Land of Lisp

More Complex Macros

Let’s suppose you need a custom my-length command. This is a classic list-eating function that will count the length of a list. We’ll write it in the proper “tail call optimized” style (discussed in Chapter 14), where the recursive function call is in the tail position. Here’s the code:

(defun my-length (lst)
   (labels ((f (lst acc)
              (if lst
                (f (cdr lst) (1+ acc))
                 acc)))
      (f lst 0)))

As you can see, this function has tons of repetitive ...