Introduction

Macros have been a distinctive feature of Lisps for decades. They are sometimes presented as a superpower native to the world of Lisp. While macros do exist in other languages, for many decades, Lisps have had the most complete macro systems. Why is this? Languages from the Lisp family share the ability to write code that modifies itself. People often talk about "code as data": Lisp programs, with their nested sets of parentheses called s-expressions, are in fact lists. And Lisps, as languages, are good at manipulating lists. The name "Lisp" originally came from "LISt Processor" when the language was first invented in 1958. As a result, Lisps can be made to operate on the code of Lisp programs. Usually, this means that a program modifies its own code.

Note

The term homoiconicity is often applied to Lisps. While the exact meaning of this term depends on who is talking, it generally means that Lisps are written in forms that they can manipulate...