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]
2014-03-21 0:57 GMT+01:00 Stephen Gilardi <scgila...@gmail.com>:
> 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
>
> On Mar 20, 2014, at 6:28 PM, Jakub Holy <jakub.h...@iterate.no> wrote:
>
> 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!
>
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--
*Forget software. Strive to make an impact, deliver a valuable change.*
* (**Vær så snill og hjelp meg med å forbedre norsken **min –** skriftlig
og muntlig. Takk!**)*
Jakub Holy
Solutions Engineer | +47 966 23 666
Iterate AS | www.iterate.no
The Lean Software Development Consultancy
- http://theholyjava.wordpress.com/ -
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