Enlighten

Enlighten Mode displays the value of locals in realtime, as your code runs. This feature is somewhat similar to a feature in the Light Table editor.

To turn it on, issue M-x cider-enlighten-mode. Then, evaluate your functions one at a time using C-M-x or C-x C-e. Note that C-c C-k won’t work.

That’s it! Once your code executes, the regular old buffer on the left will turn into the brilliant show of lights on the right.

Here’s an enlightened function lighting up as it runs with different inputs:

When you’ve had enough, cider-enlighten-stop turns everything off at once: it disables cider-enlighten-mode, removes the overlays, and ignores any further values still streaming from definitions you instrumented earlier - all without re-evaluating anything. (Disabling the mode on its own only stops new instrumentation; already-instrumented definitions keep lighting up until you re-evaluate them clean.) To just clear the overlays from a buffer (they will come back the next time the code runs), use cider-enlighten-clear.

If you only want to enlighten a single definition, you don’t have to toggle the global mode at all. Put point in the form and run cider-enlighten-defun-at-point: it evaluates just that top-level form with enlightenment enabled, leaving the global mode untouched. You can also write #light before the (def and re-evaluate it.