linfo2335-programming-paradigms/project-2/step3.rkt

(define (point x y)
  (define (getx) (lambda () x))
  (define (gety) (lambda () y))
  (define (type) (lambda () 'point))
  (define (setx!) (lambda (nx) (set! x nx)))
  (define (sety!) (lambda (ny) (set! y ny)))
  (define (info) (lambda () (list 'point x y)))
  (define (add) (lambda (p)
                  (let ((px ((p 'getx)))
                        (py ((p 'gety))))
                    (point (+ x px) (+ y py))
                   )
                 ))
  (define (sub) (lambda (p)
                  (let ((px ((p 'getx)))
                        (py ((p 'gety))))
                    (point (- x px) (- y py))
                   )
                 ))
  (define (self m)
    (cond ((eq? m 'getx) (getx))
          ((eq? m 'gety) (gety))
          ((eq? m 'type) (type))
          ((eq? m 'setx!) (setx!))
          ((eq? m 'sety!) (sety!))
          ((eq? m 'info) (info))
          ((eq? m 'add) (add))
          ((eq? m 'sub) (sub))
          (else (lambda args (display (string-append "Message not understood: " (symbol->string m) "\n"))))))
  self
 )

(define (method-lookup reciever message)
  (if (= (length message) 1)
      ((reciever (car message)))
      ((reciever (car message)) (car (cdr message)))
  ))

(define (send reciever . message)
  (if (procedure? reciever)
      (method-lookup reciever message)
      (display (string-append "Inappropriate receiver object: " (symbol->string reciever) "\n"))
  )
 )


(define p1 (point 1 2))
(define p2 (point 3 4))
(send p1 'getx) ; 1
(send p1 'gety) ; 2
(send p2 'getx) ; 3
(send p2 'gety) ; 4
(define p (send p1 'add p2))
(send p 'info) ; (point 4 6)
(send 'not-a-point 'info) ; should display "Inappropriate receiver object"
(send p 'foo) ; should display "Message not understood"
(send p 'bar 2) ; should display "Message not understood"
(send p1 'setx! 5)
(send p1 'getx) ; returns 5