When writing or debugging tests in Elixir, I often need to focus on just one test.

The mix test documentation shows a few ways to accomplish that using Filters. Probably the most common one, the test's filename and line number, for example mix test test/some/particular/file_test.exs:12, can be used to run that specific test.

If I need to edit the test and rerun, my test may not contain line 12 anymore, so I would need to change the line number in the mix test command. With a more difficult test, changing the line number over and over gets annoying.

A line-number-agnostic way to focus uses tags and filters.

I add a @tag focus: true above the test I'm focusing on, like:

@tag focus: true
test "the best feature ever" do
  assert my_feature == "best"
end

Then, on the command line, I use the --only flag to say only run the tests with my tag:

$ mix test --only focus:true
Excluding tags: [:test]
Including tags: [focus: "true"]

.

Finished in 0.06 seconds
4 tests, 0 failures, 3 excluded

Randomized with seed 418541