#lang scheme

(require test-engine/scheme-tests 2htdp/universe)

(define posn% 
  (class object%
    (init-field x y)
    
    ;; -> Number
    (define/public (get-x) x)
    
    ;; -> Number
    (define/public (get-y) y)
    
    ;; Number Number -> posn%
    ;; move this by adding deltax, deltay to the coordinates
    (define/public (translate deltax deltay)
      (new posn% [x (+ x deltax)] [y (+ y deltay)]))
    
    ;; Scene -> Scene 
    ;; add this posn's image to the given scene 
    (define/public (scene+ s) (place-image IMG x y s))
    
    (field [IMG (circle 3 'solid 'red)])

    (super-new)))

(define ex-c (new posn% [x 3][y 4]))

;; what doesn't work 
; (check-expect (send ex-c translate 0 0) ex-c)

;; what works 
(check-expect (send (send ex-c translate 0 0) get-x)
              (send ex-c get-x))

(check-expect (send (send ex-c translate 0 0) get-y)
              (send ex-c get-y))

;; alternative: 
(check-expect (local ((define n (send ex-c translate 0 0)))
                (list (send n get-x) (send n get-y)))
              (list (send ex-c get-x) (send ex-c get-y)))

(test)