Category Errors

[Dick Gabriel <RPG@SAIL.STANFORD.EDU>]

One minor point that came up at the Symbolics meeting last Thursday
was category errors and whether we should try to outlaw them somehow.
Here is an example of a category error:

Suppose that MAKE-INSTANCE is a generic function. Its definition
might look like this:

	(defmethod make-instance ((class standard-class) &rest args)...)

Notice that the class of the argument CLASS is STANDARD-CLASS, which
indicates that the operation of MAKE-INSTANCE is controlled at the
meta-object level. Suppose someone were to write:

	(defmethod make-instance ((class apple-pie) &rest args)...)

	(make-instance (class-prototype (class-named 'apple-pie)) ...)

Here someone would be making a category error by defining a method on
MAKE-INSTANCE at the wrong level - the class level. What makes this easy
for a programmer to do this is the availability of CLASS-PROTOTYPE.
However, it is never hard to do because the programmer can write:

	(defvar *apple-pie-prototype* (make-instance 'apple-pie))

to get his class prototype. 

We have two alternatives:

	1. Let sea of classes/metaclasses remain a miasma and inform
	   programmer that he ought not mix things up.

	2. Define a notion of class category or classs level and have a
	   category of generic function for each class category.

			-rpg-