(module towers-structure mzscheme
  (require (planet "inference.ss" ("williams" "inference.plt")))
  (provide (all-defined))
  ;; Example Inference Model
  (define-struct ring (size peg) (make-inspector))
  (print-struct #t)
  ;; Towers of Hanoi from Artificial Intelligence: Tools, Techniques,
  ;; and Applications, Tim O'Shea and Marc Eisenstadt, Harper & Rowe,
  ;; 1984, pp.45
  ;; The rules of the game are: (1) move one ring at a time and (2)
  ;; never place a larger ring on top of a smaller ring.  The object
  ;; ive isto transfer the entire pile of rings from its starting
  ;; peg to either of the other pegs - the target peg.
  (define-ruleset towers-structure-rules)
  ;; If the target peg hld all the rings 1 to n, stop because according
  ;; to game rule (2) they must be in their original order and so the
  ;; problem is solved.
  (define-rule (rule-1 towers-structure-rules)
      (all #(struct:ring ? right))
      (printf "Problem solved!~n"))
  ;; If there is no current goal - that is, if a ring has just been
  ;; successfully moved, or if no rings have yet to be moved - generate
  ;; a goal.  In this case the goal is to be that of moving to the
  ;; target peg the largest ring that is not yet on the target peg.
  (define-rule (rule-2 towers-structure-rules)
      (no (move . ?))
      #(struct:ring ?ring (?peg (not (eq? ?peg 'right))))
      (no #(struct:ring (?ring-1 (> ?ring-1 ?ring))
                        (?peg-1 (not (eq? ?peg-1 'right)))))
      (assert `(move ,?ring from ,?peg to right)))
  ;; If there is a current goal, it can be achieved at once of there is
  ;; no small rings on top of the ring to be moved (i.e. if the latter
  ;; is at the top of its pile), and there are no small rings on the
  ;; peg to which it is to be moved (i.e. the ring to be moved is
  ;; smaller that the top ring on the peg we intend to move it to).  If
  ;; this is the case, carry out the move and then delete the current
  ;; goal so that rule 2 will apply next time.
   (define-rule (rule-3 towers-structure-rules)
       (?move-assertion <- (move ?ring from ?from to ?to))
       (?ring-assertion <- #(struct:ring ?ring ?from))
       (no #(struct:ring (?ring-1 (< ?ring-1 ?ring)) ?from))
       (no #(struct:ring (?ring-2 (< ?ring-2 ?ring)) ?to))
       (printf "Move ring ~a from ~a to ~a.~n" ?ring ?from ?to)
       (replace ?ring-assertion (make-ring ?ring ?to))
       (retract ?move-assertion))
  ;; If there is a current goal but its disc cannot be moved as in rule
  ;; 3, set up a new goal: that of moving the largest of the obstructing
  ;; rings to the peg that is neither of those specified in the current
  ;; goal (i.e. well out of the way of the current goal).  Delete the
  ;; current goal, so that rule 2 will apply to the new goal next time.
  (define-rule (rule-4 towers-structure-rules)
      (?move-assertion <- (move ?ring from ?from to ?to))
      (peg (?other (not (memq ?other (list ?from ?to)))))
      #(struct:ring (?ring-1 (< ?ring-1 ?ring))
                    (?peg-1 (not (eq? ?peg-1 ?other))))
      (no #(struct:ring (?ring-2 (< ?ring-1 ?ring-2 ?ring))
                        (?peg-2 (not (eq? ?peg-2 ?other)))))
      (replace ?move-assertion `(move ,?ring-1 from ,?peg-1 to ,?other)))
  (define (solve-towers n)
     (activate towers-structure-rules)
     ;(current-inference-trace #t)
     ;; Create pegs.
     (assert '(peg left))
     (assert '(peg middle))
     (assert '(peg right))
     ;; Create rings.
     (do ((i 1 (+ i 1)))
       ((> i n) (void))
       (assert (make-ring i 'left)))
     ;; Start inferencing.
  (solve-towers 6)