#lang racket
(require "chez-init.rkt")
(require racket/trace)

(define length
  (lambda (ls)
    (cond [(null? ls) 0]
	  [else (+ 1 (length (cdr ls)))])))

;;; Add a continuation parameter and apply it to the body:
(define length-cps
  (lambda (ls k)
    (apply-cont k (cond [(null? ls) 0]
			[else (+ 1 (length (cdr ls)))]))))

;;; Drive the continuation in. (null? ls) and else are simple:
(define length-cps
  (lambda (ls k)
    (cond [(null? ls) (apply-cont k 0)]
	  [else (apply-cont k (+ 1 (length (cdr ls))))])))

;;; Use Capp to move the recursive call to length up:
(define length-cps
  (lambda (ls k)
    (cond [(null? ls) (apply-cont k 0)]
	  [else (length-cps (cdr ls) ...

;;; Encapsulate the continuation k in a continuation that
;;; tells you to add two values:	     
(define length-cps
  (lambda (ls k)
    (cond [(null? ls) (apply-cont k 0)]
	  [else (length-cps (cdr ls) (add-cont 1 k))])))
			

(define-datatype continuation continuation?
  (halt-cont)
  (add-cont
   (n number?)
   (next-cont continuation?)))


(define apply-cont
  (lambda (cont val)
    (cases continuation cont
	   [halt-cont ()
		      val]
	   [add-cont (n next-cont)
		     (apply-cont next-cont (+ val n))])))