add-all.lisp

;; Rewrite add-all for clarity why mantaining the functional approach
(defun add-all (list)
  (cond
    ((null list) 0)
    ((listp (car list)) (+ (add-all (car list)) (add-all (cdr list))))
    ((stringp (car list)) (+ (add-all (cdr list))))
    (t (+ (car list) (add-all (cdr list))))))

;; First the ideal case

(defun add-pair (x y)
  (+ x y))

(reduce #'add-pair '(1 2 3)) => 6

;; Now to handle strings in the lists

(defun zero-if-string (x)
  (if (stringp x)
      0
      x))

(defun add-pair (x y)
  (let ((x (zero-if-string x))
        (y (zero-if-string y)))
    (+ x y)))

(reduce #'add-pair '(1 2 "s" 3)) => 6

;; If there are lists inside of lists, reduce them first

(defun reduce-if-list (x)
  (if (listp x)
      (add-all x)
      x))

(defun add-pair (x y)
  (let ((x (reduce-if-list x))
        (y (reduce-if-list y)))
    (+ x y)))

(defun add-all (list)
  (reduce #'add-pair list))

(add-all '(1 (2 3 (4 5 6)) 3))

;; Finally, we combine both cases

(defun add-pair (x y)
  (let ((x (reduce-if-list (zero-if-string x)))
        (y (reduce-if-list (zero-if-string y))))
    (+ x y)))

(add-all '(1 (2 "s" 3 (4 5 6) "x") "s" 3))