How to update an atom & return the change?

[Jakub Holy]

I have couple of times run into a situation where I want to update a state map held in an atom

and return the change, not the new value. I haven't found a good way to do it so either I am missing

something obvious or there are more idiomatic ways to achieve what I need. Could you advise me?

A concrete example: In ma webapp I want to assign a unique random ID to each user. Creating that ID is simple:

(def state (atom {:teams {}}))

;; Remove already used IDs from a lazy seq of random IDs (=> unique), take the 1st one

(defn unique-rand-id [id-set]

  (first (remove id-set (repeatedly #(rand-int Integer/MAX_VALUE))))))

;; Add a new team with a unique random ID to the teams map

(defn make-team [teams]

  (let [id (unique-rand-id (set (keys teams)))]

    (assoc teams id {})))

;; Create a new team; TODO: How to get the new team's ID?!

(swap! state #(update-in % [:teams] make-team))

So I can generate and remember a new unique random ID but there is no way to find out

what ID it was (I cannot just take diff of state before and after since other threads could

have also added new IDs to it in the meanwhile.)

Any advice is appreciated. Thank you!

[Stephen Gilardi]

There was a stackoverflow question recently that requested a solution for a similar problem:

https://stackoverflow.com/questions/22409638/remove-first-item-from-clojure-vector-atom-and-return-it

One solution there is similar to this:

(defn swap*!

  "Like swap! but returns a vector of [old-value new-value]"

  [atom f & args]

  (loop [old-value @atom]

    (let [new-value (apply f old-value args)]

      (if (compare-and-set! atom old-value new-value)

        [old-value new-value]

        (recur @atom)))))

This will return the correct old-value and new-value which you can diff.

Another note: 

(swap! state #(update-in % [:teams] make-team))

can be written more succinctly:

(swap! state update-in [:teams] make-team)

—Steve

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[Jakub Holy]

Thanks a lot, Stephen!

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[Jakub Holy]

For the interested, this is my (certainly pretty imperfect) solution for changing a value in a state map and returning the old and new value (that can be safely diffed to get the change):

(defn swap-in!

  "Combination of update-in and swap! returning the value at the path before and after."

  [atom path f & args]

  (loop [] 

    (let [old-a @atom

          old-val (get-in old-a path)

          new-val (apply f (cons old-val args))

          new-a (assoc-in old-a path new-val)]

      (if (compare-and-set! atom old-a new-a)

        [old-val new-val]

        (recur)))))

;; example:

(swap-in! (atom {:k 1}) [:k] + 2) ;; => [1 3]

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Jakub Holy
Solutions Engineer | +47 966 23 666
Iterate AS | www.iterate.no
The Lean Software Development Consultancy
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[Jarrod Swart]

Just to add to this because I ran across this reading some code a few days ago:

https://github.com/overtone/at-at/blob/master/src/overtone/at_at.clj#L43-51