Objects in Scheme

In our explorations of Scheme, we've seen a number of data structures that allow us to organize information. A list is a dynamic data structure with a variable number of components. A vector is a data structure with a fixed number of components, each of which you can quickly access by number.

As you've seen, we often build data structures for a particular kind of data. For example, in describing a circle that we might later render, we might store four pieces of information. (There are other useful pieces of information, but these are four that can get us started.)

We then choose whether we want to store those attributes in a list or a vector. Suppose we want our circles to be mutable, so we store those components in a vector, with the color at index 0, the x coordinate at index 1, the y coordinate at index 2, and the radius at index 3.

Once we've designed a representation, we could then tell other programmers about this representation, and let them rely on the structure (e.g., someone who wants to change the color of a circle would do a vector-set! at index 0).

However, suppose we change the representation (e.g., we design a position type and decide to use that type to represent the center of the circle). In that case other programmers must then change their code to match the new representation, which seems time consuming and error-prone. Hence, it is often better to provide a set of procedures to give other programmers access to the key components of our structure. If they use only our procedures, then we can change the representation freely (as long as we change our procedures correspondingly) and their code will continue to work.

Here are some sample procedures for this simple representation of a circle.

(define make-circle
  (lambda (color x-center y-center radius)
    (cond
      [(not (color? color))
       (error "make-circle: parameter 1 (color) must be a color")]
      [(not (real? x-center))
       (error "make-circle: parameter 2 (x-center) must be real")]
      [(not (real? y-center))
       (error "make-circle: parameter 3 (y-center) must be real")]
      [(or (not (real? radius)) (not (positive? radius)))
       (error "make-circle: parameter 4 (radius) must be a positive real")]
      [else
       (vector color x-center y-center radius)])))

(define circle-get-color
  (lambda (circle)
    (vector-ref circle 0)))

(define circle-get-x
  (lambda (circle)
    (vector-ref circle 1)))

(define circle-set-color!
  (lambda (circle newcolor)
    (cond
      [(not (color? newcolor))
       (error "circle-set-color!: parameter 2 (newcolor) must be a color")]
      [else
       (vector-set! circle 0 newcolor)])))

(define circle-set-center!
  (lambda (circle newx newy)
    (cond
      [(not (real? newx))
       (error "circle-set-center!: parameter 2 (newx) must be real")]
      [(not (real? newy))
       (error "circle-set-center!: parameter 3 (newy) must be real")]
      [else
       (vector-set! circle 1 newx)
       (vector-set! circle 2 newy)])))

It is now possible to write new procedures that work with circles. For example, we can render one of these circles as follows.

(define image-render-circle!
  (lambda (image circle)
    (let ([radius (circle-get-radius circle)])
      (context-set-fgcolor! (circle-get-color circle))
      (image-select-ellipse image REPLACE
                            (- (circle-get-x circle) radius)
                            (- (circle-get-y circle) radius)
                            (* 2 radius)
                            (* 2 radius))
      (when (image-has-selection? image)
        (image-fill-selection! image)
        (image-select-nothing! image)))))

Similarly, we can compute the area of a circle with

(define circle-area
  (lambda (circle)
    (* pi (square (circle-radius circle)))))

If we later change the underlying representation to use a position for the center, we'll need to change circle-get-x and such, but we will not need to change image-render-circle or circle-area.

