(defvar *state* nil “The current state: a list of conditions.”)

(defvar *ops* nil “A list of available operators.”)

(defstruct op “An operation”

 (action nil) (preconds nil) (add-list nil) (del-list nil))

(defun GPS (*state* goals *ops*)

 “General Problem Solver: achieve ail goals using *ops*.”

 (if (every #’achieve goals) ‘solved))

(defun achieve (goal)

 “A goal is achieved if it already holds,

 or if there is an appropriate op for it that is applicable.”

 (or (member goal *state*)

  (some #’apply-op

   (find-all goal *ops* :test #’appropriate-p))))

(defun appropriate-p (goal op)

 “An op is appropriate to a goal if it is in its add list.”

 (member goal (op-add-list op)))

(defun apply-op (op)

 “Print a message and update *state* if op is applicable.”

 (when (every #’achieve (op-preconds op))

  (print (list ‘executing (op-action op)))

  (setf *state* (set-difference *state* (op-del-list op)))

  (setf *state* (union *state* (op-add-list op)))

 t))

(defparameter *school-ops*

 (list

  (make-op :action ‘drive-son-to-school

   :preconds ‘(son-at-home car-works)

   :add-list ‘(son-at-school)

   :del-list ‘(son-at-home))

  (make-op :action ‘shop-installs-battery

   :preconds ‘(car-needs-battery shop-knows-problem shop-has-money)

   :add-list ‘(car-works))

  (make-op :action ‘tel 1-shop-problem

   :preconds ‘(in-communication-with-shop)

   :add-list ‘(shop-knows-problem))

  (make-op raction ‘telephone-shop

   :preconds ‘(know-phone-number)

   :add-list ‘(in-communication-with-shop))

  (make-op .-action ‘look-up-number

   :preconds ‘(have-phone-book)

   :add-list ‘(know-phone-number))

  (make-op :action ‘give-shop-money

   :preconds ‘(have-money)

   :add-list ‘(shop-has-money)

   :del-list ‘(have-money))))

> (gps ‘(son-at-home car-needs-battery have-money have-phone-book)

  ‘(son-at-school)

  *school-ops*)

(EXECUTING LOOK-UP-NUMBER)

(EXECUTING TELEPHONE-SHOP)

(EXECUTING TELL-SHOP-PROBLEM)

(EXECUTING GIVE-SHOP-MONEY)

(EXECUTING SHOP-INSTALLS-BATTERY)

(EXECUTING DRIVE-SON-TO-SCHOOL)

SOLVED