MIT Scheme Message Passing Abstraction -

in computer science course taking, homework, tasked several different questions pertaining message passing. have been able solve one, asks following:

write mailman object factory (make-mailman) takes in no parameters , returns message-passing object responds following messages:

• 'add-to-route: return procedure takes in arbitrary number of mailbox objects , adds them mailman object's “route”
• 'collect-letters: return procedure takes in arbitrary number of letter objects , collects them future distribution
• 'distribute: add each of collected letters mailbox on mailman's route address matches letter's destination , return list of letters destinations did not match mailboxes on route (note: after each passing of 'distribute mailman object should have no collected letters.)

some remarks given make code easier include:

• if multiple letters distributed same mailbox in 1 distribution round, 1 of them may “latest” letter message returned passing 'get-latest-message mailbox.

• no 2 mailboxes have same address.

• no mailbox or letter passed mailman more once.

• the bad letters returned distribute not need in specific order.

• use . args syntax accepting arbitrary amount of arguments.

this have been able figure out myself:

(define (make-mailman)   (let ((t '()))     (define (route-adder . mobjects)       (assoc mobjects t))     (define (letter-collecter . lobjects)       (assoc lobjects t))     (define (add-to-route mobjects)       (begin (set! t (cons (route-adder . mobjects) t)) 'done))     (define (collect-letters lobjects)       (begin (set! t (cons (sort-strings (letter-collecter . lobjects)) t)) 'done))     (define (dispatch z)       (cond ((eq? z 'add-to-route) add-to-route)         ((eq? z 'collect-letters) collect-letters)         ((eq? z 'distribute) "unsure of here")         (else "invalid option")))     dispatch)) 

any can given me here appreciated, have tried looking @ problem while, , cannot figure out here.

your code has kinds of mix-ups. :) let's proceed step step.

the dispatch bit ok:

(define (make-mailman)  (let ...   ...   (define (dispatch msg)                ;; use short suggestive var names    (cond      ((eq? msg 'add-to-route)    add-to-route)     ((eq? msg 'collect-letters) collect-letters)     ((eq? msg 'distribute)         ;; "unsure of here" <<-- distribute letters, else?                                 distribute-the-letters)     (else "invalid option")))   dispatch)) 

with such objects, sample call (define ob (make-mailman)) , ((ob 'add-to-route) box1 box2 ... boxn) etc. add-to-route procedure must defined way:

(define (make-mailman)  (let ((self (list '(route)           ; each mailman has route, , mailbag                    '(mailbag))))      ; use suggestive name here (t, t?)   ...   (define (add-to-route . mailboxes)     (let ((route (assoc 'route self)))        (set-cdr! route           (append mailboxes           ; there no duplicates             (cdr route)))       'done)) 

right? same letters:

  (define (collect-letters . letters)     (let ((mailbag (assoc 'mailbag self)))       .....       'done)) 

now can deal missing part, distribute-the-letters:

  (define (distribute-the-letters)     ;; each letter in mailbag     (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 ()))     ;;   access address,            (let* ((address (letter 'get-address))             ;; (we assume supports interface,              ;;   or maybe that's part of previous assignment)     ;;     , find mailbox on route such                  (mbx (find-mailbox address mailboxes)))     ;;     address matches letter's     ;;     , if so,              (if .....     ;;        put letter mailbox:                 ((mbx 'put-letter) letter)     ;;            (we assume supports interface,      ;;             or maybe that's part of previous assignment)     ;;     if not, add letter "not-delivered" list                ..... )             (if (null? letters)     ;; having emptied mailbag, return "not-delivered" list               (begin (set-cdr! mailbag nil) not-delivered)               (loop (car letters) (cdr letters) not-delivered))))))) 

we assume both letter , mailbox objects support message type 'get-address both return same comparable address type of object, , mailbox objects support 'put-letter message.