(define make-circle
  (lambda (color x-center y-center radius)
    (cond
      [(not (color? color))
       (error "make-circle: parameter 1 (color) must be a color")]
      [(not (real? x-center))
       (error "make-circle: parameter 2 (x-center) must be real")]
      [(not (real? y-center))
       (error "make-circle: parameter 3 (y-center) must be real")]
      [(or (not (real? radius)) (not (positive? radius)))
       (error "make-circle: parameter 4 (radius) must be a positive real")]
      [else
       (vector color (position-new x-center y-center) radius)])))

(define circle-get-color
  (lambda (circle)
    (vector-ref circle 0)))

(define circle-get-x
  (lambda (circle)
    (vector-ref (position-col circle 1))))

(define circle-set-color!
  (lambda (circle newcolor)
    (cond
      [(not (color? newcolor))
       (error "circle-set-color!: parameter 2 (newcolor) must be a color")]
      [else
       (vector-set! circle 0 newcolor)])))

(define circle-set-center!
  (lambda (circle newx newy)
    (cond
      [(not (real? newx))
       (error "circle-set-center!: parameter 2 (newx) must be real")]
      [(not (real? newy))
       (error "circle-set-center!: parameter 3 (newy) must be real")]
      [else
       (vector-set! circle 1 (position-new newx newy))])))

If we did not want to permit other programmers to change particular parts (e.g., we might not want client programmers to recolor circles) or to to limit the kinds of possible access (e.g., you can set the x and y coordinates of a turtles, but you can't determine them). we would not provide procedures that gave that extra access.

This technique (of providing both representation and procedures that use that representation) has many advantages, as suggested above. For many, the most compelling advantage is that client code (procedures that use our student representation) need not change when the representation changes.

Another possible advantage is that we prevent naive programmers from doing inappropriate things to our structure (e.g., storing something other than a color at index 0). Unfortunately, we haven't really prevented such inappropriate behavior, since clients can still determine the representation we use and then modify things directly.

> (define circle0 (make-circle "blue" 0 0 10))
> (circle-set-color circle0 'red)
make-circle: parameter 1 (color) must be a color
> circle0
#("blue" 0 0 10)
> (vector-set! circle0 0 'red)
> (image-render-circle! canvas circle0)
context-set-fgcolor!: expects type <color> for 1st parameter, given red
  in (context-set-fgcolor! red)

We'd like to encapsulate our implementation so that we can hide how our circles are implemented and restrict how they're used.

One of the basic ideas of the programming paradigm called object-oriented programming is to encapsulate the data so as to intercept low-level interventions and treat them as errors. An object is a data structure that permits access to and modification of its elements only through a fixed set of procedures, the object's methods. One cannot “peek inside” an object; one is limited to the procedures provided.

To request the execution of one of these methods, one sends the object a message that names the desired method, providing any additional parameters that the object will need as part of the message. Attempting to send an object a message that does not name one of its methods simply causes an error. The custom is to precede the message names with colons.

The Scheme standard does not include objects. However, you can implement an object as a procedure that takes messages as parameters and inspects them before acting on them. Since Scheme typically does not allow one to look inside procedures, procedures provide an appropriate form of encapsulation.

How do we store data for use within the procedure? We can use vectors to build the storage locations that are protected by the procedure.

Here's a simple example -- an object named sample-box that contains only one field,contents, and responds to only one message, ':get-contents.

;;; Value
;;;   sample-box
;;; Type:
;;;   object
;;; Purpose:
;;;   To provide a sample "box"; something whose value you
;;;   can look at but not change.
;;; Valid Messages:
;;;   :get-contents
;;;     Get the contents of the box.
(define sample-box
  (let ([contents (vector 42)])
    (lambda (message)
      (cond
        [(eq? message ':get-contents)
         (vector-ref contents 0)]
        [else
         (error "sample-box: unrecognized message" message)]))))

That is,

We can test our sample object by trying to set the contents to 0.

> (sample-box ':get-contents)
42
> (sample-box ':set-contents-to-zero!)
sample-box: unrecognized message :set-contents-to-zero!
> (sample-box ':set-contents! 0)
sample-box: unrecognized message :set-contents!
> (vector-set! contents 0)
reference to unidentified identifier: contents
> (vector-set! sample-box 0 0)
vector-set!: expects type <mutable vector> as 1st argument, given: #<procedure:sample-box>; other arguments were: 0 0
> (sample-box ':get-contents)
42

All these attempts to modify the contents field of sample-box fail, as will all attempts. Sending it the message ':set-contents-to-zero! doesn't work, because the procedure is not set up to receive such a message. And you can't reach the actual contents variable from outside the sample-box procedure because that identifier is bound to the storage location that contains 42 only inside the body of the let-expression.

