;; dynamic-wind

;; A function that safely processes an input file
(define (safe-process input-file process-fn)
  (let ((p (open-input-file input-file)))
    (dynamic-wind
     (lambda () #f)
     (lambda () (process-fn p))
     (lambda () 
       (begin
         (display "closing input port...")
         (newline)
         (close-input-port p))))))

;; Define a processing function that always exits to toplevel.
(define toplevel-exit 'dummy)
(call/cc (lambda (cont) (set! toplevel-exit cont)))
(define (process p) (toplevel-exit))
  
;; Observe that dynamic-wind still executes the code which
;; closes the input port.
(safe-process "lecture14-dynamic-wind.ss" process)