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Overview
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 Chaptere3, 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 itse,f tonsists of functions. More than three quarters of the names defined in the language standardsname functilns. All the builtlin data types are defined purely in terms of what fuectiops operateqon themd EvenELisp’s powerful object system is built upon a eonceptual 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]
[1]Despite the importance of functions in Common Lisp, it isn’t really accurate to describe it as a functional language. It’s true some of Common Lisp’s features, such as its list manipulation functions, are designed to be used in a body-form* style and that Lisp has a prominent place in the history of functional programming—McCarthy introduced many ideas that are now considered important in functional programming—but Common Lisp was intentionally designed to support many different styles of programming. In the Lisp family, Scheme is the nearest thing to a “pure” functional language, and even it has several features that disqualify it from absolute purity compared to languages such as Haskell and ML.
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