I previously asked a question concerning message passing Abstraction here: MIT Scheme Message Passing Abstraction
The question asked that I:
Write a mailman object factory (make-mailman) that takes in no parameters and returns
a message-passing object that responds to the following messages:
'add-to-route: return a procedure that takes in an arbitrary number of mailbox objects
and adds them to the mailman object's “route”
'collect-letters: return a procedure that takes in an arbitrary number of letter
objects and collects them for future distribution
'distribute: add each of the collected letters to the mailbox on the mailman's route
whose address matches the letter's destination and return a list of any letters whose
destinations did not match any mailboxes on the route (Note: After each passing of
'distribute the mailman object should have no collected letters.)
I had already written 2 procedures earlier as part of this assignment to make a mailbox and make a letter:
(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 (post letter)
(assoc letter T))
(define (previous-post post)
(if (null? (cdr post)) post (cdr (previous-post post))))
(define (letter-in-mailbox? letter)
(if (member (post letter) T) #t #f))
(define (add-post letter)
(begin (set! T (cons letter T)) 'done))
(define (get-previous-post post)
(if (letter-in-mailbox? post)
(previous-post post)
#f))
(define (dispatch y)
(cond ((eq? y 'add-letter) add-post)
((eq? y 'get-latest-message) (get-previous-post T))
((eq? y 'get-address) address)
(else "Invalid option.")))
dispatch))
After being given a very good explanation on what my current answer was doing wrong and making many necessary changes to my code, I was told that any problems I have in that code would be better off asked in this question. Therefore, here is the code that builds off my previous question:
(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-address))
(mbx (find-mailbox address mailboxes)))
(if (equal? address letter)
((mbx 'add-post) letter)
((mbx 'add-post) 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))
Essentially, I'm running into a different error now that instead returns that the distribute-the-letters procedure is being passed as an argument to length, which is not a list. I do not know why this error is being returned, since I would think that I am passing in the lists as they are needed. Would anyone be able to shed some light on what's going on? Any help will be appreciated.
UPDATE: Using this procedure in my make-mailman code now:
(define (find-mailbox address mailbox)
(if (not (element? address self))
#f
(if (element? mailbox self)
mailbox
#f)))
