Recently, I joined the mentor program on Exercism¹. When we practiced mentoring, I chose Elm as my test language as a student. And I discovered this useful feature from elm-test²:

elm test --watch

It's like guard-rspec³, which will automatically rerun all your tests after you change a file. And elm-test has great Emacs support⁴.

It really helps since I don't need to manually hit the same keystrokes again and again to rerun my tests, which speeds up my TDD workflow.

Then I started looking for something similar in Elixir. And there is one called mix-test.watch⁵. But it's not bundled with the official ExUnit package and we need to add it as a dependency to our project, which I don't want to do. And it doesn't have nice Emacs support. Though I still found something useful from its README file:

mix test --stale

This command will attempt to run only those tests which reference modules that have changed since the last time you ran mix test --stale.

Documentation from mix help test:

• --stale - runs only tests which reference modules that changed since the last test --stale. You can read more about this option in the "Stale" section below.

## "Stale"

The --stale command line option attempts to run only those test files which reference modules that have changed since the last time you ran this task with --stale.

The first time this task is run with --stale, all tests are run and a manifest is generated. On subsequent runs, a test file is marked "stale" if any modules it references (and any modules those modules reference, recursively) were modified since the last run with --stale. A test file is also marked "stale" if it has been changed since the last run with --stale.

And alchemist.el also supports this command via alchemist-mix-test-stale.

It's very useful to run this command when I'm changing a feature which I don't know how many clients are there. So that I can make sure every change I make won't break any client and I don't need to run the whole test suite.

But when I'm TDD a feature, I think it's not as that useful. Since when I TDDing, I always know (or try to predict) what test would fail/success for some changes I make. Running stale tests sometimes would break this workflow because it would run more tests than I expected.

Anyway, it's a useful tool to add to your toolbox. And it again shows that Elixir has a great development tooling ecosystem.