Question

I would like to watch for changes to different parts of a Clojure Hash map (accessed via a STM ref), which forms quite a large tree, and on changes to those parts I would like to invoke some registered listeners. How can I do this in clojure as I understand that “add-watch” only works on an entire reference?

Answer 1

I, too, would watch the whole tree and check subsets with get-in. You can quickly test whether the subtree has been changed by using an identical? test against the previous state. Something like

(defn change-tester [tree path]
  (let [orig (get-in tree path)]
    (fn [tree]
      (not (identical? (get-in tree path) orig)))))

I don’t use watchers very often so I don’t know the syntax, but you could attach the above function somehow, I’m sure.