Implementation of an iteration macro

This article describes the implementation of a set of iteration macros. These macros are found in the Portable Common Loops (PCL) implementation of the Common Lisp Object System, though they are not dependent on PCL in any way. The macros were designed to be more functional than the small set of iteration macros found in Common Lisp (e.g., do), while still having a "Lisp-like" syntax that is easy to understand and extend. Pavel Curtis is responsible for the essential insights about program transformation that make feasible an efficient implementation of this conceptually simple iteration form. The implementation makes extensive use of the PCL code walker, which was described in the previous issue of Lisp Pointers .