An easier way to try out Clojure libraries with ob-clojure and cider

I keep notes in an org-roam based knowledge base. When I am learning something new I create a new note, give it a tag and write down my thoughts and useful references.

Lately I have had some time reviewing and experimenting with interesting Clojure libraries. I enjoy to use CIDER for my Clojure development, so to test libraries out I have to create a mini project with a project.clj (for lein) or deps.edn (for clojure-cli) for CIDER to open an useful REPL for me.

This approach has been unsatisfactory: even by having a template for creating the project, I would find the whole setup being a distraction.

Rather, I decided to enhance Org Babel for my use case.

The way I test Clojure code is by creating a block looking like this in an Org Mode file:

#+begin_src clojure
(+ 1 1)
#+end_src

Given you have set (setq org-babel-clojure-backend 'cider), running this block (C-c C-c) will start a CIDER session and eventually produce a result.

To test libraries I aimed for this instead:

#+begin_src clojure :deps '(("org.clojure/test.check" "1.1.1"))
(require '[clojure.test.check.generators :as tgen])
(tgen/sample (tgen/fmap #(apply str %) (tgen/vector tgen/char-alpha 10)))
#+end_src

In the above block I want to try out test.check, a library for property testing. (In the block I am producing a sample of alphabetical strings that are minimum 10 characters.)

The problem is: ob-clojure doesn't know about the :deps header! Furthermore, CIDER doesn't know about the dependencies I want to add.

Luckily there is a hack to make this work.

First, let's redefine ob-clojure's runner function:

(defun ob-clojure-eval-with-cider (expanded params)
  "Evaluate EXPANDED code block with PARAMS using cider."
  (condition-case nil (require 'cider)
    (user-error "cider not available"))
  (let ((connection (cider-current-connection (cdr (assq :target params))))
        (result-params (cdr (assq :result-params params)))
        result0)
    ;; Andrea: below is where we add dependencies!
    (unless connection (let ((my/cider-extra-deps (alist-get :deps params))) (sesman-start-session 'CIDER))) 
    (if (not connection)
        ;; Display in the result instead of using `user-error'
        (setq result0 "Please reevaluate when nREPL is connected")
      (ob-clojure-with-temp-expanded expanded params
        (let ((response (nrepl-sync-request:eval exp connection)))
          (push (or (nrepl-dict-get response "root-ex")
                    (nrepl-dict-get response "ex")
                    (nrepl-dict-get
                     response (if (or (member "output" result-params)
                                      (member "pp" result-params))
                                  "out"
                                "value")))
                result0)))
      (ob-clojure-string-or-list
       ;; Filter out s-expressions that return nil (string "nil"
       ;; from nrepl eval) or comment forms (actual nil from nrepl)
       (reverse (delete "" (mapcar (lambda (r)
                                     (replace-regexp-in-string "nil" "" (or r "")))
                                   result0)))))))

The line I commented extracts the list of dependencies and sets a local binding my/cider-extra-deps.

All is left is to make CIDER use those:

;; support a :deps header in ob-clojure blocks (can't work for bb and nbb, because they manage deps natively)
(defcustom my/cider-extra-deps nil "Extra deps to add to cider startup")

(defun my/cider-add-extra-deps (orig-fun &rest args)
  (append (apply orig-fun args) my/cider-extra-deps))

(advice-add 'cider--jack-in-required-dependencies :around #'my/cider-add-extra-deps)

Advicing cider--jack-in-required-dependencies is a bit fragile because is a non-public function, still for a hack this is enough.

The idea is to inject the dependencies we need into the startup dependencies that CIDER injects to work.

Loading these few lines makes the example above work nicely and is making my explorations much simpler!

Hope this will help you as well.

Happy exploring!