Previously, we looked at the core of the Common Lisp language and admired its succinctness and elegance. However, there are also some darker, seedier parts of Lisp built around this core that have a certain charm of their own. They may lack the beauty of the Lisp core, but they easily make up for it with their power. These parts of the language are a real delight for any budding Lisp hacker.

The extensions we’ll cover in this section, loop and format, place a strong emphasis on power over mathematical elegance. This has led to occasional controversy among Lisp programmers, some of whom question whether the power provided by these commands is worth the trade-off in elegance. These programmers believe that loop and format should be avoided when writing any serious code.

But there is one great reason to learn and use these commands: They embody the flexibility and extensibility of Lisp. Since Lisp is (arguably) the most flexible programming language available, hackers have been extending it with thousands of their own hacks for decades. loop and format, which are among the most successful of these extensions, had to be really spectacular to survive in the Darwinian battlefield.