CSC151.02 2014S, Class 30: Preconditions, Revisited
===================================================

_Overview_

* Preliminaries.
    * Admin.
    * Questions.
* Topics.
    * Verifying preconditions.
    * The `error` procedure.
    * Husk and Kernel programming.
* Lab.

Preliminaries
-------------

### Admin

* Pick new partners.  Work with someone you have not worked with before.
* Extra review session tonight at 9 pm in tragic CS commons
* Extra credit: 
    * ?

### Upcoming Work

* Reading for tomorrow: Naming Local Procedures
* NO LAB WRITEUP TODAY
    * Was CSC 151 Writeup 20: Local Procedure Bindings
* Exam due THURSDAY night/Friday class.

### Exam Questions

Verifying preconditions
-----------------------

* When we design procedures, we make assumptions about the inputs (and, maybe
  the state of the world).
    * (+ val0 val1 ... valn)
        * parameters must be real numbers
        * Error message when the precondition is not met
    * (largest values)
        * parameter must be a nonemptylist of real numbers
    * (hshift-drawing amt drawing)
        * first parameter must be a real number
        * second parameter must be a drawing
    * (image-fill! image)
        * parameter must be an image
        * potentially: must have something selected in the image
* What should happen if the assumptions (preconditions) don't hold?
    * If the preconditions are not met, the procedure must report an error
    * If the preconditions are not met, the procedure can do whatever it wants
       * "It's the user's fault."  It must be a design by lawyers from Micro...
       * "You might get something cool."
       * It's expensive to check preconditions.

   (define square
     (lambda (x)
       (* x x)))

   (define square
     (lambda (x)
       (if (not (number? x))
           (report-an-error "Excuse me, sir/maam/sam, but you seem to be confused about the appropriate input to this procedure.")
           (* x x))))

* Sam's conclusion: It doesn't matter which you do, as long as you document it.

The `error` procedure
---------------------

   (error  "Message" additional values)

Husk and Kernel programming
---------------------------

* When you're iterating a list, you probably don't want to check the same
  preconditions at every step.
* Protect the part that does the work (the kernel) with one outer precondition 
  check (the husk)