Extending built-in Clojure protocols is not easy: usually there are tens of them for simple stuff like map or atom, with multiple methods in each. And then you’ll have to do it again for ClojureScript.

Look no further! With extend-clj only need to implement the absolute semantic minimun, and the library will take care of the rest:

(extend-clj.core/deftype-atom Cursor
  [*atom path]

  (deref-impl [this]
    (get-in @*atom path))

  (compare-and-set-impl [this oldv newv]
    (let [atom   @*atom
          before (if (= oldv (get-in atom path))
                   atom
                   (assoc-in atom path oldv))
          after  (assoc-in atom path newv)]
      (compare-and-set! *atom before after))))

To construct new Cursor, use

(->Cursor (atom {:x {:y 1}}) [:x :y])

Result will behave as close to clojure.lang.Atom as possible: swap!/reset!/swap-vals!/reset-vals!/validators/meta/watchers — all of it will work as expected.

In addition to that, ILookup is implemented so you can access fields:

(let [a (atom {:x 1})
      cursor (->Cursor a [:x])]
  (:atom cursor) ; => a
  (:path cursor) ; => [:x])

Supported in Clojure:

• clojure.lang.Atom

Supported in ClojureScript:

• cljs.core.Atom

Known issues:

• reset-vals! and swap-vals! don’t work in ClojureScript

Add this to deps.edn:

io.github.tonsky/extend-clj {:mvn/version "0.1.0"}

Copyright © 2023 Nikita Prokopov

Licensed under MIT.