CSC151 2009F, Class 30: Preconditions, Revisited

Admin:
* Welcome back from break!  I hope you are feeling refreshed.
* Reading for tomorrow: Local Procedure Bindings and Recursion.
* The reading for today accidentally included some notes on an image 
  model (spots) that we have not covered this semester.  
  * I apologize.  
  * That issue has been addressed.
  * We'll go over the key issues in the reading.
* The distribution on the last quiz was bimodal, so we'll go over it a bit.

Overview:
* Notes on the quiz.
* Verifying preconditions.
* The error procedure.
* Husk and Kernel programming.

QUIZ STUFF

(define rgb-dark?
  (lambda (rgb)
    (> 192 (+ (rgb-red rgb) (rgb-green rgb) (rgb-blue rgb)))))

(define tally-darks
  (lambda (colors)
    (tally-darks-helper 0 colors)))

(define tally-darks-helper
  (lambda (tally remaining-colors)
    (write (list 'tally-darks-helper tally remaining-colors)) (newline)
    (if (null? remaining-colors)
        tally
        (if (rgb-dark? (car remaining-colors))
            (tally-darks-helper (+ 1 tally) (cdr remaining-colors))
            (tally-darks-helper tally (cdr remaining-colors))))))

Let's document tally-darks

;;; Procedure:
;;;   tally-darks
;;; Parameters:
;;;   colors, a list of colors
;;; Purpose:
;;;   Count the number of colors in the list that rgb-dark? considers
;;;   dark.
;;; Produces:
;;;   tally, an integer
;;; Preconditions:
;;;   colors should include only RGB colors.
;;;   rgb-dark? needs to be defined
;;; Postconditions:
;;;   0 <= tally <= (length colors)

An important question for the designer of procedures: What should the
procedure do when the client fails to meet the preconditions?
* Don't worry about it, let the program crash or return awkward results
  or ...
  * Sam thinks this is dangerous 
    * It might still return something, and the client won't notice that
      what it got back is wrong.
      * It can interepret numbers as RGB colors.
    * In a lot of procedures that have the form
      (if (null? (cdr lst))
          (car lst)
          ...)
* Report an error message

(define rgb-reddest
  (lambda (colors)
    (if (null? (cdr colors))
        (car colors)
        (let ((reddest-remaining (rgb-reddest (cdr colors))))
          (if (> (rgb-red (car colors)) (rgb-red reddest-remaining))
              (car colors)
              reddest-remaining)))))

> (rgb-reddest (list "Grinnell"))
"Grinnell"

Um ... that's not quite the answer we expect, since "Grinnell" is not an
RGB color.

The solution?  Use the error procedure.

(define rgb-reddest
  (lambda (colors)
    (when (not (rgb? (car colors)))
      (error "You idiot, that's not an RGB color.  I refuse to answer your question.  You gave me the strange value" (car colors)))
    (if (null? (cdr colors))
        (car colors)
        (let ((reddest-remaining (rgb-reddest (cdr colors))))
          (if (> (rgb-red (car colors)) (rgb-red reddest-remaining))
              (car colors)
              reddest-remaining)))))

[Not covered in class: What happens with the empty list?  Another
incomprehensible error.  We can fix that, too.

    (when (null? colors)
      (error "What part of NON-EMPTY LIST did you not understand?"))