Question

现在我们可以定义种种方法，或返回 self，或在方法体中使用 self：

(define (make-point init-x init-y)
 (OBJECT
  ([field x init-x]
   [field y init-y])
  ([method x? () x]
   [method y? () y]
   [method x! (new-x) (set! x new-x)]
   [method y! (new-y) (set! y new-y)]
   [method above (other-point)
           (if (> (-> other-point y?) y)
               other-point
               self)]

   [method move (dx dy)
           (begin (-> self x! (+ dx (-> self x?)))
                  (-> self y! (+ dy (-> self y?)))
                  self)])))

(define p1 (make-point 5 5))
(define p2 (make-point 2 2))

> (-> (-> p1 above p2) x?)
5
> (-> (-> p1 move 1 1) x?)
6