Automatically format Clojure buffers

Contrary to my experience with free or open source projects, when it comes to the proprietary software I have been developing for work since late 2004, code formatting has never been matter of discussion. There was a team, back in my days in the UK, that enforced some strict rules via Ant, but until today that was the sole exception.

Luckily the people I am coding with nowadays are more open to this kind of problem. Yes, I used the word problem. As I explained to a co-worker when I raised the question of formatting tools, I read code as I read a book. I cannot imagine sitting through different books each written according to the authors' own ideas of stylistic conventions. It’s about having the text, whether words or code,¹ accessible to anyone by making the assumption that the first step to understanding is ease of legibility.

The team agreed on using cljfmt² for this task. Most of the developers here use Clojure LSP and, from what I understand, the integration with cljfmt is already there for them. I, on the other hand, went with a more manual approach. I first built the cljfmt binary with GraalVM thanks to Rune Juhl Jacobsen’s useful cljfmt-graalvm.³ Rune offers a snippet for use-package and Projectile users, but I leveraged project.el and a CIDER hook instead.

(defvar mu-cljfmt-edn "cljfmt.edn"
  "The name of `cljfmt' configuration file.")
  
(defun mu-cljfmt-format-buffer ()
  "Format current buffer with `cljfmt'."
  (when (derived-mode-p 'clojure-mode)
    (let* ((pr (project-root (project-current)))
           (cfg-dir (locate-dominating-file pr mu-cljfmt-edn))
           (cfg-edn (expand-file-name mu-cljfmt-edn cfg-dir))
           (cmd (if (file-exists-p cfg-edn)
                    (format "cljfmt -c %s %s" cfg-edn buffer-file-name)
                  (format "cljfmt %s" buffer-file-name))))
      (shell-command-to-string cmd))
    (revert-buffer t t)))

(defun mu-cider-format-buffer ()
  "Automatically format current buffer after saving."
  (add-hook 'after-save-hook #'mu-cljfmt-format-buffer nil t))

(add-hook 'cider-mode-hook #'mu-cider-format-buffer)

Some things worth considering:

• the location and the name of the cljfmt.edn file matter, so check your projects if you follow my approach. For instance, the snippet provided by Rune uses .cljfmt.edn. With mu-cljfmt-edn I can customize it per-project via directory variables, if needed.

• clojurec-mode and clojurescript-mode are both derived from clojure-mode. This explains the use of derived-mode-p;

• in buffers handled by clojure-mode I always have CIDER enabled, even if not necessarily jacked-in. For a more general approach, clojure-mode-hook should be preferred;

• CIDER provides an integration with cljfmt.⁴ Depending on your needs that may be already enough.

With a tool tweaked to format code idiomatically⁵ for us, now it’s time to argue about names. Wait, let me refresh my hadoukens first.