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It cannot have escaped the astute reader that classes themselves look like they could be the instances of some class (a metaclass, if you will). Note that all classes have some common behavior: each of them has slots, a superclass, a list of method names, and a method vector. make-instance looks like it could be their shared method. This suggests that we could specify this common behavior by another class (which itself should, of course, be a class instance too).

In concrete terms, we could rewrite our class implementation to itself make use of the object-oriented approach, provided we make sure we don't run into chicken-and-egg problems. In effect, we will be getting rid of the class struct and its attendant procedures and rely on the rest of the machinery to define classes as objects.

Let us identify standard-class as the class of which other classes are instances of. In particular, standard-class must be an instance of itself. What should standard-class look like?

We know standard-class is an instance, and we are representing instances by vectors. So it is a vector whose first element holds its class, ie, itself, and whose remaining elements are slot values. We have identified four slots that all classes must have, so standard-class is a 5-element vector.

(define standard-class
  (vector 'value-of-standard-class-goes-here
          (list 'slots
          (vector make-instance)))

Note that the standard-class vector is incompletely filled in: the symbol value-of-standard-class-goes-here functions as a placeholder. Now that we have defined a standard-class value, we can use it to identify its own class, which is itself:

(vector-set! standard-class 0 standard-class)

Note that we cannot rely on procedures based on the class struct anymore. We should replace all calls of the form

(standard-class? x)
(standard-class.slots c)
(standard-class.superclass c)
(standard-class.method-names c)
(standard-class.method-vector c)
(make-standard-class ...)


(and (vector? x) (eqv? (vector-ref x 0) standard-class))
(vector-ref c 1)
(vector-ref c 2)
(vector-ref c 3)
(vector-ref c 4)
(send 'make-instance standard-class ...)

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