In fact, we can't even see that vector (as we could with circles)

> sample-box
#<procedure:sample-box>

Of course, a value that you cannot change is not always so useful. Hence, we might revise the procedure so that it would accept the message ':set-contents-to-zero!:

;;; Value
;;;   zeroable-box
;;; Type:
;;;   object
;;; Purpose:
;;;   Provides a sample "box"; something whose value you
;;;   can look at and change to 0
;;; Valid Messages:
;;;   :get-contents
;;;     Get the contents of the box.
;;;   :set-to-zero!
;;;     Set the contents of the box to 0.
(define zeroable-box
  (let ([contents (vector 57)])
    (lambda (message)
      (cond 
        [(eq? message ':get-contents) 
         (vector-ref contents 0)]
        [(eq? message ':set-contents-to-zero!) 
         (vector-set! contents 0 0)]
        [else 
         (error "zeroable-box: unrecognized message" message)]))))

Here's a simple interaction with the box.

> (zeroable-box ':get-contents)
57
> (zeroable-box ':set-contents-to-zero!)
> (zeroable-box ':get-contents)
0 

Of course, there is no way for anyone to set the contents of this particular object to anything except zero. Now that the box has been zeroed its contents will remain zero forever. If we want the box to change, we might add an ':increment! message.

;;; Value
;;;   another-box
;;; Type:
;;;   object
;;; Purpose:
;;;   Provides a sample "box"; something whose value you
;;;   can look at, set to 0, and increment
;;; Valid Messages:
;;;   :get-contents
;;;     Get the contents of the box.
;;;   :set-to-zero!
;;;     Set the contents of the box to 0.
;;;   :increment!
;;;     Add 1 to the contents of the box.
(define another-box
  (let ([contents (vector 0)])
    (lambda (message)
      (cond 
        [(eq? message ':get-contents)
         (vector-ref contents 0)]
        [(eq? message ':set-contents-to-zero!)
         (vector-set! contents 0 0)]
        [(eq? message ':increment!)
         (vector-set! contents 0 (+ (vector-ref contents 0) 1))]
        [else (error "zeroable-box: unrecognized message")]))))

Our interactions with this box are similar.

> (another-box ':get-contents)
0
> (another-box ':increment!)
> (another-box ':increment!)
> (another-box ':increment!)
> (another-box ':get-contents)
3
> (another-box ':set-contents-to-zero!)
> (another-box ':get-contents)
0
> (another-box ':increment!)
> (another-box ':get-contents)
1

What if we want to include a value with a message, such as when we want to change the boxed value to a particular new value? We'll see later in this reading.

In the preceding examples, we have created only one object of each type, but it is not difficult to write a higher-order constructor procedure that can be called repeatedly, to build and return any number of objects of a given type. Suppose, for example, that we want to build several switches, each of which is an object with one field (a Boolean value) and responding to only two messages: ':get-position, which returns 'on if the field contains #t and 'off if it contains #f, and ':toggle!, which changes the field from #t to #f or from #f to #t.

We might start by building a single switch to think about the design.

(define switch
  (let ([state (vector #f)])
    (lambda (message)
      (cond 
        [(eq? message ':get-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" message)]))))

However, when we want more than one, we need a procedure that builds switches. Hence, we need to write a procedure that returns something like the previous object. We call something that returns objects a constructor. Here's a constructor for switches.

;;; 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:
;;;     :get-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 ':type)
           'switch]
          [(eq? message ':->string)
           (string-append "#<switch>(" 
                          (if (vector-ref state 0) "on" "off")
                          ")")]
          [(eq? message ':get-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" message)])))))

The ordering of lambdas and lets is important. Because the make-switch procedure enters the let-expression to create a new binding each time it is invoked, each switch that is returned by make-switch gets a separate static state variable to put its state in. This static variable retains its contents unchanged even between calls to the object and independently of calls to any other object of the same type.

