Re: around method for make-instance

[Ora Lassila <ora@ISL1.RI.CMU.EDU>]

I haven't really been following this discussion, but something caught
my eye:

Shannon V Spires <svspire@somnet.sandia.gov> writes:

> But if you just want to specialize make-instance (or any other method that
> takes a class as an argument) for a _single_ class, you have to use the
> (eql...) syntax because you're then defining a method on a single instance
> [of a metaclass].

Using EQL-specializers will not work if you create a subclass of the
class for which MAKE-INSTANCE has been specialized (obviously the
subclass is not eql to the superclass). This is where metaclasses can
be used. If you want to define a class (say, FOO) such that it has a
specialized MAKE-INSTANCE and all of FOO's subclasses also have this
specialized MAKE-INSTANCE, you have to define a new metaclass:

  (DEFCLASS FOO-METACLASS (STANDARD-CLASS)
    ())

  (DEFCLASS FOO ()
    ()
    (:METACLASS FOO-METACLASS))

  (DEFMETHOD MAKE-INSTANCE :AROUND ((CLASS FOO-METACLASS) &REST INITARGS)
    ...)

The problem is that all subclasses of FOO that are to share the new
MAKE-INSTANCE, also have to have FOO-METACLASS (or its subclass) as
their metaclass.

(This example ignores the fact that, according to AMOP, a
VALIDATE-SUPERCLASS method has to be defined for FOO-METACLASS. In MCL
this is not an issue.)

	- Ora Lassila
	  The Robotics Institute
	  Carnegie Mellon University
	  ora@cs.cmu.edu