Interactively Evaluating Clojure Code in Comments in Emacs

There’s a loose convention in Clojure of leaving top-level (comment ...)-style comments in source files to give examples of how a particular piece of code works or to provide a convenient means of invoking functionality contained in the file. You can even see this in the source files to Clojure proper.

Even though leaving commented out code can seem a bit messy, it has also saved me a ton of time relearning how to invoke something, so I have somewhat mixed feelings about the practice. But, regardless of the merits of using this in production code, it’s unarguably useful in development, and I use it extensively as I work to test out function invocations with different arguments, and to store little bits of test data.

Frustratingly, this form does not play nicely with CIDER’s C-M-x (cider-eval-defun-at-point) and C-c M-; (cider-eval-defun-to-comment) commands, which expect the target form to be at the top-level of the file. The containing (comment ...) form means that technically isn’t so, leading me to perpetually evaluate the wrong form…

  (+ 1 1) <point>
  ) ; => nil

…and be told for the millionth time that comments evaluate to nil.

I finally discovered that there is an easy way to change this behavior, with just a single settings tweak:

(setq clojure-toplevel-inside-comment-form t)

Now things will work as you (might) expect…

  (+ 1 1) <point> ; => 2

… which is certainly more useful, and destined to become a permanent part of my Emacs config.