CSC151 2009F, Class 20: Naming Local Values

or ... 
  The day everything you've learned about parenthesization goes out
  the window.

Admin:
* I will not have office hours today.  Sorry.
* Please keep your partners from Monday.
* Assignment 5 is now ready.
* EC for attending some part of the Rosenfield anniversary symposium 
  on Thursday.
* Don't forget about the history of programming languages talk at noon 
  on Friday in 3821.
* Reading for Friday: Boolean Values and Predicate Procedures.
* Quiz Friday: Turtles, for-each, let, plus past concepts
* Firefox seems to be having some problems for some people.  Ask me
  for help.

Overview:
* Reminder of algorithm components
* Parenthesization in Scheme.
* Naming things with let.
* Naming things with let*.
* Naming procedures.
* Lab.

Algorithm components:
* Basic values and operations on those values
* [UM ness, the universal machine and its application to algorithms]
* Procedures and functions: Parameterized sequences of operations
  * [Defined with lambda in Scheme]
* [Parentheses, at least in Scheme]
* Ways to name values
* Ways to sequence operations
* Loops: Repeat operations
  * Scheme: (repeat n action parameter)
  * Scheme: (map operation list)
  * [Scheme: (iota n) - make a list]
  * (for-each operation list)
* Conditionals: Choose between operations

Paranetheses
  (foo bar baz)
* If typed by the user, likely to be "apply procedure foo to bar
  and baz"
  * If used in sectioning or compose, it could be "build a procedure"
  * If foo is "define" it could be the indication taht we could use the
    name bar for baz.
* If typed by the computer, likely to be "the list of the symbols
  foo, bar, and baz"
* If typed by the user as part of (lambda (foo bar baz) ...) foo bar
  and baz name parameters
 
Problem: Want to name values (as with define) but
* Want to keep the names safe from other people
* Want them to have limited lifetimes

Define doesn't suffice
The solution: let

NOT THE FORM
(let name value
  expression)

THE REAL FORM
(let ((name value)
      (another-name another-value)
      ...)
  expression)

Where do we put this in a procedure
(define rgb-grey
  (lambda (rgb)
    (rgb-new
      (/ (+ (rgb-red rgb) (rgb-green rgb) (rgb-blue rgb)) 3)
      (/ (+ (rgb-red rgb) (rgb-green rgb) (rgb-blue rgb)) 3)
      (/ (+ (rgb-red rgb) (rgb-green rgb) (rgb-blue rgb)) 3))))


(define rgb-grey
  (lambda (rgb)
    (let ((brightness 
           (/ (+ (rgb-red rgb) (rgb-green rgb) (rgb-blue rgb)) 3)))
    (rgb-new brightness brightness brightness))))

(define radians-to-degrees
  (let ((conversion-factor (/ 180 pi)))
    (lambda (rad)
      (* rad conversion-factor))))