Board 1:

(PICS-1.0  "http://www.rsac.org/ratingsv01.html"
	   l gen true comment "RSACi North America Server"
	   by "rodgerb@playboy.com"
	   for "http://www.playboy.com"
	   on "1996.04.04T08:15-0500"
	   r (n 4 s 3 v 0 l 4)))

Board 2:

((PICS-version 1.1)
 (rating-system "http://www.rsac.org/ratingsv01.html")
 (rating-service "http://www.rsac.org/")
 (name "The RSAC Ratings Service")
 (description "...Dr. Roberts of Stanford University...")
 ...
 (category  (transmit-as "v")
	    (name "Violence")
	    (label (name "Conflict")
		   (description "Harmless conflict;...")
		   (value 0))
	    ...)
 ...)

Symbols and PICS

10:05 - 10:10 Public debate about free speech on the Internet, focussed in the U.S. on the issue of children's access to materials on the net that are considered normal for adults but are not generally for children. Work started in June 1995 and still continues, with a White House meeting this past July and an "Internet Summit" in December. Example slide from PC.  Part of the issue is the technology: can we allow everyone to say what they wish, but allow parents some degree of control over what their children see?  

PICS Statement of Principles (overhead slide)

PICS is a cross-industry working group whose goal is to facilitate the development of technologies to give users of interactive media, such as the Internet, control over the kinds of material to which they and their children have access. PICS members believe that individuals, groups and businesses should have easy access to the widest possible range of content selection products, and a diversity of voluntary rating systems.

In order to advance its goals, PICS will devise a set of standards that facilitate the following:

Self-rating:

enable content providers to voluntarily label the content they create and distribute.

Third-party rating:

enable multiple, independent labeling services to associate additional labels with content created and distributed by others. Services may devise their own labeling systems, and the same content may receive different labels from different services.

Ease-of-use:

enable parents and teachers to use ratings and labels from a diversity of sources to control the information that children under their supervision receive.

Clinton "if I can use it anyone can"

PICS members believe that an open labeling platform which incorporates these features provides the best way to preserve and enhance the vibrancy and diversity of the Internet. Easy access to technology which enables first- and third-party rating of content will give users maximum control over the content they receive without requiring new restrictions on content providers.

Membership in PICS includes a broad cross-section of companies from the computer, communications, and content industries, as well as trade associations and public interest groups. PICS member will deploy products and services based on these standards.

10:10 - 10:20 PowerPoint presentation.

10:20 - 10:25 Show sample Rating Service Description and Label already on boards 1 and 2 (and in handout).  Slide of overall PICS system on overhead, swapping with full label and rating service description from handout.

10:25 - 10:35 Introducing symbols, EQ?, and quotation

• Data types we have already seen: numbers, booleans, strings, procedures, pairs
• Introduce EQ? as the fastest and most discerning test for equality. Two things are eq? only if there is no way whatsoever to tell them apart. The exact behavior of eq? is very difficult to describe (see the Scheme manual); for now you only need to understand how it works on symbols and pairs (two pairs are eq? if and only if they are the same pointer in a box-and-pointer diagram).

  Examples and Box-and-Pointer on board 4

  (define x (list 1 2))
  (define y (list 1 2))
  (eq? x x)  => #T
  (eq? x y)  => #F

• Consider the PICS label above. It looks like a list of 15 elements, some of which are strings (they have double quotes around them, like "1996.04.04T08:15-0500") and one of which is another list. But what are things like l, gen, true, and comment? Answer: a new data type, symbol.
• The only thing special about symbols is how they are treated by (and, of course, the new predicate symbol? is true for symbols but not for anything else):

If two symbols look the same, they are the same.

• Are there any Scheme expressions whose values are symbols? There aren’t any variables built in to Scheme whose values are symbols. But there is a new special form that can be used to create symbols.

The value of ‘X is the symbol that prints out as X

Some examples: more on board 4 and 5

(define A ‘X)
A => X
(eq? a ‘X)         => #T
(eq? ‘X ‘X)        => #T
(define B (list a 2))
B => (X 2)
(eq? (car b) ‘X)   => #T
(eq? b (list a 2)) => #F note: CONS always creates a new pair!

One final note. As a convenience, the single quote can be used to create data structures as well as symbols. Examples: on board 6

(define C ‘(X 2))
C                  => (X 2)
(pair? C)         => #T
(eq? (car c) ‘X)  => #T
(define d ‘(A 1 B))
(car d)           => A
(length d)        => 3
(list-ref d 2)    => B

10:35 - 10:40 Selectors from the PICS Service Description data structure

Board 7

(define (assq name a-list)
  (cond ((null? a-list) #F)
        ((eq? name (car (car a-list)))
         (car a-list))
        (else (assq name (cdr a-list)))))

(define (alist->value name a-list)
  (let ((result (assq name a-list)))
    (if result (second result) #F)))

Board 8

(define (category->name category)
  (alist->value 'NAME (cdr category)))

(define (category? entry) (eq? (car entry) 'CATEGORY))

10:40 - 10:50 Code to expand a label given a service description

Board 9

(define (expand picslabel service)
  (let ((ratings (picslabel->rating picslabel)))  ; Magic, on handout
    (map (lambda (category)
           (get-name-and-value category ratings))
         (filter category? service))))

Following from slide:

(define (get-name-and-value category ratings)
  (list (category->name category)
        (category->description category)
        (value->name category
         (the (filter (lambda (rating)
                        (eq? (category->transmit-name category)
                             (rating->name rating)))
                      ratings)))))

(define (the l)
  (cond ((null? l) '())
        ((null? (cdr l))
         (first l))
        (else (error "Not unique" l))))

Following from slide if there is time:

(define (value->name category rating)
  (if (null? rating)
      (list 'NO 'VALUE 'FOR 'THIS 'CATEGORY)
      (let ((value (rating->value rating)))
        (let ((label
               (the (filter
                     (lambda (label)
                       (let ((values (filter value? label)))
                         (if (null? values)
                             #F
                             (= value (second (first values))))))
                     (map cdr (filter label? (cdr category)))))))
          (if label
              (list (label->name label) (label->description label))
              (list 'VALUE 'HAS 'NO 'NAME))))))

10:50 - 10:55 Wrap-up list of data types in Scheme: number, boolean, string, pair, symbol
New today: special form QUOTE, data type SYMBOL, primitive procedures EQ? and SYMBOL?