Ok, I think I've got it, but just to make sure I'm not missing any
subtleties, or else for the benefit of posterity, here's the version with
thread cells.
In "common.rkt":
#lang racket
(provide my-val
call-with-my-val)
(define cell:my-val
(make-thread-cell #f #t))
(define (my-val)
(thread-cell-ref cell:my-val))
(define (call-with-my-val new thunk)
(let ([old (my-val)])
(dynamic-wind (λ () (thread-cell-set! cell:my-val new))
thunk
(λ () (thread-cell-set! cell:my-val old)))))
In "servlet.rkt":
#lang web-server
(require "common.rkt")
(provide start)
(define (start req)
(response/xexpr
`(html (head (title "Example"))
(body (p ,(format "~v" (my-val)))))))
In "run.rkt":
#lang racket
(require "common.rkt"
"servlet.rkt"
web-server/http
web-server/servlet-dispatch)
(define ((wrap-dispatcher inner) connection request)
(call-with-my-val (request-uri request)
(λ () (inner connection request))))
(serve/launch/wait (λ (quit-sema)
(wrap-dispatcher
(dispatch/servlet start
#:stateless? #t)))
#:banner? #t)
Thanks so much!
Philip
-Philip
On Thu, Jun 22, 2017 at 9:31 AM, Jay McCarthy <jay.mccar...@gmail.com>
wrote:
> On Thu, Jun 22, 2017 at 10:21 AM, Philip McGrath
> <phi...@philipmcgrath.com> wrote:
> > I've come up with a (relatively) minimal example.
> >
> > In "common.rkt":
> >>
> >> #lang racket
> >> (provide my-param)
> >> (define my-param
> >> (make-parameter #f))
> >
> >
> > In "servlet.rkt":
> >>
> >> #lang web-server
> >> (require "common.rkt")
> >> (provide start)
> >> (define (start req)
> >> (response/xexpr
> >> `(html (head (title "Example"))
> >> (body (p ,(format "~v" (my-param)))))))
> >
> >
> > In "run.rkt":
> >>
> >> #lang racket
> >> (require "common.rkt"
> >> "servlet.rkt"
> >> web-server/http
> >> web-server/servlet-dispatch)
> >> (define ((wrap-dispatcher inner) connection request)
> >> (parameterize ([my-param (request-uri request)])
> >> (inner connection request)))
> >> (serve/launch/wait (λ (quit-sema)
> >> (wrap-dispatcher
> >> (dispatch/servlet start
> >> #:stateless? #t)))
> >> #:banner? #t)
> >
> >
> > If you visit a bunch of different URLs, the page will always show the
> first
> > URL you visited.
> >
> > From what you said, I'm guessing the problem is that "the stateless Web
> > language guarantees that the parameters will have values they had when
> the
> > servlet was initialized", which I was not aware of.
>
> Yup
>
> > I haven't worked with thread cells before, if that is indeed where I
> want to
> > store this data. One difference that immediately occurs to me is that the
> > value would not be propagated to any new child threads, right?
>
> If you want it propagated, then create the cell with #t for preserved?
>
> http://docs.racket-lang.org/reference/threadcells.html?q=
> thread-cell#%28def._%28%28quote._~23~25kernel%29._make-thread-cell%29%29
>
> > Thanks,
> > Philip
> >
> > -Philip
> >
> > On Thu, Jun 22, 2017 at 8:39 AM, Jay McCarthy <jay.mccar...@gmail.com>
> > wrote:
> >>
> >> Hi Philip,
> >>
> >> I think I'd have to see more code to really tell what the problem is,
> >> but here are some thoughts:
> >>
> >> - A continuation captures the values of parameters at the time it is
> >> captured. So, if your parameterize is outside of the dispatcher that
> >> calls the continuation, then the code inside the continuation will not
> >> see the parameter change. That's a pretty important feature of
> >> continuations.
> >>
> >> - Web parameters would work the same way as normal parameters and not
> help
> >> you.
> >>
> >> - The stateless Web language guarantees that the parameters will have
> >> values they had when the servlet was initialized.
> >>
> >> - I think that you may want to use a thread-cell as the storage place.
> >>
> >> Jay
> >>
> >>
> >> On Thu, Jun 22, 2017 at 3:56 AM, Philip McGrath
> >> <phi...@philipmcgrath.com> wrote:
> >> > I'm encountering some strange behavior when using parameters with my
> >> > (stateless) web servlets.
> >> >
> >> > I want to check the value of a cookie every time a request is made to
> my
> >> > server and make a Racket value derived from the cookie's value
> available
> >> > for
> >> > use within the servlet code. Since this value shouldn't be stored in
> the
> >> > continuation — it should be generated fresh for every request — it
> >> > seemed
> >> > like writing a dispatcher (in the sense of serve/launch/wait, not of
> >> > dispatch-rules) was the right place to do this. Since the servlet code
> >> > itself does not need to (and indeed should not) modify the value, I
> >> > thought
> >> > I could use a standard parameter to store the value. Essentially I
> have
> >> > a
> >> > function shaped like this:
> >> >
> >> > (define ((wrap-dispatcher inner) connection request)
> >> > (parameterize ([my-param ...])
> >> > (inner connection request)))
> >> >
> >> > where inner is a dispatcher produced by dispatch/servlet.
> >> >
> >> > All of the work being done by wrap-dispatcher seems to be happening
> >> > correctly (e.g. setting and reading the cookie), but within the
> servlet
> >> > itself, the parameter seems to get "stuck" on the first value
> installed
> >> > by
> >> > wrap-dispatcher after the server is launched (which is not the default
> >> > value
> >> > for the parameter).
> >> >
> >> > I'm not sure why this is happening. I understand that plain parameters
> >> > cannot be set within stateless servlets for serialization reasons
> (hence
> >> > the
> >> > existence of web-parameters), but I didn't think that impacted code
> that
> >> > merely accesses the value of a parameter.
> >> >
> >> > More urgently, I'm not sure how to fix it. Would making my-param a web
> >> > parameter solve the problem (without storing the value in the
> >> > continuation)?
> >> > Or is there some other method that should be used for this sort of
> >> > per-request work?
> >> >
> >> > Thanks,
> >> > Philip
> >> >
> >> > --
> >> > 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.
> >>
> >>
> >>
> >> --
> >> -=[ Jay McCarthy http://jeapostrophe.github.io ]=-
> >> -=[ Associate Professor PLT @ CS @ UMass Lowell ]=-
> >> -=[ Moses 1:33: And worlds without number have I created; ]=-
> >
> >
>
>
>
> --
> -=[ Jay McCarthy http://jeapostrophe.github.io ]=-
> -=[ Associate Professor PLT @ CS @ UMass Lowell ]=-
> -=[ Moses 1:33: And worlds without number have I created; ]=-
>
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