parallel-chan-map: Read in-chan and put result of `(apply f in-chan-value args)` on out-chan, running items in "parallel" and yielding results out-of-order. Works with Clojure or Clojurescript.

parallel-chan-map.clj

(ns parallel-chan-map
  (:require-macros [cljs.core.async.macros :refer (go alt!)])
  (:require [cljs.core.async :refer (>! close! put! chan )]))

(defn parallel-chan-map
  "Read in-chan and put result of `(apply f in-chan-value args)` on out-chan,
  potentially running items in parallel and yielding results out-of-order.

  Function f must return something takeable (e.g. a go-block, channel, promise),
  in essence spawning a \"subprocess\" which this function manages as a pool
  internally."
  [in-chan out-chan f & args]
  (go (loop [chans [in-chan]]
        (when-not (empty? chans)
          (let [[v c] (alts! chans)]
            (if (nil? v)
              (recur (remove #{c} chans))
              (if (identical? c in-chan)
                (recur (conj chans (apply f v args)))
                (do (>! out-chan v) (recur chans))))))))
  out-chan)