Using delimited continuations to implement RAC's signal type

delim-cont-reactive.rkt

#lang scheme
(require racket/control)
(require racket/match)

(define (create-signal didSubscribe)
  (lambda (next error complete)
    (match (shift k (didSubscribe k))
      [(list 'next v)   (next  v)]
      [(list 'error e)  (error e)]
      [(list 'complete) (complete)])))
      
;; helpers

(define (add-subscriber s n e c) (s n e c))
(define (send-next k v) (k (list 'next v)))
(define (send-error k e) (k (list 'error e)))
(define (send-complete k) (k (list 'complete)))

;; example usage. prints "hello, ""world!""done"

(let ([signal (create-signal (lambda (subscriber)
                               (send-next subscriber "hello, ")
                               (send-next subscriber "world!")
                               (send-complete subscriber)))])
     (add-subscriber signal
       (lambda (next)  (displayln next))
       (lambda (error) (displayln (string-append "! " error)))
       (lambda ()      (displayln "done"))))