the REPL? Too much ceremony, too much time.

Sadly, the (waning?) dominance of OO languages means many of us are missing out in this department.


Using a REPL is great, and naturally we would want to save the code snippets as reference for our later selves, and what better place to put it than right alongside the code under test, as comments?

And wouldn't it be DREAM if we can then execute such (code as) comments directly? This is exactly the idea of Python's doctest. Example from this link.

Below is a test case for the function, factorial()!

>>> factorial(5)

def factorial(n):

    import math
    # calculates factorial for n
    return result

Wikipedia also lists out doctest implementation in other languages. For example, Rustlang features full support plus a great summary,

Nothing is better than documentation with examples. Nothing is worse than examples that don't actually work...

We are not done with Rust yet as it supports yet another alternative that's equally exciting...

Rust again

Consider this idiomatic code snippet from the documentation of Rust,

pub fn add_two(a: i32) -> i32 {
    a + 2

mod tests {
    use super::add_two;

    fn it_works() {
        assert_eq!(4, add_two(2));

Code and test live harmoniously in the same file. In no small surprise, and to the irk of testing doctrinaires, private functions are eligible too. Way to put an end to such farcical menial debates. Well played Rust.

C# again

Those of us using C# need not feel left behind here. Doctest may not be a thing in .Net (yet), but existing testing libraries such as NUnit already make it possible to inline tests right next (or above) to code.

Anecdotally, I used to think the simplest form of unit test one can reach is something like this,

[TestCase("bar", 2, "bar,bar")]
public void TestFoo(string arg1, int arg2, string expected)
    var actual = Foo(arg1, arg2);
    Assert.AreEqual(actual, expected);

Apparently that's NOT EVEN CLOSE. Look at this lovely example from this stackoverflow reply by Andrej Adamenko.

public static class Ext
     [TestCase(1.1, Result = 1)]
     [TestCase(0.9, Result = 1)]
     public static int ToRoundedInt(this double d)
         return (int) Math.Round(d);

This really brings all the goodies together - code and test as one, and maintenance becomes a joy. Why look elsewhere?!

There are a few restrictions though - for good reasons.

In another word, it's only possible with pure functions. Incidentally, pure functions are the best type of functions!

But we need not restrict ourselves here. Putting tests (either inlined or as separate test suites) alongside classes under test affords us many extra benefits on top of those above,

One would argue the size of the package may increase with all tests being included. Sure thing if a few extra MBs really make a difference, as it may be for drivers, embedded or some systems software. But is it really the case for most web, desktop or even mobile applications?

We get all these benefits by bringing existing code and test closer, without introducing new libraries or dependencies. Why not?!