You'll note that we've added support for two other messages, ':type and ':->string message. It is good practice to regularly include those two methods.

> (define overhead-lights (make-switch))
> (define board-lights (make-switch))
> (overhead-lights ':get-position)
off
> (board-lights ':get-position)
off
> (board-lights ':toggle!)
> (board-lights ':get-position)
on
> (overhead-lights ':get-position)
off
> (overhead-lights ':toggle!)
> (overhead-lights ':->string)
"#<switch>(on)"
> (overhead-lights ':type)
switch

In all of the preceding examples, the messages received by the object have not included any additional parameters. Suppose that we want to define an object similar to sample-box except that one can replace the value in the contents field with any integer that is larger than the one that it currently contains, by giving it the message ':set-value! and including the new, larger value.

Scheme supports a special lambda syntax that permits variable numbers of parameters. If you include a period in the parameter list and follow it with an identifier, when the procedure is called, all remaining arguments are packed up into a list and associated with that identifier.

> (define param-test (lambda (message . parameters) parameters))
> (param-test 'hello)
()
> (param-test 'hello 'goodbye)
(goodbye)
> (param-test 'hello 'goodbye 1 2 3)
(goodbye 1 2 3)
> (param-test)
procedure param-test: expects at least 1 argument, given 0

As the examples suggest, this form allows us to require one parameter (the message) and leave the remaining parameters optional. We often use this form when supporting parameterized messages.

;;; Procedure:
;;;   make-growing-box
;;; Parameters:
;;;   [None]
;;; Purpose:
;;;   Creates a new box whose values you can change to larger values.
;;; Produces:
;;;   newbox, a box whose contents can change to larger values.
;;; Preconditions:
;;;   [None]
;;; Postconditions:
;;;   newbox is an object which responds to two messages:
;;;     :get-value
;;;        Get the contents of the box.
;;;     :set-value! val
;;;        Set the contents of the box to val, provided val
;;;        is larger than the current contents of the box.
(define make-growing-box
  (lambda ()
    ; Build a new vector that contains the one value
    ; accessed by the object.
    (let ([contents (vector 0)])
      ; Respond to messages with additional parameters
      (lambda (message . parameters)
        (cond
          ; [type]
          ;   Get the type
          [(eq? message ':type)
           'growing-box]
          ; [:->string]
          ;   Convert to a sting (typically for output)
          [(eq? message ':->string)
           (string-append "#<growing-box>("
                          (number->string (vector-ref contents 0))
                          ")")]
          ; [:get-value]
          ;   Show the current contents of the box
          [(eq? message ':get-value) 
           (vector-ref contents 0)]
          ; [:set-value! val]
          ;   Replace the contents of the box with val
          [(eq? message ':set-value!)
           (cond
             ; We need at least one parameter
             [(null? parameters)
              (error "growing-box:set-value!: requires an argument")]
             ; But no more than one
             [(not (null? (cdr parameters)))
              (error "growing-box:set-value!: only one argument allowed")]
             [else
              (let ([new-contents (car parameters)])
                (cond 
                  ; That parameter needs to be an integer
                  [(not (integer? new-contents))
                   (error "growing-box:set-value: "
                          "the argument must be an integer")]
                  ; Precondition: The new value must be larger
                  [(<= new-contents (vector-ref contents 0))
                   (error "growing-box:set-value: "
                          "the argument must exceed the current contents")]
                [else (vector-set! contents 0 new-contents)]))])]
          ; [OTHER MESSAGE]
          ;   No other messages are allowed
          [else 
           (error "#<growing-box>: unrecognized message " message)])))))

