MIT Scheme Message Passing Abstraction

Question

In a Computer Science course I am taking, for homework, we were tasked with several different questions all pertaining to message passing. I have been able to solve all but one,

Answer 1

Other than the specifics of the message functionality, it looks like you've nailed it. There are however some errors:

This (route-adder . mobjects) should be (router-adder objects) and similarly for (letter-collector . lobjects).

The use of begin is unneeded. The body of a (define (func . args) ...) is implicitly enclosed in a begin.

Idiomatically your code could be written as:

(define (make-mailman)
  (let ((T '()))
    ;; ...
    (lambda (z)
      (case z
        ((add-to-route)    add-to-route)
        ((collect-letters) collect-letters)
        ((distribute)      distribute)
        (else (error "Invalid option"))))))

[but you may not know about case nor lambda yet...]

As for solving the actual messaging functionality. You are going to need to maintain a set of mailboxes where each mailbox is going to hold a set of letters. A letter will presumably consist of an address and some content (extra credit for a return-address). The distribute behavior will check the address on each letter and deposit it in its mailbox. The mailman will need to hold letters (while on his route collecting-letters) until instructed to distribute.

For this you might start by building up the lower-levels of the functionality and then using the lower-levels to build up the actual message passing functionality. Starting like, for example:

(define (make-letter addr content)
  `(LETTER ,addr ,content))
(define letter-addr cadr)
;; ...

(define (make-mailbox addr)
  '(MBOX ,addr))
(define mailbox-letters cddr)
(define (mailbox-letters-add mailbox letter)
  (set-cdr! (cdr mailbox) (cons letter (mailbox-letters mailbox))))

;;...