If you need to visit a tree in some way, there are two built-in functions that, while not explicitly designed for the purpose, can walk a tree effectively.

First, subst-if takes a predicate function that is called for each cons and atom in a tree. As long as the predicate returns nil, no substitution will actually take place.

Here’s a simple way to turn it into a function:

(defun walk-tree (fun tree) (subst-if t (constantly nil) tree :key fun)) * (walk-tree 'print '(a b (1 2 . 10) c)) (A B (1 2 . 10) C) A (B (1 2 . 10) C) B ((1 2 . 10) C) (1 2 . 10) 1 (2 . 10) 2 10 (C) C NIL (A B (1 2 . 10) C)

Second, tree-equal is normally considered for comparing two trees, but it can also be used to call a function for each atom in a single tree; just pass the same tree to tree-equal, and use the test function to do something with each atom. As long as the test function returns true, the walking continues.

Here’s a function that wraps it up:

(defun walk-tree-atoms (fun tree) (tree-equal tree tree :test (lambda (element-1 element-2) (declare (ignore element-2)) (funcall fun element-1) t))) * (walk-tree-atoms 'print '(a b (1 2 . 10) c)) A B 1 2 10 C NIL T

(This tip inspired by a few articles from Rob Warnock.)