&REST lists

[Eric Benson <edsel!eb@labrea.Stanford.EDU>]

It's not only smashing a &rest argument that's a problem, it's
smashing any list that has been given as the last argument to APPLY as
well.  Consider the following in an implementation that doesn't copy
the last argument to APPLY when it is passed as a &rest argument:


> (defvar *message*)
*MESSAGE*
> (defun set-message (&rest mess)
    (setq *message* mess))
SET-MESSAGE
> (let ((winner (list 'a 'winner)))
    (apply #'set-message winner)
    (setf (cdr winner) (list 'loser))
    winner)
(A LOSER)

Is *message* (A WINNER) or (A LOSER)?  (It might be
(#<DTP-LOCATIVE 76123756> #<DTP-ODD-PC 12313453> ...)
but that's a different problem.)  This suggests that once a list has
been given as the last argument to APPLY it is no longer OK to modify
it.