CSC151.01 2006S, Class 18: Documentation, Preconditions and Postconditions

Admin:
* The Joe Rosenfield '(25 center)
* Q&A on Exam?
* Reminder: No CS Extra tomorrow.
* I'll try to do less introductory lecture today.
* Reading for Tuesday: Testing (ready after noon)
* Lab partners: Choose your own, keep for the week

Overview:
* Quick summary of topics:
  * Documentation
  * The error procedure
  * Husk and Kernel
* Lab

* (1) It is important to document your code: It helps others, it helps you
* (2) Think about preconditions, particularly 
       "for what values will my code work?"
* (3) When the parameters are wrong, you might 
  (error "You idiot, you called this procedure incorrectly")
* (4) It may be inefficient to repeatedly check parameters, so use "The Iowa Method"

; The Husk
(define proc
  (labmda (params)
    (if (the-client-chose-bad-parameters param)
        (error "You idiot")
        (kernel params))))
; The Kernel
(define proc-kernel
  (lambda (params)
    (go-ahead-with-your-business-without-checking params)))

/Reflection/

(define all-real?
  (lambda (lst)
    (if (not (list? lst))
        (error "all-real?: expected a list, received something else")
        (all-real-kernel? lst))))

(define all-real-kernel?
  (lambda (lst)
    (or (null? lst)
        (and (real? (car lst))
             (all-real-kernel? (cdr lst))))))