continuation hackery

gistfile1.rkt

#lang racket

;; 03. pvar a = 0;
;; 04. [lbl] topOfLoop:
;; 05. a++;
;; 06. if (a < 292 ) {
;; 07. goto topOfLoop;
;; 08. }

(let ((a 0))
  ;; (let/cc k k) returns the continuation at this point, which is to say that
  ;; calling (top-of-loop VALUE) will return to the let form and bind
  ;; top-of-loop to VALUE and then do the set! and the if, again
  (let ((top-of-loop (let/cc k k)))
    (set! a (add1 a))
    (if (< a 292)
        ;; we want to go back to the top of the loop and we want the label to
        ;; stay the same. we could change the continuation as we're going, but
        ;; that's not how labels work
        (top-of-loop top-of-loop)
        ;; when we're finally done, just return some value
        (list 'done a))))

(define-syntax lbl
  (syntax-rules ()
    ((_ name)
     (define name (let/cc k k)))))

(define-syntax goto
  (syntax-rules ()
    ((_ label-name)
     (label-name label-name))))


(define (tryit)
  (define a 0)
  (lbl topOfLoop)
  (set! a (add1 a))
  (if (a . < . 292)
      (goto topOfLoop)
      (list 'done a)))

(tryit)

;; or

(let ((a 0))
  (lbl topOfLoop)
  (set! a (add1 a))
  (if (a . < . 292)
      (goto topOfLoop)
      (list 'done a)))

;; or

(define (tryit)
  (define a 0)
  (lbl topOfLoop)
  (set! a (add1 a))
  (when (a . < . 292)
    (goto topOfLoop))
  (list 'done a))

;; or

(let ((a 0))
  (lbl topOfLoop)
  (set! a (add1 a))
  (when (a . < . 292)
    (goto topOfLoop))
  (list 'done a))

You could also implement lbl and goto like this, with the slight exception that you no longer capture the binding in the continuation.

(define-syntax lbl
  (syntax-rules ()
    ((_ name)
     (define name #f)
     (let/cc k (set! name k)))))

(define-syntax goto
  (syntax-rules ()
    ((_ label-name)
     (label-name))))