robert zubek / blog

(Edit: Welcome reddit readers. The following is a howto on setting up an embedded Jetty server in Clojure, and writing a minimal servlet that serves up dynamic content.)

Last week I discovered a very nice language named Clojure. It’s based on Lisp, but hosted on the Java platform, and running inside the JVM. It has some lovely features, like native support for Java collections, and a clean API that ditches a lot of the old ANSI CL library cruft.

But the best part for me was language interop: Clojure can effortlessly load and use any external Java library. And there is a ton of good ones out there, including a rich set that comes with the Java platform. (This may seem like a minor point, but most free Lisps lack good libraries; only the non-free Allegro CL has really extensive, cross-platform ones.)

So I decided to take the language for a spin, especially exercising the language interop bit. For example, by building a little web server to serve up some dynamic content. You know, web development 101 kind of stuff. This turns out to be really easy to do.

Since Clojure is backed by Java, we can use an existing server library to do the heavy lifting for us. Several high-quality ones are available - I went with Jetty, a servlet-based engine that’s really easy to embed in custom applications. Using a servlet container will take care of all the mundane bookkeeping for us.

In the following posting, I’m reproducing the steps required to get a Clojure-based web server up and running, in much less than 100 lines of code. Hope you find it interesting!

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Here are my steps for setting up SBCL (Steel Bank Common Lisp) and Slime (Emacs Lisp mode) to work under Windows Vista.

It’s mostly straightforward, except for dealing with spaces in path names. It turns out that slime.el uses the (split-string) function to pull apart the Lisp command line, which won’t work with the default “c:\program files\…” location. Here’s how to fix that, using symbolic links (yes, Windows supports them too!).

The following was tested on a Vista box, SBCL 1.0.13, and Slime 2.0 on Emacs 22.2.1.

Installing SBCL:

1. Download and install SBCL. By default, it will install to c:\Program Files\Steel Bank Common Lisp\1.0.13
2. Run cmd.exe as Administrator (required for symbolic links)
3. Set up a link from SBCL install directory, to some location without spaces. Note that the syntax is an inverse of the Unix ‘ln’ command.
   

   C:\Users\Rob\Documents>ver
   Microsoft Windows [Version 6.0.6000]

   C:\Users\Rob\Documents>mklink /d SBCL “c:\Program Files\Steel Bank Common Lisp\1.0.13″
   symbolic link created for SBCL <<===>> c:\Program Files\Steel Bank Common Lisp\1.0.13

   C:\Users\Rob\Documents>dir sbcl*
   …
   04/09/2008 09:07 PM <SYMLINKD> SBCL [c:\Program Files\Steel Bank Common Lisp\1.0.13]

Installing Emacs and Slime:

1. Download Emacs, unzip it to any location (eg. c:\Program Files\emacs). If desired, run the emacs\bin\addpm.exe program to add a link to the start menu.
2. Download Slime, and unzip it under your emacs/site-lisp directory.
3. Add the following to your ~/.emacs file:
   

   (setq inferior-lisp-program “c:/users/rob/documents/sbcl/sbcl.exe –core c:/users/rob/documents/sbcl/sbcl.core”)
   (require ’slime)
   (slime-setup) 

4. Restart Emacs, and then run M-x run-lisp to test whether Lisp starts up.
5. Start slime with M-x slime. Happy hacking!