Help: producer/consumer fiber example

[uvtc]

I want to create a classic producer/consumer using fibers. The producer fills up an array until it's full (3 items), and then passes control to the consumer which uses up elements one by one until the array is empty, and then passes control back to the producer. Control just bounces back and forth unless/until I break in and tell it to stop fooling around. I've got this so far:

#!/usr/bin/env janet

(def arr @[])


(defn producer
  ``Fills up `arr` with three "@" signs.``
  []
  (while (< (length arr) 4)
    (pp arr)
    (array/push arr "@"))
  (print "---")
  (yield))


(defn consumer
  ``Uses up the elements in `arr` until empty.``
  []
  (while (not (empty? arr))
    (array/pop arr)
    (pp arr))
  (print "---")
  (resume producer))


(defn main
  [&]
  (def fbr (fiber/new producer))
  (resume fbr))

but I know it's not right, and anyway, it just ends after producer finishes:

$ ./foo.janet 
@[]
@["@"]
@["@" "@"]
@["@" "@" "@"]
---

How can I fix this and get it to work? I think I want producer to yield to consumer, and consumer to likewise yield to producer...