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

(define fac
  (lambda (n)
    (if (zero? n)
	1
	(* n (fac (- n 1))))))


;;; Add a continuation parameter and apply it to the body:
(define fac
  (lambda (n k)
    (apply-cont k (if (zero? n)
		      1
		      (* n (fac (- n 1)))))))


;;; Drive the continuation in. (zero? n) is simple:
(define fac
  (lambda (n k)
    (if (zero? n)
	(apply-cont k 1)
	(apply-cont k (* n (fac (- n 1)))))))


;;; Use Capp to move the recursive call to fac up:
(define fac
  (lambda (n k)
    (if (zero? n)
	(apply-cont k 1)
	(fac (- n 1) ...

;;; Encapsulate the continuation k in a continuation that
;;; tells you to multiply two values:	     
(define fac
  (lambda (n k)
    (if (zero? n)
	(apply-cont k 1)
	(fac (- n 1) (multiplication-cont n k)))))


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


(define apply-cont
  (lambda (cont val)
    (cases continuation cont
	   [halt-cont ()
		      val]
	   [multiplication-cont (n next-cont)
				(apply-cont next-cont (* val n))])))