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DOESN'T WORK YET

; counter-maker1.scm - 
; - Program 12.7, pg. 391 -

(define for-effect-only
  (lambda (item-ignored)
    "unspecified value"))

(define base-object
  (lambda msg
    (case (1st msg)
      ((type) "base-object")
      (else invalid-method-name-indicator))))

(define box-maker
  (lambda (init-value)
    (let ((cell (cons init-value "any value")))
      (lambda msg
        (case (1st msg)
          ((type) "box")
          ((update!) (for-effect-only (set-car! cell (2nd msg))))
          ((swap!) (let ((ans (car cell)))
                     (set-car! cell (2nd msg))
                     ans))
          ((show) (car cell))
          ((reset!) (for-effect-only (set-car! cell init-value)))
          (else (delegate base-object msg)))))))

(define counter-maker
  (lambda (init-value unary-proc)
    (let ((total (box-maker init-value)))
      (lambda msg
        (case (1st msg)
          ((type) "counter")
          ((update!) (let ((result (unary-proc (send total 'show))))
                       (send total 'update! result)))
          ((swap!) (delegate base-object msg))
          (else (delegate total msg)))))))

(define send
  (lambda (args)
    (let ((object (car args)) (message (cdr args)))
      (let ((try (apply object message)))
        (if (eq? invalid-method-name-indicator try)
            (error 
              (string-append (symbol->string (car message)) ": "
                             "Bad method name sent to object of " 
                             (object 'type) " type."))
            try)))))