How to use guardians

["Kjetil S. Matheussen" <k.s.matheussen@notam02.no>]

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))))