[racket] Engineering Tradeoffs of ANF transforms and the Stateless Server

From: Galler (lzgaller at optonline.net)
Date: Sat Dec 31 21:18:42 EST 2011



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 

A very useful and valuable suggestion.  I think that may very well be 
plan B.


(define (nextproc)
  ;need an intermediate function, or it will replace prior mark 
    (with-continuation-mark 'state '(S2 beforeproc afterproc) ((λ ()
"in S2 ~A\n" (continuation-mark-set->list
(current-continuation-marks) 'state)))))
    (with-continuation-mark 'state '(S3 beforeproc afterproc) ((λ ()
"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 <lzgaller at 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])))
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