协程

coroutine.rkt

#lang racket
(define ready-queue '())

(define list-append
  (lambda (ls x)
    (if (null? ls)
        (cons x '())
        (cons (car ls) (list-append (cdr ls) x)))))

;; 定义队列操作的函数
(define enqueue list-append)
(define queue-empty? null?)
(define queue-head car)
(define dequeue cdr)

(define yield
  (lambda ()
    (call/cc
     (lambda (cont)
       (if (queue-empty? ready-queue)
           (cont) ;;继续运行下去，yield不生效
           (let ((run (queue-head ready-queue))
                 (newq (enqueue (dequeue ready-queue) cont)))
             (set! ready-queue newq)
             (run)))))))

(define print
  (lambda (x)
    (let loop ((i 0))
             (if (= i 10)
                 42
                 (begin
                   (printf "~a\n" x)
                   (yield)
                   (loop (+ i 1)))))))
(set! ready-queue (enqueue ready-queue (lambda () (print "a"))))
(set! ready-queue (enqueue ready-queue (lambda () (print "b"))))
(print "c")