6.2 Metaobject Protocol

SBCL supports a metaobject protocol which is intended to be compatible with AMOP; present exceptions to this (as distinct from current bugs) are:

• compute-effective-method only returns one value, not two.

  There is no record of what the second return value was meant to indicate, and apparently no clients for it.

• the arguments :declare and :declarations to ensure-generic-function are both accepted, with the leftmost argument defining the declarations to be stored and returned by generic-function-declarations.

  Where AMOP specifies :declarations as the keyword argument to ensure-generic-function, the Common Lisp standard specifies :declare. Portable code should use :declare.

• although SBCL obeys the requirement in AMOP for validate-superclass for standard-class and funcallable-standard-class to be compatible metaclasses, we impose an additional requirement at class finalization time: a class of metaclass funcallable-standard-class must have function in its superclasses, and a class of metaclass standard-class must not.

  At class finalization, a class prototype which is accessible by a standard mop function sb-mop:class-prototype. The user can then ask whether this object is a function or not in several different ways: whether it is a function according to typep; whether its class-of is subtypep function, or whether function appears in the superclasses of the class. The additional consistency requirement comes from the desire to make all of these answers the same.

  The following class definitions are bad, and will lead to errors either immediately or if an instance is created:

            (defclass bad-object (funcallable-standard-object)
              ()
              (:metaclass standard-class))
       

            (defclass bad-funcallable-object (standard-object)
              ()
              (:metaclass funcallable-standard-class))
       

  The following definition is acceptable:

            (defclass mixin ()
              ((slot :initarg slot)))
            (defclass funcallable-object (funcallable-standard-object mixin)
              ()
              (:metaclass funcallable-standard-class))
       

  and leads to a class whose instances are funcallable and have one slot.