Green threads in the browser in 20 lines of Wat

This page shows 5 independent, cooperatively scheduled Wat green threads (view source for full Wat code).

Each thread has an ID and is defined as a function that loops forever, repeatedly printing its ID, and then sleeping for a (randomly long) while.

(define (run-thread (id String))
  (loop
    (@appendChild (.body $document)
                  (@createTextNode $document (+ "Active thread: " id " ")))
    (sleep (* 1000 (@random $Math)))))

So, how can a Wat thread sleep inside a loop when JavaScript forbids sleeping? Why, with good ole delimited continuations:

To spawn a thread, I just wrap the call to RUN-THREAD in a prompt (which is just a unique object used as an identifier):

(define default-prompt 'default-prompt)

(define (spawn-thread (id String))
  (push-prompt default-prompt
    (run-thread id)))

Where it gets interesting is the SLEEP function which captures the continuation up to the prompt, and sets up a callback with JavaScript's setTimeout that will reinstall the continuation later:

(define (sleep (ms Number))
  (take-subcont default-prompt k
    (define (callback . #ignore)
      (push-prompt-subcont default-prompt k))
    ($setTimeout (js-callback callback) ms)))

So, first, SLEEP aborts up to and including the default prompt using TAKE-SUBCONT. It receives the continuation in the variable K. Once it has K, it defines a CALLBACK function, that will reinstall the default prompt with PUSH-PROMPT, and then continue with K again with PUSH-SUBCONT. All that's left is to give this callback to setTimeout.

Then I can spawn independent threads:

(spawn-thread "thread-1")
(spawn-thread "thread-2")
(spawn-thread "thread-3")
(spawn-thread "thread-4")
(spawn-thread "thread-5")

Wat is very new, but it's already practical for adding concurrency and metaprogramming to JavaScript. Deployment is very easy. Include the single wat.js file, put some Lisp code in a <script> tag, and run it.