[plt-scheme] PLT Web Server

From: Noel Welsh (noelwelsh at gmail.com)
Date: Sat Jul 25 16:03:19 EDT 2009

On Sat, Jul 25, 2009 at 8:30 PM, Emmanuel
Delaborde<emmanuel.delaborde at cimex.com> wrote:
> I want to add a file upload form but I got stuck...
> The application raised an exception with the message:
> you broke the contract xexpr? on start; Not an Xexpr. Expected a string,
> symbol, number, comment, processing instruction, or list, given #<void>
> Context:
> #
> Stack trace:

Two things:

1. Your function add-image didn't return a xexpr; instead it returned
void. It must return something to the browser, or you'll get this
error message. A simple result might be:

  '(html (head (title "Thanks")) (body (h1 "Thanks. Your file has been

2. Using extract-binding/single and friends is bad style; better to
use bindings-assq -- search Help Desk for the docs (press F1 in
DrScheme, or choose Help from the menu).

Finally, I don't know of any sizable public projects written using the
web server.  (There are several sizable private projects written using


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