Various Decisions

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

On condition handlers instead of NEXT-METHOD:

It seems that condition handlers are an appropriate vehicle for dealing
with exceptional conditions, or at least with conditions that are not
routinely encountered. For example, in a Lisp in which CAR and CDR of NIL
signal errors, one would be surprised to see the naive SUBST function
written like this:

	(defun subst (x y z)
         (cond ((eq x z) y)
	       (t (cons (subst x y (car z))
			(subst x y (cdr z))))))

with condition handlers for the null and non-CONS cases.

Similarly, if it is the case (and I think it likely) that a programmer
frequently wants to CALL-NEXT-METHOD when there might not be one, then he
will likely want to write:

	(when (next-method) (call-next-method))

or use a non-error-signaling version of CALL-NEXT-METHOD rather than
program a condition handler.

			-rpg-