(and robot baby)

robot-baby.el

;; (quoted-from (short (story (title (and robot baby)))))
(setq line "(Implies (Non-literal-interpretation x) y) (Value (Obey x) (* 0.5 (Value (Obey y)))))")
"(Implies (Non-literal-interpretation x) y) (Value (Obey x) (* 0.5 (Value (Obey y)))))"

;; check if expr (list of chars of some expression) is balanced
(defun balanced (expr)
  (defvar lparen (string-to-char "("))
  (defvar rparen (string-to-char ")"))

  (defun is-open (c) (eq lparen c))
  (defun is-close (c) (eq rparen c))
  (defun is-empty (ls) (eq ls nil))
  (defun is-matching (l r) (and (is-open l) (is-close r)))

  (defun is-balanced (list stack)
    (cond ((is-empty list) (is-empty stack))
	   
	  ((is-open (car list))
	   (is-balanced (cdr list) ;; 'push' open-paren onto stack
			(cons (car list) stack)))

	  ((is-close (car list))
	  
	   (if (is-empty stack) nil
	     (and 
	      (is-balanced (cdr list) (cdr stack))
	      (is-matching (car stack) (car list)))))

	  (t (is-balanced (cdr list) stack))))

  (is-balanced (string-to-list expr) nil))
balanced


(balanced "(balanced () (() )")
nil

(balanced line)
nil