The new macro, define-tagged-binary-class, will look like define-binary-class with the addition of a :dispatch option used to specify a form that should evaluate to the name of a binary class. The :dispatch form will be evaluated in a context where the names of the slots defined by the tagged class are bound to variables that hold the values read from the file. The class whose name it returns must accept initargs corresponding to the slot names defined by the tagged class. This is easily ensured if the :dispatch form always evaluates to the name of a class that subclasses the tagged class.

The expansion of a define-tagged-binary-class will contain a **DEFCLASS** and a write-object method just like the expansion of define-binary-class, but instead of a read-object method it’ll contain a read-value method that looks like this:

Now you can define both define-binary-class and define-tagged-binary-class to expand into a call to define-generic-binary-class. Here’s a new version of define-binary-class that generates the same code as the earlier version when it’s fully expanded: