Using reduce over a tree in Lisp

Question

To fold a flat list in Lisp you use reduce:

* (reduce #\'+ \'(1 2 3 4 5))
15

But what if I have an arbitrarily complex tree, a

Answer 1

Common Lisp does not have tree versions of map or reduce. In fact, the only tree functions I can remember off-hand are tree-equal and subst.

However, it should not be too hard to do something like:

(defun reduce-tree (function tree &key (key #'identity))
  (if (atom tree)
      (funcall key tree)
      (funcall function
               (reduce-tree function (car tree) :key key)
               (reduce-tree function (cdr tree) :key key))))

try it:

> (reduce-tree #'+ '(1 . ((2 . 3) . ((4 . 5) . 6))))
==> 21
> (reduce-tree #'+ '(1 (2) (3 (4) 5)) :key (lambda (x) (or x 0)))
==> 15