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How to use guardians
From: |
Kjetil S. Matheussen |
Subject: |
How to use guardians |
Date: |
Thu, 13 Mar 2008 15:12:10 +0100 (CET) |
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))))
- How to use guardians,
Kjetil S. Matheussen <=