core.async

async.clj

(let [start (.getTime (java.util.Date.))
        c-inputs (async/chan)
        c-outputs (async/chan)]

    (async/onto-chan c-inputs (range 10))

    (async/pipeline-async 1
                          c-outputs
                          (fn [x c-out]
                            (async/thread
                              (Thread/sleep 2000)
                              (async/put! c-out x)
                              (async/close! c-out)))
                          c-inputs)

    (loop [res (<!! c-outputs)]
      (if (nil? res)
        (println (- (.getTime (java.util.Date.)) start) "done")
        (do
          (println (- (.getTime (java.util.Date.)) start) res)
          (recur (<!! c-outputs))))))

;; Using [org.clojure/core.async "0.3.443"]
;; Output indicates that three async tasks are running at the same time, even though `pipeline-async` n = 1:
;;
;; 2004 0
;; 2004 1
;; 2004 2
;; 4005 3
;; 4005 4
;; 4006 5
;; 6007 6
;; 6009 7
;; 6009 8
;; 8009 9
;; 8010 done
          
          

Issue here: https://dev.clojure.org/jira/browse/ASYNC-163 Thanks hiredman in clojure slack channel for the pointer.