compatible-metaclasses validates superclasses according to a simple substitution model,
thereby greatly simplifying the definition of class mixins.

⚓ Overview

Traditionally, defining "class mixins" (subclasses of metaclasses, for instance if you just want to add a slot and/or a few methods) has been surprisingly hard due to the combinatorial explosion of c2mop:validate-superclass methods needed when using the naive approach. This library defines a simple substitution model that completely solves this problem.

Here is an example usage:

(defclass my-class-mixin (compatible-metaclasses:standard-class metaclass)
  ()
  (:metaclass compatible-metaclasses:standard-metaclass)
  (:validate-as metaclass))

Note that compatible-metaclasses:standard-metaclass, being used as the metaclass of a metaclass, is thus a metametaclass!

Also note that compatible-metaclasses:standard-class will be effectively ignored in any subclasses of my-class-mixin that don't have compatible-metaclasses:standard-metaclass as their metaclass, which is exactly what we want since we can't safely make any assumptions about how c2mop:validate-superclass should behave (i.e. metaclass compatibility) for those subclasses.

⚓ Dictionary

((validate-as :initarg :validate-as
              :reader compatible-metaclasses:validate-as
              :type class
              :initform (find-class 'standard-class)
              :documentation "The metaclass to use for purposes of validation."))

((metasubstitute :initarg :metasubstitute
                 :reader compatible-metaclasses:metasubstitute
                 :type class))