Call some function asynchronously, reduce the results, and call the callback with the result

gistfile1.clj

(ns user
  (:require [cljs.core.async :refer [chan <! >!]]
            [cljs.core.async :as async]
  (:require-macros [cljs.core.async.macros :refer [go]]))

(defn async-reduce
  "Reduce a value over a collection of functions asynchronously
   and merge the results.

   Example:
   (async-reduce + 0 [inc inc] println)"
  [merge-fn init-val fns callback]
  (let [work (count fns)
        ch (chan work)
        counter (chan 1)]
    (go (>! counter 0))
    (doseq [f fns]
      (go (let [c (<! counter)]
             (if (= (inc c) work)
               (do (close! counter)
                   (close! ch)
                   (callback (<! (async/reduce merge-fn init-val ch))))
               (do (>! ch (f init-val))
                   (>! counter (inc c)))))))))