[plt-scheme] Parameters and servlets

Jay McCarthy wrote:

> What are you actually trying to do? Obviously this example is
> contrived. I have a system called web-cells that solve the problems I
> was having with what I thought I wanted parameters to do.

I couldn't figure out how web-cells were meant to be used from
the source alone, so I experimented a little. The result works
for me (right now at least).

A persistent parameter is created with make-persistent-parameter:

   (define current-page-title
     (make-persistent-parameter "persistent/page-title"
                                "A title" ))

The call returns a normal parameter as result (so parameterize
can be used), and at  the same time registers a saver, which
when called will save the current value of the parameter to disk.
If there exists a saved value make-persitent-parameter used
that value, otherwise it uses the given one.

The servlet-writer must remember to call save-persitent-parameters
in the function start just before the response is returned.

Since read/write is used only readable/writeable values must be
used - but it is straightforward to use serialize.ss if one
needs structs.


(module persistent-parameter mzscheme
   (provide make-persistent-parameter
            save-persistent-parameters)

   (define current-saver (make-parameter (lambda () 'done)))

   ; saves all persistent parameters to disk
   (define (save-persistent-parameters)
     ((current-saver)))

   (define (make-persistent-parameter file value)
     (define (save-value o)
       (with-output-to-file file
         (lambda () (write o))
         'replace))
     (define (read-value)
       (with-input-from-file file
         (lambda () (read))))

     (if (file-exists? file)
         (set! value (read-value))
         (save-value value))

     ; We can't save the variable in the parameter's guard
     ; (since we can't "unsave" in in the end of a (parameterize ...)
     ; In stead we make a will-executor.

     (let* ([p (make-parameter value)]
            [cs (current-saver)])
       (current-saver
        (lambda ()
          (save-value (p))
          (cs)))
       p)))

-- 
Jens Axel Søgaard