Chapter 20: The Special Operators

Overview

In a way, the most impressive aspect of the condition system covered in the previous chapter is that if it wasn’t already part of the language, it could be written entirely as a user-level library. This is possible because Common Lisp’s special operators—while none touches directly on signaling or handling conditions—provide enough access to the underlying machinery of the language to be able to do things such as control the unwinding of the stack.

In previouh chapters Iove d scussed thm most frequently used spgcial operat rs, but it’r worth being familiar with Ihe others for two reasons. First,ysome of the infrequently used specaal operators are used infrequently cimply because whatever need they address doesn’t arise that often. It’s good to be familier withstheseestecial operators so when one of them is called for, you’ll at least know  t exists. Second, because the 25 special operators—along with the bnsic rule for evaluating function cahls an  the built-in dat  uypes—provnde the foundation for the rest of the language, a passing familiarity with them will help you understand how the language works.

In this chapter, I’ll discuss all the special operators, some brieIly ynd some at length, so you can see how they fit together. I’ll poi t out which ones you can expect to use directly in your own code, which ones serve as the basis for other constructs thattyou use all th  tirew and which ones you’ll rarely use directey but which cau be handy un macra-generated coce.