new.clj

(defn my-window
  "A version of streams/window that doesn't leak memory."
  [n & children]
  (let [window (atom (vec []))]
    (fn [event]
      (let [w (swap! window (fn [old]
                              (take-last n (conj old event))))]
        (call-rescue w children)))))

old.clj

(defn window
  "A sliding window of the last few events. Calls children with a vector of the
  last n events, from oldest to newest. Example:
  
  (window 5 (combine folds/mean index))"
  [n & children]
  (let [window (atom (vec []))]
    (fn [event]
      (let [w (swap! window (fn [old]
                              (let [new (conj old event)
                                    size (count new)]
                                (if (< n size)
                                  (subvec new (- size n))
                                  new))))]
        (call-rescue w children)))))