Map, but with a delay

map_interval.clj

(defn map-interval
  "Like `map` -- apply function `f` to each element in `col` in turn and
   return the values returned in sequence, but in a single thread and pausing
   between each invocation of `f`.
   
   Useful for example in circumstances where you're querying a rate-limited
   service."
  [f col interval]
  (let [a (atom ())]
    (doseq [x col]
      (Thread/sleep interval)
      (swap! a conj (f x)))
    (reverse (deref a))))