#lang racket
  (provide
    error-obj
    make-obj)

(define error-obj
  (lambda (msg . args)
    (error 'dispatch "no field ~a" msg)))

  (define make-obj
    (lambda maybe-parent
      (let ([delegate (get-arg-or-default maybe-parent error-obj)])
        (let ([table (make-hash)])
          (lambda msg
             (match msg  
                                     
               [(list (? symbol? x)) ;; a list with one symbol
                 (hash-ref table x   ;; ;; it's a get
                   (lambda ()
                     (delegate x)))] ;; look up x in the delegate

               [(list (? symbol? x) v) ;; it's  a set
                (hash-set! table x v)]

               [else (error 'make-obj "invalid object lookup syntax ~a" msg)]))))))
  
  (define get-arg-or-default
    (lambda (args default)
      (if (null? args)
        default
        (car args))))

;;; demonstrating inheritance
;;; -------------------------
(define b (make-obj))  ;; base object
(b 'x 5)  ;; install the binding (x 5) in b
(define c (make-obj b)) ;; c inherits from b
(require rackunit)

;; install the binding (y 7) in c
(c 'y 7)  
;; return y's binding => 7
(check-eq? (c 'y) 7 "c7")
;; return x's binding => 5
(check-eq? (c 'x) 5 "c5")
;; create the binding (x 3) in c
(c 'x 3)  
;; => 3
(check-eq? (c 'x) 3 "c3")
;; => 5  b's binding for x remains unchanged
(check-eq? (b 'x) 5 "b5")