Experiment with persistent continuations

counters-with-countinuations.rkt

#lang racket
;; Experiment with persistent continuations

(require db)

;; read-char* : like read-char but ignores \r and \n
(define (read-char* [in (current-input-port)])
  (let ([v (read-char in)])
    (case v
      [(#\newline #\return) (read-char* in)]
      [else v])))

;; A simple interactive counter.
;; Type + to increment, - do decrement, anything else to stop.
;; When stopped, the counter returns a continuation which
;; let's you continue using your counter from where you stopped.
(define (counter [init 0])
  (displayln init)
  (let/ec e
    (counter
     (+ init
        (let/cc k
          (case (read-char*)
            [(#\+) 1]
            [(#\-) -1]
            [else (displayln "bye...") (e k)]))))))

(define sqlc (sqlite3-connect #:database 'memory))
(query-exec sqlc "create table counters (continuation)")
(query-exec sqlc "insert into counters values (?)"
            (with-input-from-string "++++-+--." (thunk (counter))))
(query-list sqlc "select continuation from counters")