First thing, thank you all for the consideration you're giving to my request.
A bit of background. I'm studying the meaning of continuations and their role in building useful constructs; think at generators/iterators/coroutines, as they're coming to JavaScript. I am kind of smelling already that call/cc isn't the holy grail but nevertheless I can't see anything else around that may assist my study. There is a paper ( http://matt.might.net/articles/programming-with-continuations--exceptions-backtracking-search-threads-generators-coroutines/) that uses call/cc idiomatically for doing apparently workable/useable stuff. That's where I am at the moment, kinda stuck, and where I hoped the stepper could help me understand. I'd kept very kind memories of using the stepper along with the Little Schemer ;-) and here I am. I couldn't help asking the list why call/cc ain't supported but I'm afraid I cannot provide high-level feedback in this discussion. However, I'd be glad to be of assistance as an alpha/beta tester. I am also studying John's non-trivial example and thinking what I'd like to be highlighted in that case. I do believe A LOT of people in various communities (JavaScript, Haskell, Java/Scala, you name it...) are wandering in the mist when it comes to continuations, and it's a pity. I reckon the stepper could really come to the rescue :-) Have a good day, Marco On Wed, Nov 4, 2015 at 10:48 PM, Robby Findler <ro...@eecs.northwestern.edu> wrote: > I think it woudl be kind of cool to add abort and call/cc (plus a good > set of delimited operators, of course) to ASL or to define ASL+ as > including those. (Or maybe ASL-- and take out state?) > > I can see John's point about the work involved, but it is hard to > imagine a better learning environment for continuations than ASL-- > plus John's stepper! > > Robby > > > On Wed, Nov 4, 2015 at 11:48 AM, Matthias Felleisen > <matth...@ccs.neu.edu> wrote: > > > > On Nov 4, 2015, at 12:21 PM, "'John Clements' via users-redirect" < > us...@plt-scheme.org> wrote: > > > >> Following up on this, here’s a simple test case, written in the > stepper’s test case language. One amusing UI note: what should be > highlighted as the “before” of the invocation-of-continuation step? The > application of the continuation, or the whole program? > >> > >> ;; this is just a sketch... > >> (t 'call/cc m:mz > >> (+ 3 (call/cc (λ (k) (+ 1 (k 2))))) > >> :: (+ 3 {(call/cc (λ (k) (+ 1 (k 2))))}) > >> ;; not sure what KONT-REP should be... > >> -> (+ 3 (+ 1 ({KONT-REP} 2))) > > > > > > According to my old work, two representations would work best here: > > > > (A) -> (call/cc (lambda (abort) (+ 3 (+ 1 ((lambda (x) (abort (+ 3 x))) > 2))))) > > > > Option A is completely faithful to the language and essentially > > the execution (if you think of 'goto somewhere global' is the > > callcc-lambda-abort at the top). > > > > > > (B) -> (+ 3 (+ 1 ((lambda (x) (ABORT (+ 3 x))) 2))) > > > > Option B brings in a new feature, which is now available in > > racket/control but wasn't at the time. I consider this easier > > to understand and it is faithful to the library. > > > > > > Everything else John says stands. call/cc is a cute game and essentially > > useless. Use prompt/control/shift/reset/foo/bar instead. > > > > > > > > > >> ;; interesting how the highlight falls apart... what should be > highlighted? > >> :: {(+ 1 (KONT-REP 2))} > >> -> {(+ 3 2)} > >> -> 5) > >> > >> John > >> > >>> On Nov 4, 2015, at 9:02 AM, 'John Clements' via users-redirect < > us...@plt-scheme.org> wrote: > >>> > >>> First, apologies for bouncing this back to the list, hope that’s okay… > >>> = > >>>> On Nov 4, 2015, at 2:20 AM, Marco Faustinelli < > marco.faustine...@onebip.com> wrote: > >>>> > >>>> > >>>> What I definitely AM missing (I can't see why you contradict my > personal statement at the same time you are actually taking it into > consideration :-) is the educational value of being able to "see" the code > contained in a continuation, which is the core problem of whoever > approaches call/cc. > >>>> > >>>> myCont = (lambda(_) ...) > >>>> > >>>> As the stepper is there for educational purposes, this is something > that would be spot-on with its core business; I am ready to believe that a > text dump of the code contained in the continuation is not easy to achieve > (I assume that the code has been interpreted on the fly into a tree or > something). > >>> > >>> Forgive me; I didn’t mean to suggest that it