(require (lib "gui.ss" "frtime"))

(define op-names (list "+" "-" "*" "/"))
(define ops (list + - * /))

(define (str->num s)
  (cond
    [(string->number s)]
    [else 0]))

(define x
  (str->num (make-text "First number:")))

(define op
  (make-choice "Op:" op-names))

(define y
  (str->num (make-text "Second number:")))

(make-message
 (format "Result = ~a" ((list-ref ops op) x y)))