Back-of-the-envelope cooperative threading library in Racket

coroutines.rkt

#lang racket

;;; adapted from: https://youtu.be/BAMtstt3Jp8?t=1466

(define threads '())

(define (extend lst item)
  (append lst (list item)))

(define (spawn func)
  (set! threads
        (extend threads (λ ()
                          (abort-current-continuation
                           (default-continuation-prompt-tag)
                           (func))))))

(define (switch)
  (unless (empty? threads)
    (define next (first threads))
    (set! threads (rest threads))
    (next)))

(define (yield)
  (let/cc k
    (spawn k)
    (switch)))

;; Example usage
(define (looper id n)
  (for ([i (in-range n)])
    (printf "[~a ~a] " i id)
    (yield))
  (switch))

(spawn (λ () (looper 'a 5)))
(looper 'b 7)