CSC151 2007S, Class 16: Local Bindings

Admin:
* Assigned: Exam 1
* Due: HW 7
* I've made a few short adjustments to the syllabus.
* Today's outline is fairly long; I don't intend to go into 
  that much detail in class.
* Reading for Monday: Preconditions and Postconditions.

Overview:
* Things we might talk about
  * Why name things.
  * Naming things with let.
  * Naming things with let*.
  * Naming procedures.
  * Q & A.
* Lab.

Issue in all algorithms: We want to name values, including computed values
* define provides a perfectly nice way to name things
  * Name values:
    (define time-of-day 1016)
  * Name computed values:
    (define ave-hw (/ (sum hw) (length hw)))
  * name procedures
    (define id (lambda (x) x))
  * Name recursive procedures
    (define factorial (lambda (n) (if (= 0 n) 1 (* n (factorial (- n 1))))))
* At times, we want to name more than what is nameable with define
  * We want to name a value *in the middle of a procedure*
    * Define not formally allowed in that context
  * We want a name that no one else can change
    * Define provides a "globally accessible" name

We need another way to name values: let

(let ((name value)
      (name value)
      (name value))
  exp)

* exp can use the names
* Nothing else can use those names

(define silly
  (lambda (a b c)
    (sqrt (+ (* a a) (* b b) (* c c)))))

(define silly
  (lambda (a b c)
    (let ((square (lambda (x) (* x x))))
      (sqrt (+ (square a) (square b) (square c))))))