Lab: Objects in Scheme

Summary: In this lab, we consider techniques for building objects, collections of data that support operations on those data.

Contents:

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Testing Switches
  • Exercise 2: A Single Tally Object
  • Exercise 3: Making Generic Tallys
  • Exercise 4: Tallys with Initial Values
  • Exercise 5: Monitored Tallys
• Notes

Exercises

Exercise 0: Preparation

Start DrScheme.

Exercise 1: Testing Switches

Here's the make-switch procedure from the reading.

;;; Procedure:
;;;   make-switch
;;; Parameters:
;;;   None
;;; Purpose:
;;;   Creates a new switch in the off position.
;;; Produces:
;;;   newswitch, a switch
;;; Preconditions:
;;;   None
;;; Postconditions:
;;;   newswitch is an object which responds to two messages:
;;;     :show-position
;;;       Shows the current position ('on or 'off)
;;;     :toggle!
;;;       Switches the current position
(define make-switch
  (lambda ()
    (let ((state (vector #f)))   ; All switches are off when manufactured.
      (lambda (message)
        (cond ((eq? message ':show-position) 
               (if (vector-ref state 0) 'on 'off))
              ((eq? message ':toggle!) 
               (vector-set! state 0 (not (vector-ref state 0))))
              (else (error "switch: unrecognized message")))))))

Test the switches created by the make-switch procedure. Here are a few possible instructions.

> (define lamp-switch (make-switch))
> (define vacuum-cleaner-switch (make-switch))
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)
> (lamp-switch ':toggle!)
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)
> (lamp-switch ':toggle!)
> (vacuum-cleaner-switch ':toggle!)
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)

Exercise 2: A Single Tally Object

Define a one-field object, tally, that responds to exactly four messages:

• ':show-contents,
• ':set-contents-to-zero!,
• ':increment!, which has the effect of increasing the number stored in the contents field by 1.
• ':decrement!, which has the effect of decreasing the number stored in the contents field by 1.

The initial value of the field should be 0.

Note that you are creating an object, not a procedure that creates objects.

Exercise 3: Making Generic Tallys

a. Define a make-tally procedure that constructs and returns objects similar to the tally object you defined in exercise 2.

b. Create two tally objects and demonstrate that they can be incremented and reset independently.

Exercise 4: Tallys with Initial Values

Write a new make-tally procedure that allows the client to create new tallys with a specied initial value. For example, I might say that a starting grade is 90 with

> (define grade (make-tally 90))

I would then increment and decrement it as students do good or bad work.

Exercise 5: Monitored Tallys

a. Define a constructor procedure, make-monitored-tally, for objects similar to the tally objects from exercise 2 above, except that each such object keeps track of the total number of messages that it has received.

Hint: For this exercise, you will want to make a two-element vector. Element 0 of that vector will be the value of the tally. Element 1 of that vector will be the count of operations.

b. Test your procedure.

Notes