Testing on the Emacs Lisp track

Learn how to test your Emacs Lisp exercises on Exercism

Elisp tests are run using ERT, an Emacs Lisp library for regression/unit testing. Tests can be run several ways.

To run the tests interactively from within Emacs:

  1. Open test file, M-x eval-buffer RET
  2. Interactively and individually, with M-x ert RET test-name RET
  3. Interactively and all at once, with M-x ert RET t RET Note that this will run all tests currently loaded into Emacs!
  4. Rerun M-x eval-buffer RET before testing whenever you've made changes

To only execute the tests in the current buffer you can add this to your .emacs or init.el:

(defun my-eval-and-run-all-tests-in-buffer ()
  "Delete all loaded tests from the runtime, evaluate the current buffer and run all loaded tests with ert."
  (ert 't))

After restarting Emacs:

  1. Open test file
  2. M-x my-eval-and-run-all-tests-in-buffer RET

Interactive testing is great while you're working on an exercise, but batch mode testing is preferable for when you want to check that an exercise is ready for submission.

To run run all tests from the terminal, in batch mode, execute emacs -batch -l ert -l *-test.el -f ert-run-tests-batch-and-exit

The above command is a bit unwieldy, so if you like:

  1. Create a file on your $PATH (probably in ~/bin) called ert-run
  2. The contents of the file should be as follows:
    #!/usr/bin/env sh
    emacs -batch -l ert -l $1 -f ert-run-tests-batch-and-exit
  3. Make the file executable with chmod +x ert-run

You should be able to simply call ert-run *-test.el and run the tests in batch mode.

Other options can be found in the docs, C-h i m ert RET.

Working on exercises

Since Emacs is, itself, an elisp interpreter, your working code is always in its native execution environment. You can evaluate any form by pressing C-x e at the end of the form, or a selection with M-x eval-region or the whole buffer with M-x eval-buffer. This can be extremely useful for quickly debugging your code.


Since both your code and tests are valid elisp, it is suggested to work with your exercise code in a buffer pane side-by side with its test, like so:

Screenshot showing Emacs with the Frame vertically split into two Windows

Split the frame vertically with C-x 3, switch to the new window with C-x o, and open the file with C-x C-f /path/to/file.