Test Autodiscovery is a Silver Bullet

I tend to write a lot more tests in Rust than in OCaml. Why? Not because Rust is so unsafe it needs more tests. Rust and OCaml are at roughly equivalent levels of safety. It’s because of ergonomics.

The cargo developers have done enough work that the entire process for writing and running a unit test in Rust, from scratch, is:

$ cargo new hello-world
$ cd hello-world
$ mkdir tests
$ echo '#[test]
fn test_two_plus_two() {
    assert_eq!(2+2, 4);
}' > tests/test.rs
$ cargo test
running 1 test
test test_two_plus_two ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

That’s it. That’s all there is to it. And that includes creating a new empty project, if you’re working on an existing codebase, it’s even easier: just open a test module and write a function.

cargo will discover the tests automatically, every test is a function that starts from nothing and returns nothing. Tests are run in parallel. Suites? Modules are test suites. Setup, teardown? Function calls. If you need a more sophisticated setup, you can build it.

Compare OCaml: when I was setting up tests for Austral, it took me a lot of trial and error and reading Discourse threads and Reddit posts to write the most basic assert(true = true) unit test. dune new is useless, you’d better hope you have a project starter template that sets up unit testing for you. And even when you have the test harness set up, writing and running unit tests is unergonomic:

  1. You have to first open a test module, say, FooTest.

  2. And write a test function:

    open OUnit2
    let test_two_plus_two _ =
      assert_equal (2 + 2) 4
  3. And wire it up into a test suite:

     let suite =
       "Arithmetic" >::: [
         "two plus two" >:: test_two_plus_two;
  4. And export a function to run the suite, if you haven’t done that:

    let _ = run_test_tt_main suite
  5. And register the module in the dune file, if it’s not already there:


And then, only then, can you run dune runtest and start debugging the inevitable module visibility issues. Consider all the sources of friction:

  1. Choosing a unit testing framework.
  2. Integrating this into dune. Normally this would be an unremarkable step, but dune is very painful.
  3. Choosing both a name for the test function, and a string description of it.
  4. Wiring everything together.

If there’s a better way to do it, it’s not widely advertised, and I don’t care to investigate it because getting this setup going was already frustrating enough.

The result: in theory, OCaml should be safer than Rust, because it’s garbage-collected and high-level and you can write purely-functional code guilt-free. In practice, my OCaml code is woefully undertested, while my Rust code is tested every which way. Because in OCaml—not because of the language, but because of tooling choices—testing is painful, but in Rust it’s easy.

Python also gets this right: writing unit tests in Python is one of the few aspects of the language that is simply delightful.

This ties into programming language pragmatics: discipline doesn’t scale, and if you want people to use good practices, you have to make it ergonomic to do so.