Beautiful Code

Chapter 25. Syntactic Abstraction: The syntax-case Expander

R. Kent Dybvig

When writing computer programs, certain patterns arise over and over again. For example, programs must often loop through the elements of arrays, increment or decrement the values of variables, and perform multiway conditionals based on numeric or character values. Programming language designers typically acknowledge this by including special-purpose syntactic constructs that handle the most common patterns. C, for instance, provides multiple looping constructs, multiple conditional constructs, and multiple constructs for incrementing or otherwise updating the value of a variable.^[76]

Some patterns are less common but may occur frequently in a certain class of programs, or perhaps just within a single program. These patterns may not even be anticipated by a language’s designers, who in any case would typically choose not to incorporate syntactic constructs to handle such patterns in the language core.

Yet, recognizing that such patterns do arise and that special-purpose syntactic constructs can make programs both simpler and easier to read, language designers sometimes include a mechanism for syntactic abstraction, such as C’s preprocessor macros or Common Lisp^[77] macros. When such facilities are absent or are inadequate for a specific purpose, an external tool, such as the m4 macro expander,^[78] may be brought to bear.

Syntactic abstraction facilities differ in several significant ways. C’s preprocessor macros ...