(require 2htdp/image)
(require 2htdp/universe)
(define X/Y (vector 90 90))
(vector? X/Y)

(define BACKGROUND (empty-scene 100 100))
(define DOT (circle 3 "solid" "plum"))

(define (main X/Y)
  (big-bang X/Y
    [on-key press]
    [to-draw place-dot-at]))

(define (place-dot-at X/Y)
  (place-image DOT  (vector-ref X/Y 0) (vector-ref X/Y 1) BACKGROUND))

(define (press X/Y a-key)
  (cond
    [(key=? a-key "left") (vector-set! X/Y 0 (sub1 (vector-ref X/Y 0)))]
    [(key=? a-key "right") (vector-set! X/Y 0 (add1 (vector-ref X/Y 1)))]
    [else X/Y]))

(main X/Y)