wouldn’t be educational. > It would. > >>> > >>>> > >>>> But I see that the stepper is very good at substituting readable > expressions. That's the heart of its effectiveness, so there must be a way… > >>> > >>> Yep. There’s definitely a way, and if anyone was going to do it, it > would probably be me :). > >>> > >>> Okay, I just thought about it for five minutes, and here’s what I > think: > >>> > >>> 1) At a very basic level, the stepper is not currently enabled for any > of the languages that include call/cc. IIRC, the first language that > includes call/cc is advanced, which also includes mutation. Doing mutation > right is kind of a bear. You should be able to enable half-baked stepper > support in any language by setting the “PLTSTEPPERUNSAFE” environment > variable. > >>> 1a) Okay, I just tried it, and it turns out that call/cc isn’t in > Advanced, either. This means that you need to try this in full #lang > racket, which is really not a language where the stepper is going to be too > good at rendering. > >>> 1b) Hmm, interesting. I got a bit conservative in the 6.3 release, and > currently PLTSTEPPERUNSAFE works for advanced but not for full #lang > racket. I’ve just made a change that should allow this, but it DEFINITELY > WON’T be a part of the 6.3 release. > >>> 2) You mention that “the stepper is very good at substituting.” In > fact, for the most part, the stepper doesn’t know anything about > substitution; it simply observes the computation as it proceeds. This > actually turns out to be a big win for watching call/cc run; while the > existing stepper has no idea how to render a continuation, as soon as you > step *into* the continuation, the context should be correctly displayed. In > my opinion, this is about three quarters of the way to > >>> understanding call/cc. Maybe I’m looking at the glass as more full > than it is :). > >>> 3) The remaining piece would be to capture the current marks and > associate them with the continuation. To do this, the stepper would have to > know when a continuation was being captured. Not impossible, but > interesting. > >>> 4) Rendering the continuation: this could be horrible, or easy. If you > decide to try to render the continuation as a legal term in the language, > it would be a good deal of the work. BUT… if you just deployed the existing > mark-rendering machinery to render the set of marks as a context, and then > put a special “kont-box” around it—that is, “this isn’t a legal term in the > language, it’s a special context term that will replace the existing > context if called”—it wouldn’t be too hard. > >>> 5) This is the unpleasant bullet. I’m at a teaching school, and I’ve > got a whole bunch of exams to grade. This isn’t at the top of my list > right now, and I’m not sure when it will be, especially given that call/cc > doesn’t appear in Advanced; even if it worked as I suggest, using the > stepper for call/cc would be like a trip to the jungle: “we’re going in > here, to catch a glimpse of the jaguar. Whew! Done! Now, let’s retreat back > to civilization.” > >>> > >>> Best, > >>> > >>> John Clements > >>> > >>> -- > >>> You received this message because you are subscribed to the Google > Groups "Racket Users" group. > >>> To unsubscribe from this group and stop receiving emails from it, send > an email to racket-users+unsubscr...@googlegroups.com. > >>> For more options, visit https://groups.google.com/d/optout. > >> > >> > >> > >> -- > >> You received this message because you are subscribed to the Google > Groups "Racket Users" group. > >> To unsubscribe from this group and stop receiving emails from it, send > an email to racket-users+unsubscr...@googlegroups.com. > >> For more options, visit https://groups.google.com/d/optout. > > > > -- > > You received this message because you are subscribed to the Google > Groups "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to racket-users+unsubscr...@googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. > -- *Marco Faustinelli | Sr. developerMobile: +393473553073 Office: via Fara, 28 - 20124 Milano* marco.faustine...@onebip.com www.onebip.com CONFIDENTIALITY NOTICE. Information contained in this email is intended for the use of the addressee only, is confidential and may be legally privileged. Any further dissemination, distribution, copying or use of this communication without prior permission of the sender is strictly prohibited. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
