Trying to build a tic tac toe without any state (in pure fp style)

Question

I am learning Clojure and trying to implement a simple tic-tac-toe (or morpion). But i am working hard in order to avoid any ref/atom/agent ...

I know I could do it easily in a console application, as it would not be event-driven, so that I would exactly know when to pass in a new board value. I mean

• my board would be a vector of vectors ([[:circle 0 :cross][:any :circle :empty][:cross :none :none]] where only :circle and :cross values matter)
• I would have a simple text-based method which takes a board and return a string

But, whenever I want to implement a graphical Panel, I wonder how can I do the same thing. For example :

• I am creating an instance of javax.swing.JPanel (please don't care about the other methods that are used here) :

  (defn make-board-panel
  "Builds the board JPanel, whose state is given by board argument,
  and turn-piece argument (must be either :circle or :cross, I mean the next
  value to set in the board)."
  [board turn-piece]
  {:pre ['morpion.core/is-board? board, 'morpion.core/is-a-player-piece? turn-piece]}
    (proxy [JPanel MouseListener] []
      (paintComponent [g]
          (proxy-super paintComponent g)
          (paint-board-lines g)
          (paint-board board g)
      )
      (mouseClicked [e]
          (if (and (abs-coord-in-cell? (.getX e)) (abs-coord-in-cell? (.getY e)))
              (let [cell-x (abs-coord-to-rel (.getX e))
                      cell-y (abs-coord-to-rel (.getY e))]
                  ;; Here I compute the new board value
              )  
              nil ;; Here I wish I could return the new board value to the caller of make-board-panel, but this seems impossible !
          ) 
      )
      (mouseEntered [e])
      (mouseExited [e])
      (mousePressed [e])
      (mouseReleased [e])
     )
  )
  

• But it seems that there are no way for me to fetch the new value of the board, from the mouseClicked event of the Panel : unless I introduce a state variable.

So is there a workaround :

Sources for my complete project :

• Morpion Core
• Morpion Graphic.

(I've tried to improve thanks to Igrapenthin comment, but I am still failing.)

Answer 1

(defn make-board-panel
 [board turn-piece output-fn]
 (proxy [JPanel MouseListener] []
   ;; ...
   (mouseClicked [e]
     (when (and (abs-coord-in-cell? (.getX e)) 
           (abs-coord-in-cell? (.getY e)))
       (let [cell-x (abs-coord-to-rel (.getX e))
             cell-y (abs-coord-to-rel (.getY e))]
          ;; Here I compute the new board value
          (output-fn new-board-value))))
    ;; ...
    ))