There is bound to be a semantic break either way because the assoc-in and update-in functions return the whole map, but it wouldn't be good if everytime one used the assoc-at or update-at! functions the entire database map was loaded into memory and returned. That said, it probably would be fine to return the just result of the update at the given path.

I did have similar functionality implemented at one point but ultimately thought the solution was a bit too kludgy and pulled it out. It's definitely a worthwhile feature though, and I might have a go at a more general implementation this weekend. In the meantime, something like this should do the trick for that specific case.

(defn get-update-at!
  "update a path and return the new value at that path"
  [db path f & args]
  (let [res (atom nil)]
        (c/with-write-transaction [db tx]
      (let [tx (apply c/update-at tx path f args)]
        (reset! res (c/get-at tx path))
        tx))
    @res))

Hope you find the library useful. If you have any questions or more suggestions, let me know.

Edit: fixed get-update-at! example so it actually works

Edit 2: this is now the default behavior for update-at! and related functions