> (define growable (make-growing-box))
> box
<procedure>
> (growable ':get-value)
0 
> (growable ':set-value! 5)
> (growable ':get-value)
5
> (growable ':set-value! 3)
growing-box:set-value: the argument must exceed the current contents
> (growable ':get-value)
5
> (growable ':set-value! 'foo)
growing-box:set-value: the argument must be an integer
> (growable ':set-value!)
growing-box:set-value: an argument is required
> (growable ':get-value)
5
> (growable ':set-value! 7)
> (growable ':->string)
"#<growing-box>(7)"

All the objects that we've seen so far have stored only one value. However, since we use a vector to keep track of the value, we can certainly store more than one value in the vector. For example, suppose we want something that keeps track of the number of times we get the answers “yes” and “no”. We'll use position 0 to keep track of the yes answers and position 1 to keep track of the no answers.

(define make-yesno
  (lambda ()
    (let ([counts (vector 0 0)])
      (lambda (message)
        (cond
          ; [:type]
          [(eq? message ':type)
           'yesno]
          ; [:->string]
          ;  Convert to a string
          [(eq? message ':->string)
           (string-append "<yesno>("
                          "yes:" (number->string (vector-ref counts 0))
                          ", no:" (number->string (vector-ref counts 1))
                          ")")]
          ; [:yes!]
          ;  Increment the number of yes responses.
          [(eq? message ':yes!)
           (vector-set! counts 0 (+ 1 (vector-ref counts 0)))]
          ; [:no!]
          ;  Increment the number of no responses.
          [(eq? message ':no!)
           (vector-set! counts 1 (+ 1 (vector-ref counts 1)))]
          ; [:report!]
          ;  Print a report of responses.
          [(eq? message 'report!)
           (display "Yes appeared ")
           (display (vector-ref counts 0))
           (display " times.")
           (newline)
           (display "No appeared ")
           (display (vector-ref counts 1))
           (display " times.")
           (newline)]
          [else
           (error "#<yesno>: unrecognized message" message)])))))

For example,

> (define yn (make-yesno))
> (yn ':yes!)
> (yn ':report!)
Yes appeared 1 times.
No appeared 0 times.
> (yn ':no!)
> (yn ':no!)
> (yn ':no!)
> (yn ':report!)
Yes appeared 1 times.
No appeared 3 times.
> (yn ':no!)
> (yn ':no!)

However, we might find it clearer to build two vectors, one that keeps track of yes responses and one that keeps track of no responses.

(define make-yesno
  (lambda ()
    (let ([yes-count (vector 0)]
          [no-count (vector 0)])
      (lambda (message)
        (cond
          ; [:type]
          [(eq? message ':type)
           'yesno]
          ; [:->string]
          ;  Convert to a string
          [(eq? message ':->string)
           (string-append "<yesno>("
                          "yes:" (number->string (vector-ref yes-count 0))
                          ", no:" (number->string (vector-ref no-count 0))
                          ")")]
          ; [:yes!]
          ;  Increment the number of yes responses.
          [(eq? message ':yes!)
           (vector-set! yes-count 0 (+ 1 (vector-ref yes-count 0)))]
          ; [:no!]
          ;  Increment the number of no responses.
          [(eq? message ':no!)
           (vector-set! counts 0 (+ 1 (vector-ref no-count 0)))]
          ; [:report!]
          ;  Print a report of responses.
          [(eq? message 'report!)
           (display "Yes appeared ")
           (display (vector-ref yes-count 0))
           (display " times.")
           (newline)
           (display "No appeared ")
           (display (vector-ref no-count 1))
           (display " times.")
           (newline)]
          [else
           (error "#<yesno>: unrecognized message" message)])))))

This latter technique has the advantage of being a bit more readable - We don't have to remember what position in the vector we've used for what value.

At the beginning of this reading, we considered one mechanism for representing circles: a vector with associated procedures. Now that you've seen how to build objects, we can consider how to represent circles as objects.

