Clojure equivalent of reagent cursor to update nested map and access new value

I find myself doing a lot of (get-in (swap! nested-map update-in [:a :b] update-fn) [:a :b]) to update a new value and then access it atomically. It seems a bit clunky to me - is there a more elegant way of doing this? reagent has the cursor idea which works nicely as once you define the cursor the above just becomes (swap! cursor update-fn).

I found https://github.com/rlewczuk/clj-cursor which seems to be what I want but maybe overkill?

Thanks,

Solution

Implementing your own cursor based on the clojure.lang.IAtom interface is fairly straight-forward:

(defn my-cursor [target-atom path]
  (reify clojure.lang.IAtom
    (swap [_this f] (get-in (swap! target-atom update-in path f) path))))

Note that clojure.lang.IAtom may be considered an implementation detail of Clojure so whether you want to depend on it is up to you.

Here is an example:

(def nested-map (atom {}))
(defn update-fn [x] (inc (or x 0)))

(def c (my-cursor nested-map [:a :b]))

(swap! c update-fn)
;; => 1

(deref nested-map)
;; => {:a {:b 1}}

Implementing IDeref may also be useful:

(defn my-cursor [target-atom path]
  (reify clojure.lang.IAtom
    (swap [_this f] (get-in (swap! target-atom update-in path f) path))
    clojure.lang.IDeref
    (deref [_this] (get-in (deref target-atom) path))))

Even if you don't have to implement the other methods of the interfaces, it is nevertheless a good idea.

A previous version of this answer used proxy but reify is probably better.