🚀 go-pugleaf

    Date: Wed, 15 May 1991 22:37 EDT
    From: clamen@O.GP.CS.CMU.EDU  (Stewart Clamen)


    Section 28.1.10 in the 2nd Edition of CLtL features the following:

	    A class that is in an instance of STANDARD-CLASS can
	    be redefined if the new class will also be an
	    instance of STANDARD-CLASS.  Redefining a class
	    modifies the existing class object to reflect the
	    new class definition; it does not create a new class
	    object for the class. [rest of paragraph omitted --
	    SMC]

	    When the class C is redefined, changes are
	    propogated to its instances and to instances of any
	    of its subclasses.  Updating such an instance occurs
	    at an implementation-dependent time, but no later
	    than the next time a slot of that instance is read
	    or written.  Updating an instance does not change
	    its indentity as defined by the EQ function.  The
	    updating process may change the slots of that
	    particular instance, but it does not create a new
	    instance.  Whether updating an instance comsumes
	    storage is implementation-dependent.

	    ...

    I am curious as to how the current implementations of CLOS (both
    commercial and research) attempt to conform to this specification.  Is
    this feature generally supported yet?  Do implementations that do
    support it convert all instances at the time of class redefinition, or
    "lazily"?  Are there limitations on the types of changes that can be
    made to a class?

Symbolics' (commercial) CLOS conforms to this specification.  We convert
the instances "lazily".  There are no limitations.