postman Scheme object, http://stackoverflow.com/questions/16428640/mit-scheme-message-passing-abstraction-mailman-procedure/16430154#16430154

gistfile1.scm

(define (make-letter destination message)
  (define (dispatch x)
    (cond ((eq? x 'get-destination) destination)
          ((eq? x 'get-message) message)
          (else "Invalid option.")))
      dispatch)

(define (make-mailbox address)
  (let ((T '()))
    (define (add-post letter)
      (begin (set! T (cons letter T)) 'done)) 
    (define (get-latest-letter)
      (if T (car T)))
    (define (dispatch y)
      (cond ((eq? y 'add-letter) add-post)
            ((eq? y 'get-latest-message) ;;;;;; (get-previous-post T))
                                         get-latest-letter)
            ((eq? y 'get-address) address)
            (else "Invalid option.")))
    dispatch))

(define (find-mailbox address mailboxes)   ; general utility
      (if (not (null? mailboxes))
        (if (equal? address ((car mailboxes) 'get-address))
          (car mailboxes)
          (find-mailbox address (cdr mailboxes)))))

(define (make-mailman)
  (let ((self (list '(ROUTE)
                    '(MAILBAG))))
    (define (add-to-route . mailboxes)
      (let ((route (assoc 'ROUTE self)))
        (set-cdr! route (append mailboxes (cdr route))) 'DONE))
    (define (collect-letters . letters)
      (let ((mailbag (assoc 'MAILBAG self)))
        (set-cdr! mailbag (append letters (cdr mailbag))) 'DONE))
    (define (distribute-the-letters)
      (let* ((mailbag (assoc 'MAILBAG self))
             (mailboxes (cdr (assoc 'ROUTE self)))
             (letters (cdr mailbag)))
    (if (null? letters)
        ()
        (let loop ((letter (car letters))
                   (letters (cdr letters))
                   (not-delivered ()))
          (let* ((address (letter 'get-destination))          ;;;;;; correct name
                 (mbox (find-mailbox address mailboxes)))
        (if mbox
            ((mbox 'add-letter) letter)
            (set! not-delivered (cons letter not-delivered)))
        (if (null? letters)
            (begin (set-cdr! mailbag '()) not-delivered)
            (loop (car letters) (cdr letters) not-delivered)))))))
    (define (dispatch z)
      (cond ((eq? z 'add-to-route) add-to-route)
        ((eq? z 'collect-letters) collect-letters)
        ((eq? z 'distribute) distribute-the-letters)
        (else "Invalid option")))
    dispatch))