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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