;; ** Simulation of web interactions with Racket's built-in
;; ** continuation facility

#lang racket

(define error raise-user-error)

(define (nothing-to-do ignored)
  (error 'nothing-to-do "No computation to resume."))

(define resumer (box nothing-to-do))

(define (web-display n)
  (set-box! resumer nothing-to-do)
  (error 'web-display "~s" n))

(define (web-read prompt)
  (let/cc k
    (set-box! resumer k)
    (error 'web-read
           "enter (submit N) to continue the following\n  ~a:"
           prompt)))

(define (submit n)
  ;; to avoid mistakes, we clear out `resumer' before invoking it
  (let ([k (unbox resumer)])
    (set-box! resumer nothing-to-do)
    (k n)))