Re: foreign objects and the garbage collector

[adriano <randomlooser@riseup.net>]

Hi Maxime,

Il giorno dom, 19/09/2021 alle 20.23 +0200, Maxime Devos ha scritto:
> Olivier Dion via General Guile related discussions schreef op zo 19-
> 09-2021 om 14:11 [-0400]:
> > On Sun, 19 Sep 2021, adriano <randomlooser@riseup.net> wrote:
> > > > If you want to avoid the problem, you should explicitely bind
> > > > and
> > > > call the gps-close function and not rely on the garbage
> > > > collector to
> > > > do it for you. You can use dynamic-wind to open and close
> > > > resources as needed.
> > > 
> > > It'd be so nice to have an example
> > 
> > (define (with-my-resource token proc)
> >   (let ((resource #f))
> >     (dynamic-wind
> >       (lambda ()
> >         (set! resource (open-my-resource% token)))
> > 
> >       (proc resource)
> > 
> >       (lambda ()
> >         (when resource
> >           (close-my-resource% resource))))))
> > 
> > (with-my-resource "some-internal-token" (lambda ()))
> 
> FWIW, this doesn't work well with continuations, e.g. if you use
> guile-fibers
> for concurrency.  I'm not familiar with 'libgps', so I'm not sure if
> it would
> work for you, but another method for implementing with-my-resource
> could be to
> use the exception handling mechanism to  call 'close-my-resource' on
> both
> normal exits and abnormal exits.
> 
> Note that this alternative method does _not_ close the resource when
> calling
> an escape continuation (let/ec (with-my-resource  .. (lambda (r) (ec)
> (unreachable)))),
> so you might want to use something like guardians and after-gc-hook
> to eventually
> free the resource.
> 
> (Here, close-my-resource is like close-my-resource% except it doesn't
> do anything
> if the resource is already closed.)
> 
> Greetings,
> Maxime.


Well, it'd be wonderful to have an example of what you're describing
here ! 🙂️

The new exceptions system is still quite obscure 🤷️