List-eating functions in Lisp

“Since the Lisp philosophy strongly emphasizes the use of lists to store and manipulate information, it will come as no surprise that the design of Common Lisp favors behaviors that make it easy to slice and dice such lists. The most profound design decision made in Common Lisp, with regard to lists, is that it automatically treats an empty list as a false value when evaluating a condition […]  when we pass the empty list () into an if form, it evaluates as a false value, whereas a list that contains an item evaluates as true.

Because we can easily detect an empty list, we can process lists using recursion. With this technique, we can take items from the front of a list and send the rest of the list back to the same function until the list is empty. (It’s a good thing that detecting empty lists is so easy, because so many functions in Lisp end up being list-eaters.)

Let’s look at a common list-eating function, which calculates the length of a list:

> (defun my-length (list)
     (if list
         (1+ (my-length (cdr list)))

> (my-length '(list with four symbols))

This function is written in classic Lisp style. It calls itself recursively as it chomps items off the front of the list. Calling yourself in this way is not only allowed in Lisp, but is often strongly encouraged. Lists in Lisp are recursive (conses of conses of conses . . .), so the act of consuming a list maps naturally onto functions that are recursive.”

— Conrad Barski, The Land of Lisp: Learn to Program in Lisp, One Game at a Time (No Starch Press, 2011)