Re: define-modify-macro bug?

On 16 Nov 2003 at 16:45, Dan Muller afjeccf02-at-sneakemail.com |Y wrote:

> Is this a bug in Corman Lisp 2.5? This example is very similar to one 
> given in the Hyperspec, and the definition of appendf also exists in the 
> version of ASDF that I am trying to use:
> 
> ?(define-modify-macro appendf (&rest args) 
>     append "Append onto list")
> ;;; Warning: Symbol &REST assumed special
> APPENDF
> ?(setq x '(a b c))
> (A B C)
> ?(appendf x '(d e f) '(1 2 3))
> ;;; An error occurred in function MACROEXPAND:
> ;;; Error: The variable &REST is unbound
> ;;; Entering Corman Lisp debug loop. 
> ;;; Use :C followed by an option to exit. Type :HELP for help.
> ;;; Restart options:
> ;;; 1   Abort to top level.
> :c 1
> ;;; Returning to top level loop.
> ?
> 
Working from the assumption that the above is in fact a bug, I thought it would be an 
interesting exercise to try to fix it, seeing as I haven't written any non-trivial macros yet. I 
started from the definition in sys/control-structures.lisp, and came up with the following 
modified version, which seems to work:

(defmacro define-modify-macro (name lambda-list function &optional documentation)
	(let ((place (gensym))
              reg-args
              rest-arg)
          (flet ((get-args (ll)
                   (cond ((null ll) t)
                         ((eq (car ll) '&rest) (setq rest-arg (cadr ll)))
                         ((eq (car ll) '&optional) (get-args (cdr ll)))
                         (t (push (car ll) reg-args)))))
            (get-args lambda-list)
            `(defmacro ,name (,place ,@lambda-list &environment env)
               ,@(if documentation (list documentation))
               (multiple-value-bind (vars vals new setter getter)
                 (get-setf-expansion ,place env)
                 (declare (ignore setter))
                 `(let* (,@(mapcar 'list vars vals) (,(car new) ,getter))
                    (setf ,getter (funcall (function ,',function) ,(car new) ,,@reg-args ,@,rest-
arg))))))))
        

Critiques are welcome. The nested backquote form at the end makes my head hurt; I'm 
not sure I understand exactly *why* splicing rest-arg in using ,@, works.

I don't think this is going to let me use ASDF with Corman Lisp, yet, though. There is a 
dependence on make-broadcast-stream, which doesn't seem to be defined in Corman 
Lisp 2.5.


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