Page:AIM-453.djvu/68

 {This ain't A-lists} Page 11

Our symbol table routines are not the same as those in LISP 1.5. Their behavior is approximately the same, but the data structures involved differ. The LISP 1.5 routines (PAIRLIS and ASSOC) use the traditional "association list" format:

Our routines (BIND and LOOKUP), besides having nicer names, are more efficient because the number of conses performed to bind a given number of variables is usually smaller (we arrange for the environment structure to share the variable lists already contained in LAMBDA-expressions). Morever, the environment is organized into "frames" or "contours", which will be of some utility later. The environment is represented in this form: