;; ** Fake web interaction library (to be used with manual code CPS-ing
;; ** examples)

#lang racket

(define error raise-user-error)

(define (nothing-to-do ignored)
  (error 'REAL-ERROR "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/k prompt 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)))