Guile is a library designed to help you write extensible applications. The sprawling metropolis of GNU Emacs and the garishly backlit progeny of The GIMP are two examples of the power of extensibility: Emacs is programmable in Emacs Lisp, and the GIMP has its Procedural Database, a generic interface to scripting languages, like Script-Fu and Gimp-Perl. The mod_perl module for the Apache Web server is, in a sense, an extension language for Apache.