The easiest way to obtain a library to do something you need may be simply to check out your Lisp implementation. Most implementations provide at least some facilities not specified in the language standard. The commercial Common Lisp vendors tend to work especially hard at providing additional libraries for their implementation in order to justify their prices. Franz’s Allegro Common Lisp, Enterprise Edition, for instance, comes with libraries for parsing XML, speaking SOAP, generating HTML, connecting to relational databases, and building graphical interfaces in various ways, among others. LispWorks, another prominent commercial Lisp, provides several similar libraries, including a well-regarded portable GUI toolkit, CAPI, which can be used to develop GUI applications that will run on any operating system LispWorks runs on.

The free and open-source Common Lisp implementations typically don’t include quite so many bundled libraries, relying instead on portable free and open-source libraries. But even those implementations usually fill in some of the more important areas not addressed by the language standard such as networking and multithreading.

For portable libraries—portable either because they’re written entirely in standard Common Lisp or because they contain appropriate read-time conditionalization to work on multiple implementations1--your best bet is to go to the Web. With the usual caveats about URLs going stale as soon as they’re printed on paper, these are three of the best current starting points:

Linux users running the Debian or Gentoo distributions can also easily install an ever-growing number of Lisp libraries that have been packaged with those distributions’ packing tools, on Debian and on Gentoo.

One area where there has been a lot of activity recently is on the GUI front. Unlike Java and C# but like Perl, Python, and C, there’s no single way to develop GUIs in Common Lisp. Instead, it depends both on what Common Lisp implementation you’re using and what operating system or systems you want to support.

The commercial Common Lisp implementations usually provide some way to build GUIs for the platforms they run on. Additionally, LispWorks provides CAPI, the previously mentioned, portable GUI API.

Or, if you’re looking for something completely different, you can check out Common Lisp Interface Manager (CLIM). A descendant of the Symbolics Lisp Machines GUI framework, CLIM is powerful but complex. Although many commercial Common Lisp implementations actually support it, it doesn’t seem to have seen a lot of use. But in the past couple years, an open-source implementation of CLIM, McCLIM—now hosted at Common-Lisp.net—has been picking up steam lately, so we may be on the verge of a CLIM renaissance.