Nice! (as you can tell, I didn't see that when I wrote my first reply to you....)
Robby On Sat, Dec 31, 2011 at 9:22 PM, Galler <lzgal...@optonline.net> wrote: > Section five > > "Although the continuation-marks procedure has been a part of PLT Scheme for > years, we have not yet found a use for it" > > I think we've found a use for it. > > > >>>> > > Robby, > > Section 5 of your paper notes the function (continuation-marks cont) > > provides the continuation marks of an arbitrary continuation at time of > reification. > > Illustrative code snippet below > > That certainly takes care of a lot of the problems associated with > implementing a solution to chain exit and entry actions. > > Thanks very much for sharing the paper. > > I had no idea! > > > > #lang web-server > > (define *s2-k* #f) > > > (define (nextproc) > (list ;have to have an intermediate function, or it will replace prior mark > (tail-calling > (with-continuation-mark 'state '(S2 beforeproc afterproc) ((λ () > > (call/cc (λ (k) (set! *s2-k* k)) (default-continuation-prompt-tag)) > (printf "in > S2 ~A\n" (continuation-mark-set->list > > (current-continuation-marks) 'state))))) > (with-continuation-mark 'state '(S3 beforeproc afterproc) ((λ () > (printf "in > S3 ~A\n" (continuation-mark-set->list > > (current-continuation-marks) 'state)) > (printf "in > S3 s2's continuation marks are ~A\n" (continuation-mark-set->list > > (continuation-marks *s2-k*) 'state))))) > > )) > > > > (with-continuation-mark 'state '(S1 beforeproc afterproc) (nextproc)) > > > > > > On Sat, Dec 31, 2011 at 9:36 PM, Robby Findler wrote: > >> FWIW, this is essentially re-implementing dynamic-wind (but it is not >> too difficult if you are familiar with continuations). One thing to >> watch out for: the lists of continuation marks will, in some cases, >> share a tail. In that case you'll want to avoid exiting and >> re-entering the same things. Also worth noting here, I think, you may >> need to use some kind of unique values in the marks to be able to tell >> if the shared tails _really_ are shared or if they simply have the >> same structure. >> >> You might find this paper useful to sort out some of the subtleties: >> >> http://www.cs.utah.edu/plt/delim-cont/ >> >> hth, >> Robby >> >> On Sat, Dec 31, 2011 at 8:18 PM, Galler <lzgal...@optonline.net> wrote: >>> >>> Robby, >>> >>> Thanks >>> >>> Not at all opaque. I was experimenting with that earlier this afternoon, >>> as >>> below. >>> >>> Actuallly, the way you phrased it helps clarify something I was struggle >>> with. >>> >>> A very useful and valuable suggestion. I think that may very well be >>> plan >>> B. >>> >>> R./ >>> Zack >>> >>> >>> (define (nextproc) >>> ;need an intermediate function, or it will replace prior mark >>> (tail-calling >>> (list >>> (with-continuation-mark 'state '(S2 beforeproc afterproc) ((λ () >>> (printf >>> "in >>> S2 ~A\n" (continuation-mark-set->list >>> >>> (current-continuation-marks) 'state))))) >>> (with-continuation-mark 'state '(S3 beforeproc afterproc) ((λ () >>> (printf >>> "in >>> S3 ~A\n" (continuation-mark-set->list >>> >>> (current-continuation-marks) 'state))))))) >>> >>> >>> (with-continuation-mark 'state '(S1 beforeproc afterproc) (nextproc)) >>> >>> >>> >>> >>> On Sat, Dec 31, 2011 at 9:03 PM, Robby Findler wrote: >>> >>>> I'm not sure if the web-server supports continuation marks or not, but >>>> if it does, you could use continuation marks to note what the exit and >>>> entry operations are, and then when you would do a continuation jump, >>>> first grab the continuation marks from one side, do the jump, grab the >>>> continuation marks from the other side, and then explicitly do the >>>> actions that the marks have recorded should be done. (Let me know if >>>> that is too opaque ...) >>>> >>>> Robby >>>> >>>> On Sat, Dec 31, 2011 at 7:56 PM, Galler <lzgal...@optonline.net> wrote: >>>>> >>>>> >>>>> Sorry, should have included a brief example. >>>>> >>>>> Here's an example of what I've implemented, which is just Harel's >>>>> hierarchical state machines >>>>> >>>>> You can create a Hierarchical State Machine by defining individual >>>>> nodes >>>>> in >>>>> a flat file, as below. >>>>> >>>>> each node has 6 keyword parameters , state, parent, entry, init, exit, >>>>> and a >>>>> handler. >>>>> >>>>> The machine is very much like a web-server. It receives a signal (akin >>>>> to >>>>> a >>>>> request), processes the signal which may cause a change internal state, >>>>> and >>>>> returns a continuation to be invoked when the next signal is presented. >>>>> >>>>> The new signal is the argument provided to this continuation at >>>>> invocation. >>>>> >>>>> In the stateful server, the HSM continuation is closed over by the PLT >>>>> Server's external continuation (reified by send/suspend) >>>>> >>>>> In the stateless version, I would replace the HSM continuation with the >>>>> PLT >>>>> server's continuation (reified by send/suspend). >>>>> >>>>> I can't do that right now because the continuation is in the scope of >>>>> the >>>>> dynamic-wind function, which is an unsafe place to capture >>>>> continuations >>>>> for >>>>> #lang web-server >>>>> >>>>> The dynamic wind is used as follows: >>>>> >>>>> At present, the nodes are brought into the correct tree-like dynamic >>>>> extent >>>>> by a recursive function which invokes dynamic-wind. >>>>> >>>>> Note that the handler code supports jumping to another state (trans >>>>> 'S2 >>>>> 'S3). >>>>> >>>>> Trans invokes a continuation reified earlier associated with state S3. >>>>> In >>>>> this case, the invocation of the continuation calls the exit actions of >>>>> state S2 and the entry actions of S3. >>>>> >>>>> That's where the dynamic-wind comes in. Its necessary for transitions >>>>> between states and but used for anything else. >>>>> >>>>> That's why I think an overridden call/cc-dynamic-wind and dynamic wind >>>>> would >>>>> be sufficient. >>>>> >>>>> >>>>> >>>>> (defnode >>>>> #:state 'S2 >>>>> #:parent 'Top >>>>> #:entry (λ () (I-TLL:entry-start-TLL)) >>>>> #:init (lambda () (trans 'S2 'S3)) >>>>> #:exit (λ () (I-TLL:exit-secure-TLL)) >>>>> #:handler (λ (signal) >>>>> (cond >>>>> [(signal-eq? signal "main") (trans 'S2 'S3)] >>>>> [(signal-eq? signal "info") (trans 'S2 'S4)] >>>>> [(signal-eq? signal "about") (trans 'S2 'S5)] >>>>> [(signal-eq? signal "item") (trans 'S2 'S6)] >>>>> [(signal-eq? signal "field") (trans 'S2 'S7)] >>>>> [(signal-eq? signal "dispatch") (trans 'S2 'S8-3)] >>>>> [(signal-eq? signal "burden") (trans 'S2 'S8-2)] >>>>> [(signal-eq? signal "planning") (trans 'S2 'S8-1)] >>>>> [(signal-eq? signal "failentry") (trans 'S2 'S3)] >>>>> [else #f]))) >>>>> >>>>> >>>>> ____________________ >>>>> Racket Users list: >>>>> http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
