How to use guardians

How to use guardians

[Kjetil S. Matheussen]

Hi,

I'm trying to use guardians, but have a little bit of
trouble understanding how they work.

I have made a function "add-guardian-object" which
takes an object "object" and a function "cleanup-func".
"cleanup-func" is called before the object is freed.

Here is my implementation of "add-guardian-object":

   (define das-guardian (make-guardian))

   (define (add-guardian-object object cleanup-func)
     (let loop ((guard (das-guardian)))
       (when guard
         ((car guard) (cadr guard))
         (loop (das-guardian))))
     (das-guardian (list cleanup-func object)))



First try
---------
I run a test evaluating the following expressions:

   (define alist (list 'unique-name 2 3 4))
   (add-guardian-object alist (lambda (object)
                                (c-display object "was garbage collected")))
   (gc)
   (begin das-guardian)


But "(begin das-guardian)" shows:

   #<guardian 8a8a8c0 (reachable: 0 unreachable: 1)>

Which means that the reference to "alist" is now unreachable.
But why is that so? "alist" is a global reference
and should therefore definitely be reachable.

Is this a bug in guile or have I misunderstood something?



Second try
-----------
Does "unreachable" mean that it is only unreachable from
within the "add-guardian-object" function? In case,
why doesn't it work just changing "add-guardian-object"
into a mcaro like this?

(define-macro (add-guardian-object object cleanup-func)
   (define loop (gensym))
   `(begin
      (let ,loop ((guard (das-guardian)))
 	  (when guard
 	    ((car guard) (cadr guard))
 	    (,loop (das-guardian))))
      (das-guardian (list ,cleanup-func ,object))))

Re: How to use guardians

[Thien-Thi Nguyen]

() "Kjetil S. Matheussen" <k.s.matheussen@notam02.no>
() Thu, 13 Mar 2008 15:12:10 +0100 (CET)

       (das-guardian (list cleanup-func object)))

Here, you add the compound object (constructed using `list'),
not just `object'.  Is that what you want do, really?

thi

Re: How to use guardians

[Kjetil S. Matheussen]

Thien-Thi Nguyen:
>         (das-guardian (list cleanup-func object)))
>
>  Here, you add the compound object (constructed using `list'),
>  not just `object'.  Is that what you want do, really?


Ah, of course. Thank you very much. Also for the mysteriously
quick reply. :-)

I have now changed "add-guardian-object" into:

    (define das-guardians '())
    (define (add-guardian-object object cleanup-func)
      (set! das-guardians
            (remove (lambda (das-guardian)
                      (let ((guard ((cadr das-guardian))))
                        (and guard
                             (begin
                               ((car das-guardian) guard)
                               #t))))
                    das-guardians))
      (push! (list cleanup-func
                   (let ((guardian (make-guardian)))
                     (guardian object)
                     guardian))
             das-guardians))



And this seems to work. Great!