continuation demo.

cont.scm

(define call/cc call-with-current-continuation)

;;;;;;;;;;;;;;
(call/cc (lambda (cont) 2)) ; 2, cont do nothing

(call/cc (lambda (cont) (cont 2))) ; 2, cont do nothing

(* 3 (call/cc (lambda (cont) 
                (+ 1 2)))) ; 9, cont is not used 

(* 3 (call/cc (lambda (cont) 
                (cont 2)))) ; 6, cont = (* 3 ?) => (cont 2) = (* 3 2)

(* 3 (call/cc (lambda (cont)
                (+ 1 (cont 2))))) ; still 6, cont = (* 3 ?) => (cont 2) = (* 3 2)

;;;;;;;;;;;;;;

(define k 'void)

(+ 1 (call/cc (lambda (cont)
                (set! k cont) ;; set x to the cont (+ 1 ?) ...
                2)))

(k 3) ;; (+ 1 3) -> 4
(+ 100 (x 3)) ;; still 4, cont = (+ 100 ?)... is replaced by (+ 1 ?)...

;;;;;;;;;;;;;;

(define (fact n)
  (if (= n 1)
      1
      (* n (call/cc (lambda (cont) 
                      (cont (fact (- n 1))))))))
(fact 5) ; cont = (* n (* (- n 1) (* (- n 2) ...? ))) => n * (n - 1) ... 1 

(define (fact n)
   (let ((r 1) (k 'void))
      (call/cc (lambda (cont) 
                 (set! k cont) ))
      (set! r (* r n))
      (set! n (- n 1))
      (if (= n 1) 
          r 
          (k 'rec)))) ; 'rec does nothing , cont must be called by more than 1 arg.
(fact 5) 
; (k 'rec) => process below the call/cc ...
;; (set! r (* r n))
;; (set! n (- n 1))
;; (if (= n 1) 
;;     r 
;;     (k 'rec))
;          =>  r = n * (n - 1) ... 1 

;;;;;;;;;;;;;;

(define (foo)
  (let ((x (call/cc (lambda (cont)
                      (display "called")
                      (cont 5) ; cont is outer of the call/cc
                      (display "not called")))))
    (display "called2")
    x))

(foo) ; called called2 5