I'm inclined to the reference implementation behavior; the last example you
show seems very confusing, especially that the dynamic value of m retrieved
in seemingly the same dynamic environment (on the prompt) is affected by
the parameterization of unrelated parameters. In general you wound't know
what reparameterization is done during the inner calls.
On the other hand, the behavior of Racket (and the current version of
Gauche) can be explained with dynamic-wind.
```
(define c #f)
(define (foo)
(dynamic-wind
(lambda () (print 'pre1))
(lambda ()
(reset
(print 'pre2)
(shift k (print 'cont) (set! c k))
(print 'post2)))
(lambda () (print 'post1))))
(define (bar)
(dynamic-wind
(lambda () (print 'pre3))
(lambda () (c #f))
(lambda () (print 'post3))))
```
All implementations (Racket, srfi-226 reference impl, Gauche release,
Gauche HEAD) agree that what handlers are called:
> (foo)
pre1
pre2
cont
post1
> (bar)
pre3
post2
post3
If parameterization is implemented by dynamic-wind, the invocation of the
delimited continuation won't restore dynamic values of parameters when
reset is called (which would've been done by pre1).
On Fri, Nov 18, 2022 at 11:28 AM Marc Nieper-Wißkirchen <
marc.nie...@gmail.com> wrote:
> Am Fr., 18. Nov. 2022 um 22:12 Uhr schrieb Marc Nieper-Wißkirchen
> <marc.nie...@gmail.com>:
> >
> > At first sight, Racket's behavior looks strange (tested with 8.2):
> >
> > This expression
> >
> > (let ((m (make-parameter 0))
> > (n (make-parameter 0)))
> > (define k
> > (parameterize ((m 1))
> > (call-with-continuation-prompt
> > (lambda ()
> > (parameterize ()
> > ((call-with-composable-continuation
> > (lambda (k)
> > (lambda () k)))))))))
> > (k (lambda () (values (m) (n)))))
> >
> > evaluates to the values 0 0. The following expression, however,
> >
> > (let ((m (make-parameter 0))
> > (n (make-parameter 0)))
> > (define k
> > (parameterize ((m 1))
> > (call-with-continuation-prompt
> > (lambda ()
> > (parameterize ((n 1))
> > ((call-with-composable-continuation
> > (lambda (k)
> > (lambda () k)))))))))
> > (k (lambda () (values (m) (n)))))
> >
> > evaluates to the values 1 1.
> >
> > The only difference is that the inner parameterization is not trivial.
> >
> > It seems to be that each non-trivial parameterize installs a new
> > continuation mark (holding the complete new parameterization). This
> > is captured by c-w-c-c. However, when there is no non-trivial
> > parameterize, no continuation mark about parameterizations is
> > installed in the frames that are captured, and so reinstalling the
> > delimited continuation does not restore the relevant continuation
> > marks.
> >
> > Actually, this model happens to coincide with my wording in SRFI 226:
> > "Conceptually, each continuation contains at least one otherwise
> > inaccessible parameterization continuation mark, whose value is a
> > parameterization. The parameterization of a continuation is the value
> > of the most recent parameterization continuation mark in the
> > continuation. The parameterization of the current continuation is the
> > current parameterization. The (current) parameterization can
> > conceptually be seen as part of the dynamic environment."
> >
> > If I take this seriously, my sample implementation has to be corrected.
>
> PS I would like to hear some opinions about it. I think the sample
> implementation's behavior (namely, to record the current
> parameterization and the current exception handler stack at each
> prompt so that it will be captured) makes more sense than what I wrote
> literally (and what is Racket's behavior).
>
> >
> > Am Fr., 18. Nov. 2022 um 20:57 Uhr schrieb Marc Nieper-Wißkirchen
> > <marc.nie...@gmail.com>:
> > >
> > > Here is an example using only the primitives:
> > >
> > > (let ((m (make-parameter 0)))
> > > ((parameterize ((m 4))
> > > (call-with-continuation-prompt
> > > (lambda ()
> > > ((call-with-composable-continuation
> > > (lambda (k)
> > > (abort-current-continuation
> (default-continuation-prompt-tag)
> > > (lambda () k))))))))
> > > m))
> > >
> > > In Racket, it evaluates to 0 and not to 4.
> > >
> > > Am Fr., 18. Nov. 2022 um 20:51 Uhr schrieb Shiro Kawai <
> shiro.ka...@gmail.com>:
> > > >
> > > > Racket v8.6 behaves the same way.
