CSC151.02 2003F, Class 51: Objects

Admin:
* Mr. Park's editorial.
* Questions on the exam?
* The danger to late arrivals of early arrivals.

Overview:
* Strengths and weaknesses of records.
* An alternative: Objects.
* Key aspects of object-oriented programming: Encapsulation, Polymorphism, and Inheritance
* Implementing encapsulation in Scheme.  (I.e., "See the reading")
* Lab.

Question and answers
* How do I deal with symbols or strings?
	(if (symbol? tag) (symbol->string tag) tag)
* How do you write a procedure that takes one or two parameters.
	(define one-or-two
	  (lambda (first . rest)
	    (cond
	      ((null? rest) (display first) (newline))
	      ((null? (cdr rest)) (display (car rest)) (newline))
	      (else (error 'one-or-two "You idiot.  You should only call this procedure with one or two parameters.  You used more.  Are you sure that you understand Scheme?  Take German instead.")))))
* Boy, it seems like you're giving away answers left and right.  Will you give away more?
  * Try and see.

Tell me about records
* Records are data structures
  * Data structures provide a way to structure data
  * "Structure" = "Organize or give form to"
  * "Data" = "Information"
  * Also a way to store information.
* We've certainly seen other ways to store information in Scheme: Lists, Vectors, Association Lists, Files, ...
* What sets records apart?
  * You need to write your own procedures to access the data in records.
  * Records seem to be harder to write than vectors.
  * Records might be defined in terms of the other structures (particularly vectors).
  * Records are intended to be somewhat "encapsulated"; It is hard for someone other than the designer to make new records of a particular type or to do "inappropriate" things to the contents of the record.
  * Each kind of record is designed to store a particular kind of information (e.g., sales for a shirt shop, information on a library book, etc.).  Vectors and Lists are designed to be more general.
  * When you use a record, you don't need to worry about the internal structuring (because someone has written all the helpful procedures)
* Why would you ever want to use records instead of vectors or lists?
  * Clearer code and less worry on the part of the user.
  * Encapsulation is also important.
* How do you build records in Scheme?
  * Create a record constructor (e.g., new-compound)
  * Represent most records as vectors.
  * Write a lot of xxx-get and xxx-set! procedures for that new record (which look a lot like calls to vector-ref and vector-set! with some accompanying "safety" code).
* What's wrong with these kinds of records, given the goals above?
  * You can still use vector-set! to change parts that are not mutable.

Some smart Norwegian computer scientists (Nygaard and ...) in the 1960's decided to design a new way of structuring data to handle this problem.  They decide something called an object and started a "revolution" called object-oriented programming.
* Like records, objects collect information for a specific purpose.
* Unlike records, objects also group "capabilities" with that information.
* Objects generally provide better encapsulation than records.
* "Real" objects provide polymorphism and inheritance.
  * "Polymorphism" = "Many forms" ; Idea that you can write one procedure that can work on many similar objects. "If you know how to add things, you can sum lists of those things."
  * "Inheritance" = "To give or pass down"; Idea that you can design new objects in terms of old.  "Every library book is a book; Library books add the following characteristics."
* We will focus on encapsulation.

How to encapsulate a vector in Scheme

Rather than applying procedures to the vector, we make the vector respond to "instructions".
* We put the lambda after the let.

; 0: mood (0: particuarly grump to 10: incredibly happy)
; 1: weight (a number)
; 2: eye-color (a string)
; 3: cash on hand (an integer, represents money in cents)
(define sams-characteristics
  (let ((characteristics (vector 5 10 "brown" 2000)))
    (lambda (command)
       (cond
         ((eq? ':describe) 
          (display "Everything you needed to know about Sam Rebelsky but didn't want to ask.")
          (newline)
          (display "Sam's mood is ranked at: ")
          (display (vector-ref characteristics 0))
          (newline)
          (display "Sam's weight is ")
          (display "none of your damn business.")
          (newline)
          (display "Sam has $")
          (display (/ (vector-ref characteristics 3) 100))
          (display " on hand."))))))