Created
May 14, 2013 10:00
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mailman object , http://stackoverflow.com/questions/16428640/mit-scheme-message-passing-abstraction-mailman-procedure/16430154#1643015
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(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)) |
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