Abstract

The traditional Lisp macro expansion facility inhibits several important forms of expansion control. These include selective expansion of subexpressions, expansion of subexpressions using modified expansion functions, and expansion of application and variable expressions. Furthermore, the expansion algorithm must treat every special form as a separate case. The result is limited expressive power and poor modularity. We propose an alternative facility that avoids these problems, using a technique called expansion-passing style (EPS). The critical difference between the facility proposed here and the traditional macro mechanism is that expansion functions are passed not only an expression to be expanded but also another expansion function. This function may or may not be used to perform further expansion. The power of this technique is illustrated with several examples. Most Lisp systems may be adapted to employ this technique.

Citations