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

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Sat Dec 31 21:03:02 EST 2011

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 ...)


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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