Skip to content

Instantly share code, notes, and snippets.

@base698 base698/
Last active Mar 11, 2016

What would you like to do?
update-in, get-in, and assoc-in

update-in, assoc-in, get-in: Not using these functions is doing it wrong

I recently went back and looked at some clojure I wrote when I was learning.


(def x {:a 1 
        :b {:c 3}})

So for a case like this, where I wanted to increment {:c 3} and have the change persist in x I was doing code like this:

(let [val (:c (x :b))
      updated (inc val)]
      (set! x (assoc x :b (assoc (:b x) :c updated))))

The correct way would be this:

(def x (atom {:a 1 :b {:c 3}}))

; To run the update function:
(swap! x update-in [:b :c] inc)

; To just swap the value with seven:
(swap! x assoc-in [:b :c] 7)

Atom is hard to follow and realize why you need it in the docs, but it has to do with "software transactional memory". It's basically a way to get transactions of the database variety with setting variables. Useful for when you have state to update, like in a game world without being a pure function Nazi about state like Haskell. Atoms are related to refs, so you may see them mentioned together.

Happy clojuring....

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.