Question

Is it possible to add another function procC in here so that the sequence of evaluation is procA->procB->procC->procA … ?

(define (procA another-fun)
  (let loop ((n 5))
    (display "In Proc A \n")
    (set! another-fun (call/cc another-fun))
    (when (> n 0)
      (loop (- n 1)))))

(define (procB another-fun)
  (let loop ((n 5))
    (display "In Proc B  \n")
    (set! another-fun (call/cc another-fun))
    (when (> n 0)
      (loop (- n 1)))))

Answer 1

From “The Scheme Programming Language”

http://www.scheme.com/tspl4/further.html#./further:h3

(define lwp-list '()) ; SO's highlighter gets confused
(define lwp
  (lambda (thunk)
    (set! lwp-list (append lwp-list (list thunk)))))

(define start
  (lambda ()
    (let ([p (car lwp-list)])
      (set! lwp-list (cdr lwp-list))
      (p))))

(define pause
  (lambda ()
    (call/cc
      (lambda (k)
        (lwp (lambda () (k #f)))
        (start)))))


(lwp (lambda () (let f () (pause) (display "h") (f))))
(lwp (lambda () (let f () (pause) (display "e") (f))))
(lwp (lambda () (let f () (pause) (display "y") (f))))
(lwp (lambda () (let f () (pause) (display "!") (f))))
(lwp (lambda () (let f () (pause) (newline) (f))))
(start)  hey!
         hey!
         hey!
         hey!