On Sun, Oct 9, 2022 at 1:21 AM Marc Nieper-Wißkirchen <marc.nie...@gmail.com>
wrote:
> Am So., 9. Okt. 2022 um 12:41 Uhr schrieb Shiro Kawai <
> shiro.ka...@gmail.com>:
> >
> > 1. Common Lisp's unwind-protect allows multiple cleanup forms.
> Personally I don't see much benefit of allowing it, but to avoid
> confusions, we may adopt the same API.
>
> That's a good point. (I missed initially that there can be more than
> one cleanup form.) Does CL execute the cleanup forms in order of their
> appearance?
>
Yes. It's the same as (unwind-protect protected-form (begin cleanup-form
...))
>
> > 2. This is more involved. The cleanup forms of unwind-protect are
> literally for cleaning up, e.g. releasing resources. This poses an issue
> in Scheme; with continuations, control can reenter protected-form.
> Executing cleanup-forms in the after thunk of dynamic-wind invalides such
> applications as coroutines.
>
> Please note the appearance of `call-with-continuation-barrier` in my
> proposed definition. It is defined in SRFI 226. It is an error to
> reinstate a continuation barrier, so escaping continuation in my
> definition are allowed (triggering the cleanup forms), but returning
> through a captured continuation is not.
>
> I think I don't quite understand that part. Is this allowed? If so, does
it output "15135" and returns 11?
(call-with-continuation-prompt
(lambda ()
(+ 1
(call-with-composable-continuation
(lambda (k)
(call-with-continuation-barrier
(lambda ()
(dynamic-wind
(lambda () (write 1))
(lambda () (write (k 2)) 10)
(lambda () (write 5))))))))))
If this isn't allowed, coroutines that capture continuation outside of
unwind-protect can't be invoked within unwind-protect?
...
