5. Functions

After the rules of syntax and semantics, the three most basic components of all Lisp programs are functions, variables and macros. You used all three while building the database in Chapter 3, but I glossed over a lot of the details of how they work and how to best use them. I’ll devote the next few chapters to these three topics, starting with functions, which—like their counterparts in other languages—provide the basic mechanism for abstracting, well, functionality.

The bulk of Lisp itself consists of functions. More than three quarters of the names defined in the language standard name functions. All the built-in data types are defined purely in terms of what functions operate on them. Even Lisp’s powerful object system is built upon a conceptual extension to functions, generic functions, which I’ll cover in Chapter 16.

And, despite the importance of macros to The Lisp Way, in the end all real functionality is provided by functions. Macros run at compile time, so the code they generate—the code that will actually make up the program after all the macros are expanded—will consist entirely of calls to functions and special operators. Not to mention, macros themselves are also functions, albeit functions that are used to generate code rather than to perform the actions of the program.1