Re: Continuations: possible newbie question

[Marius Vollmer <mvo@zagadka.ping.de>]

Matt Hellige <matt@immute.net> writes:

> On the other hand, I'll probably still not end up using this technique
> to implement my call-backs, given the performance penalties and so on.
> I hope I can just shuffle things around a bit in my native code...

I would not worry about performance too early.  If you invoke one
continuation for every key press by the user, that should be no
problem.

Here is a little Gtk+ program that shows off continuations.  I still
find it quite instructive...

;; This is an example of how to use continuations with the Gtk event
;; loop.  It implements a dialog box that looks to the programmer like
;; it was modal, and to the user like it was non-modal.  The function
;; `yes-or-no?' that implements this dialog box only returns to the
;; caller when the user has aswered the dialog.  The user however can
;; pop up any number of these dialog boxes and answer them in any
;; order he likes.  The main application stays alive as well.

(use-modules (gtk gtk))

;; The callbacks that have been delayed

(define callbacks '())

;; Our own event-loop.  We remove the callbacks before invoking them
;; so that we don't get confused when the callback reenters the
;; event-loop.

(define (event-loop)
  (cond
   ((not (null? callbacks))
    (let ((c (car callbacks)))
      (set! callbacks (cdr callbacks))
	 (c)
	 (event-loop)))
   ((not (zero? (gtk-main-iteration)))
    (event-loop))))

;; Connect to a signal and arrange for PROC to be consed onto
;; CALLBACKS when the signal is emitted.

(define (gtk-signal-connect-delayed obj sig proc)
  (gtk-signal-connect obj sig 
		      (lambda () (set! callbacks (cons proc callbacks)))))

;; Now for the continuation part.  To implement the non-modal dialog box
;; that can be used from your code like a modal one, we save the
;; continuation of the YES-OR-NO? invokation and reenter the event-loop
;; (after popping up the window).  When a button has been clicked, we
;; destroy the window and invoke the saved continuation with the
;; appropriate return value.

(define (yes-or-no? title)
  (call-with-current-continuation
   (lambda (cont)
     ;; Now CONT is the part of the program that receives our
     ;; return value.

     (let* ((d (gtk-window-new 'dialog))
	    (v (gtk-vbox-new #f 0))
	    (h (gtk-hbox-new #t 0))
	    (l (gtk-label-new title))
	    (s (gtk-hseparator-new))
	    (y (gtk-button-new-with-label "Yes"))
	    (n (gtk-button-new-with-label "No"))

	    (answer (lambda (val)
		      (gtk-widget-destroy d)

		      ;; Here we return to our caller after the
		      ;; dialog has been destroyed.
		      (cont val))))

       (gtk-container-add d v)
       (gtk-box-pack-start v l #f #f 0)
       (gtk-box-pack-start v s #f #f 0)
       (gtk-box-pack-start v h #f #f 0)
       (gtk-box-pack-start h y #f #f 0)
       (gtk-box-pack-start h n #f #f 0)
       (gtk-widget-show-all d)

       ;; Bind ANSWER to the "clicked" signals of the action
       ;; buttons.
       (gtk-signal-connect-delayed y "clicked" (lambda () (answer #t)))
       (gtk-signal-connect-delayed n "clicked" (lambda () (answer #f)))

       ;; Reenter the event-loop.  You can think of this as a goto.
       (event-loop)))))

(define w (gtk-window-new 'toplevel))
(define b (gtk-button-new-with-label "Ok!"))
(gtk-container-add w b)

(gtk-signal-connect-delayed 
 b "clicked" 
 (let ((i 0))
   (lambda () 
     (set! i (1+ i))
     ;; capture I in a local environment so that nobody can alter
     ;; it while YES-OR-NO? does its thing.
     (let ((i i))
       ;; Use YES-OR-NO? as if it were a modal dialog.
       (pk i (yes-or-no? (string-append (number->string i)
					": Really?")))))))

(gtk-signal-connect w "destroy" gtk-exit)

(gtk-widget-show b)
(gtk-widget-show w)

(event-loop)

-- 
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