Periodically calling a function in Clojure

Question

I'm looking for a very simple way to call a function periodically in Clojure.

JavaScript's setInterval has the kind of API I'd like. If I reimagined it in Clojure, it'd look something like this:

(def job (set-interval my-callback 1000))

; some time later...

(clear-interval job)

For my purposes I don't mind if this creates a new thread, runs in a thread pool or something else. It's not critical that the timing is exact either. In fact, the period provided (in milliseconds) can just be a delay between the end of one call completing and the commencement of the next.

Answer 1

If you want very simple

(defn set-interval [callback ms] 
  (future (while true (do (Thread/sleep ms) (callback)))))

(def job (set-interval #(println "hello") 1000))
 =>hello
   hello
   ...

(future-cancel job)
 =>true

Good-bye.

Answer 2

There's also quite a few scheduling libraries for Clojure: (from simple to very advanced)

• at-at
• chime (core.async integration)
• monotony
• quartzite

Straight from the examples of the github homepage of at-at:

(use 'overtone.at-at)
(def my-pool (mk-pool))
(let [schedule (every 1000 #(println "I am cool!") my-pool)]
  (do stuff while schedule runs)
  (stop schedule))

Use (every 1000 #(println "I am cool!") my-pool :fixed-delay true) if you want a delay of a second between end of task and start of next, instead of between two starts.

Answer 3

This is how I would do the core.async version with stop channel.

(defn set-interval
  [f time-in-ms]
  (let [stop (chan)]
    (go-loop []
      (alt!
        (timeout time-in-ms) (do (<! (thread (f)))
                                 (recur))
        stop :stop))
    stop))

And the usage

(def job (set-interval #(println "Howdy") 2000))
; Howdy
; Howdy
(close! job)

Answer 4

The simplest approach would be to just have a loop in a separate thread.

(defn periodically
  [f interval]
  (doto (Thread.
          #(try
             (while (not (.isInterrupted (Thread/currentThread)))
               (Thread/sleep interval)
               (f))
             (catch InterruptedException _)))
    (.start)))

You can cancel execution using Thread.interrupt():

(def t (periodically #(println "Hello!") 1000))
;; prints "Hello!" every second
(.interrupt t)

You could even just use future to wrap the loop and future-cancel to stop it.

Answer 5

I took a stab at coding this up, with a slightly modified interface than specified in the original question. Here's what I came up with.

(defn periodically [fn millis]
  "Calls fn every millis. Returns a function that stops the loop."
  (let [p (promise)]
    (future
      (while
          (= (deref p millis "timeout") "timeout")
        (fn)))
    #(deliver p "cancel")))

Feedback welcomed.

Answer 6

Another option would be to use java.util.Timer's scheduleAtFixedRate method

edit - multiplex tasks on a single timer, and stop a single task rather than the entire timer

(defn ->timer [] (java.util.Timer.))

(defn fixed-rate 
  ([f per] (fixed-rate f (->timer) 0 per))
  ([f timer per] (fixed-rate f timer 0 per))
  ([f timer dlay per] 
    (let [tt (proxy [java.util.TimerTask] [] (run [] (f)))]
      (.scheduleAtFixedRate timer tt dlay per)
      #(.cancel tt))))

;; Example
(let [t    (->timer)
      job1 (fixed-rate #(println "A") t 1000)
      job2 (fixed-rate #(println "B") t 2000)
      job3 (fixed-rate #(println "C") t 3000)]
  (Thread/sleep 10000)
  (job3) ;; stop printing C
  (Thread/sleep 10000)
  (job2) ;; stop printing B
  (Thread/sleep 10000)
  (job1))

Answer 7

Using core.async

(ns your-namespace
 (:require [clojure.core.async :as async :refer [<! timeout chan go]])
 )

(def milisecs-to-wait 1000)
(defn what-you-want-to-do []
  (println "working"))

(def the-condition (atom true))

(defn evaluate-condition []
  @the-condition)

(defn stop-periodic-function []
  (reset! the-condition false )
  )

(go
 (while (evaluate-condition)
   (<! (timeout milisecs-to-wait))
   (what-you-want-to-do)))