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Dispatch On a Link Being Clicked

Problem

You are writing a servlet in the PLT web server and want to choose an action based on which link was clicked. You cannot use a form submit button.

Solution

Use send/suspend/dispatch

;; send/suspend/dispatch : Response -> void
(define (send/suspend/dispatch p-exp)
 (let/cc k0
  (send/back
   (replace-procedures
     p-exp (lambda (proc)
             (let/cc k1 (k0 (proc (send/suspend k1)))))))))

(define (replace-procedures p-exp p->a)
  (cond
    ((list? p-exp) (map (lambda (p-e) (replace-procedures p-e p->a))
                        p-exp))
    ((procedure? p-exp) (p->a p-exp))
    (else p-exp)))

A simple example of its use:

(define (callback req)
   `(html (head (title "Page Two"))
          (body (h1 "Page Two"))))

(send/suspend/dispatch
   `(html (head (title "Page One"))
          (body (h1 "Page One")
                (p (a ((href ,callback)) "Page Two")))))

Discussion

A clumsy way to handle this situation is to append a parameter to the URL:

`(a ((href ,(string-append url "?arg=suchandsuch"))) "click here")

This is not recommended: the web server will add this parameter to all the next 'continuation' URL it gives you and you will have to manually strip it out.

Comments about this recipe

Based on a posting to plt-scheme by Mike Burns. Only handles the xexpr datatype, which makes it mostly useless for those using more advanced (SXML, WebIt!) frameworks?

-- NoelWelsh - 31 Aug 2004

This is a variant on my original send/suspend/dispatch. It's clean, but doesn't give the programmer access to the generated URL. For a version that does that, please see my paper at:

http://continue.cs.brown.edu/research/

Having the URL is useful for appending anchor references to the URL, or for using the URL in JavaScript?.

-- PeteHopkins - 23 Nov 2004

I suggest renaming this solution as send/suspend/callback to avoid confusion with the original by PeteHopkins?.

-- DanielSilva - 11 Jan 2005

It would be nice if send/suspend were implemented on top of a web-server internal version of PeteHopkins?'s send/suspend/dispatch, like this:

(define (send/suspend build-page)
  (let/cc cc
    (send/suspend/dispatch
       (lambda (embed/url)
          (send/suspend (embed/url cc))))))

-- DanielSilva - 11 Jan 2005

Contributors

-- NoelWelsh - 31 Aug 2004