> > > >
> > > > On Fri, Nov 18, 2022 at 9:29 AM Shiro Kawai <shiro.ka...@gmail.com>
> wrote:
> > > >>
> > > >> I used Racket v7.2, and here's the full transcription. I'm going
> to check with the newest Racket.
> > > >>
> > > >> shiro@scherzo:~/src/srfi-226$ racket
> > > >> Welcome to Racket v7.2.
> > > >> > (require racket/control)
> > > >> > (define (print . xs) (for-each display xs) (newline))
> > > >> > (define m (make-parameter 0))
> > > >> > (define c #f)
> > > >> > (define (foo)
> > > >> (parameterize ((m 1))
> > > >> (reset
> > > >> (print 'a: (m))
> > > >> (shift k (print 'b: (m)) (set! c k))
> > > >> (print 'c: (m)))))
> > > >> > (define (bar)
> > > >> (parameterize ((m 2))
> > > >> (c #f)))
> > > >> > (foo)
> > > >> a:1
> > > >> b:1
> > > >> > (bar)
> > > >> c:2
> > > >>
> > > >>
> > > >> On Fri, Nov 18, 2022 at 9:26 AM Marc Nieper-Wißkirchen <
> marc.nie...@gmail.com> wrote:
> > > >>>
> > > >>> Thanks for the report, Shiro!
> > > >>>
> > > >>> I have to investigate Racket's behavior. In 11.3.2 of the Racket
> > > >>> reference, it says: "If a continuation is captured during the
> > > >>> evaluation of parameterize, invoking the continuation effectively
> > > >>> re-introduces the parameterization, since a parameterization is
> > > >>> associated to a continuation via a continuation mark (see
> Continuation
> > > >>> Marks) using a private key." This seems to be consistent with SRFI
> > > >>> 226 and its sample implementation, but not consistent with your
> Racket
> > > >>> experiments.
> > > >>>
> > > >>> Am Fr., 18. Nov. 2022 um 20:07 Uhr schrieb Shiro Kawai <
> shiro.ka...@gmail.com>:
> > > >>> >
> > > >>> > It seems that there's a disagreement in how a delimited
> continuation captures dynamic environment, between Racket and srfi-226.
> > > >>> >
> > > >>> > Suppose the following code:
> > > >>> >
> > > >>> > ```
> > > >>> > (define (print . xs) (for-each display xs) (newline))
> > > >>> >
> > > >>> > (define m (make-parameter 0))
> > > >>> >
> > > >>> > (define c #f)
> > > >>> >
> > > >>> > (define (foo)
> > > >>> > (parameterize ((m 1))
> > > >>> > (reset
> > > >>> > (print 'a: (m))
> > > >>> > (shift k (print 'b: (m)) (set! c k))
> > > >>> > (print 'c: (m)))))
> > > >>> >
> > > >>> > (define (bar)
> > > >>> > (parameterize ((m 2))
> > > >>> > (c #f)))
> > > >>> > ```
> > > >>> >
> > > >>> > With srfi-226 (using reset/shift as given in the srfi) reference
> implementation on Chez, I get this:
> > > >>> >
> > > >>> > ```
> > > >>> > > (run foo)
> > > >>> > a:1
> > > >>> > b:1
> > > >>> > > (run bar)
> > > >>> > c:1
> > > >>> > ```
> > > >>> >
> > > >>> > With Racket racket/control, I get this:
> > > >>> >
> > > >>> > ```
> > > >>> > > (foo)
> > > >>> > a:1
> > > >>> > b:1
> > > >>> > > (bar)
> > > >>> > c:2
> > > >>> > ```
> > > >>> >
> > > >>> > I'm switching Gauche's internals to srfi-226 based model, and I
> noticed the difference---the current released version of Gauche (relying on
> dynamic-wind to handle parameterization) works like Racket, while the
> srfi-226 based version (using dynamic env chain to keep parameters) works
> like srfi-226 reference implementation.
> > > >>> >
> > > >>> > I think srfi-226 behavior is more consistent (when the delimited
> continuation is invoked, it restores the dynamic environment of the
> continuation of reset), but is there a plausible explanation of Racket
> behavior?
> > > >>> >
> > > >>> > This difference actually caused a compatibility problem of an
> existing application so I want to understand it fully.
> > > >>> >
> > > >>> >
> > > >>> >
> > > >>> >
> > > >>> >
> > > >>> >
> > > >>> >
>