;;; Procedure:
;;;   make-circle
;;; Parameters:
;;;   initial-color, a color
;;;   x-center, a real
;;;   y-center, a real
;;;   initial-radius, a positive real
;;; Purpose:
;;;   Creates a circle object.
;;; Produces:
;;;   circle, an object
;;; Preconditions:
;;;   [No additional]
;;; Postconditions:
;;;   Initially, when rendered, circle has the given color, 
;;;     center, and radius.
;;;   circle is an object that responds to the following messages
;;;     :area
;;;        Get the area of the circle.
;;;     :clone
;;;        Make a copy of the circle.
;;;     :render-on image
;;;        Render the circle on an image.
;;;     :set-center! x y
;;;        Set the position of the circle.
;;;     :set-color! new-color
;;;        Set the color of the circle.
;;;     :set-radius! new-radius
;;;        Set the radius of the circle.
(define make-circle
  (let ([get-field (lambda (field) (vector-ref field 0))]
        [set-field! (lambda (field val) (vector-set! field 0 val))])
    (lambda (initial-color x-center y-center initial-radius)
      (cond
        ((not (color? initial-color))
         (error "make-circle: parameter 1 (initial-color) must be a color"))
        ((not (real? x-center))
         (error "make-circle: parameter 2 (x-center) must be real"))
        ((not (real? y-center))
         (error "make-circle: parameter 3 (y-center) must be real"))
        ((or (not (real? initial-radius)) (not (positive? initial-radius)))
         (error "make-circle: parameter 4 (initial-radius) must be a positive real"))
        (else
         (let ([color (vector initial-color)]
               [x (vector x-center)]
               [y (vector y-center)]
               [radius (vector initial-radius)])
           (lambda (message . parameters)
             (cond
               ; [:type]
               ((eq? message ':type)
                'circle)
               
               ; [:->string]
               ;  Convert to a string
               ((eq? message ':->string)
                (string-append "<circle>("
                               "color:" (color->string (get-field color))
                               ", center:(" (number->string (get-field x))
                               "," (number->string (get-field y)) ")"
                               ", radius:" (number->string (get-field radius))
                               ")"))
               
               ; [:area]
               ((eq? message ':area)
                (* pi (square (get-field radius))))
               
               ; [:clone]
               ((eq? message ':clone)
                (make-circle (get-field color)
                             (get-field x)
                             (get-field y)
                             (get-field radius)))
               
               ; [:render-on image]
               ;  Render the circle on an image
               ((eq? message ':render-on)
                ; Sanity checking skipped
                (let ([image (car parameters)]
                      [r (get-field radius)])
                  (context-set-fgcolor! (get-field color))
                  (image-select-ellipse! image REPLACE
                                         (- (get-field x) r) (- (get-field y) r)
                                         (* 2 r) (* 2 r))
                  (when (image-has-selection? image)
                    (image-fill-selection! image)
                    (image-select-nothing! image)
                    (context-update-displays!))))
               
               ; [:set-center! x y]
               ;   Set the center to (x,y)
               ((eq? message ':set-center!)
                ; No sanity checking.
                (set-field! x (car parameters))
                (set-field! y (cadr parameters)))
               
               ; [:set-color! newcolor]
               ;   Set the color of the circle.
               ((eq? message ':set-color!)
                (error "<circle>:set-color! is not yet implemented"))
               
               ; [:set-radius! new-radius]
               ;   Set the radius to new-radius
               ((eq? message ':set-radius!)
                ; Most of the sanity checking skipped
                (let ([new-radius (car parameters)])
                  (cond
                    ((or (not (real? new-radius)) (not (positive? (new-radius))))
                     (error "<circle>:set-radius! invalid radius" new-radius))
                    (else
                     (set-field! radius new-radius)))))
               
               ; [DEFAULT]
               ;  No other messages supported right now.
               (else
                (error "<circle>: invalid message" message))))))))))