Emacs Web Servlets

Remember that Emacs web server I wrote back in May? Well, I got an e-mail last night from Chunye Wang containing a patch with a variant of my dynamic lisp idea, called "servlets" (not to be confused with Java servlets). Chunye had similar concept for an Emacs web server for a long time, but never implemented because Emacs lacked network functionality until recently (Specifically, make-network-process in Emacs 22.1, June 2007). This led Chunye to find my implementation.

Again, you can clone/view the code here. I turned the patch into a series of commits,

git clone git://github.com/skeeto/emacs-http-server.git

This is some cool stuff here.

The servlets are simply functions installed under an "httpd/" namespace, where the trailing slash represents the server root. So, the function httpd/example-servlet will be executed when "/example-servlet" is requested from the server. The servlet runs on a temporary buffer, whose contents are served when the servlet function returns.

To assist in HTML generation, Chunye also wrote a function to turn an S-expression into HTML, similar to the one I described in the web server previous post. Symbols are converted into strings, alists are attributes, and the elisp symbol indicates code to be executed, and the results used to generate HTML. For a simple hello word,

(html (head (title "hello world")) (body "hello world"))

And for some dynamic content, a die roller,

(defun httpd/roll-die (uri-query req uri-path)
"Rolls a die with the requested number of sides (default 6)."
(let ((sides
(1- (string-to-number (or (cadr (assoc "sides" uri-query)) "6")))))
(title "Die Roll Servlet"))
(h1 "Die Roll Servlet")
"You rolled a "
(elisp (list (number-to-string (1+ (random sides)))))))))))

That one would be accessed from the browser with with "/roll-die" or "/roll-die?sides=100".

Chunye provided some sample servlets that list the buffers, with links that serve them up. There is also another servlet that will switch the current buffer, which I find compelling. All of Emacs' functionality is available to the servlet.

Now, to write a servlet that runs the Emacs psychiatrist ...

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Chris Wellons