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)