Clojure fork-join example

fork-join-recur.clj

(ns fork-join-recur
  (:import [java.util.concurrent ForkJoinPool
            ForkJoinWorkerThread ForkJoinTask RecursiveTask]))

;; API
;; ===
(def ^:dynamic ^ForkJoinPool *pool* (ForkJoinPool/commonPool))

(defmacro fork [& body]
  `(let [^ForkJoinTask task# (proxy [RecursiveTask] []
                               (compute []
                                 (do ~@body)))]
     (if (ForkJoinTask/inForkJoinPool)
       (.fork task#)
       (.execute *pool* task#))
     task#))

(defn join [^ForkJoinTask task]
  (.join task))


;; Test
;; ====
(defn rec:factorial [n]
  (fork
   (if (< n 2)
     1
     (*' n (join (rec:factorial (dec n)))))))

(defn factorial [n]
  (join (rec:factorial n)))


#_(

   (factorial 5) ;; => 120
   
   (factorial 5000) ;; => StackOverflow!

   )