Am So., 9. Okt. 2022 um 14:46 Uhr schrieb Shiro Kawai <shiro.ka...@gmail.com>: > > > > 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 > ...))
Thanks; with my definition, I would have to code it like (unwind-protect protected-form (begin (values) cleanup-form ...)) as otherwise definitions would be allowed by the definition making it too easy to write non-portable code. >> >> >> > 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? Your example prints "135" and evaluates to 11. The composable continuation captured in the variable K is an ordinary procedure. Invoking it does not remove any continuation frames. In particular, the dynamic extent of the THUNK in dynamic-wind is not left. On the other hand, coroutines implemented through call/cc would not be allowed because this would mean that a non-composable continuation is captured within the dynamic extent of call-with-continuation-barrier (which itself is fine) and then later reinstated outside of that dynamic extent (which would raise a &continuation condition object). When SRFI 226 is available, the thread mechanism should be used to implement coroutines. This does not inter with dynamic-wind and unwind-protect. (In fact, coroutines through first-class continuations are often too costly because of the presence of dynamic-wind.) Feel free to modify and repost your example in case not everything is clear. Thanks, Marc
