Re: foreign objects and the garbage collector

[Maxime Devos <maximedevos@telenet.be>]

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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.

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