Here is the skeleton for a queue that uses message passing.

Note that you do not pass a queue as an input to a queue object. It is already a queue!

Peek is already written. Blanks do not imply that only one line of code is needed. They do imply that some amount of code is needed.

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))

    ; empty-queue? is written to align with the way front-ptr
    ; and rear-ptr were given, above
    (define (empty-queue?)

    ; peek returns the datum at the front of the queue
    ; peek returns #f if the queue is empty
    (define (peek)
      (cond ((empty-queue?) (error "Empty queue.  :-("))
            (else (car front-ptr))))

    ; insert-queue! plays out differently depending on whether the queue
    ; is currently empty or not
    (define (insert-queue! datum)
      (let ((new-node (cons datum ())))

    ; delete-queue! has three possibilties:
    ; * empty queue
    ; * one element in queue
    ; * more than one element in queue
    (define (delete-queue!)
      (cond ((empty-queue?) (error "Empty queue.  :-("))
                    ; store the datum at the head of the queue
                  (let ((return-value (peek)))
                    ; update the front pointer
                    ; If there was only one thing in the queue, then the
                    ; rear-ptr will need to be set to nil
                    (if (null? front-ptr) ________)
                    ; Now return the element of the queue (or #f)

    (define (dispatch message)
      (cond ((eq? message 'insert-queue!) insert-queue!)
            ((eq? message 'delete-queue!) delete-queue!)
            ((eq? message 'peek) peek)
            ((eq? message 'empty?) empty-queue?)))