[clisp-list] Re: New patch.

[Kaz K. <ka...@as...>]

On 27 Mar 2003, Sam Steingold wrote:

> > On 27 Mar 2003, Sam Steingold wrote:
> > > > * Honorable Kaz Kylheku <ka...@as...> writes:
> > > >
> > > > http://users.footprints.net/~kaz/clisp-cvs-2003-04-26-backquote.diff
> > > 
> > > read-from-string "`,@x")
> > > -ERROR
> > > +(SYSTEM::BACKQUOTE (SYSTEM::SPLICE X))
> > > 
> > > this is not good:  backquote is a reader macro, so the error must be
> > > signalled at read time, not macroexpand time (CMUCL and unpatched CLISP
> > > signal at read time).
> > 
> > If you are really adamant about catching `,@FORM at read time,
> > it can be achieved trivially. After the the backquote reader
> > reads the following object, if it's a cons whose car is the symbol
> > SYSTEM::SPLICE or SYSTEM::NSPLICE, then signal the error.
> > But I think that the macro-expansion-time check should stay in place,
> > to trap cases when the error occurs in ``synthetic'' backquotes.
> 
> maybe the BACKQUOTE macro should be expanded at read time, just like
> CLISP does right now.

You just need something like this version of backquote-reader:

(defun backquote-reader (stream char)
  (declare (ignore char))
  (let* ((*unquote-occured* nil)
	 (*reading-array* nil)
	 (*backquote-level* (1+ (or *backquote-level* 0)))
	 (object (read stream t nil t)))

    ;; Check for unquotes in wrong type objects.
    (unless (or (and (vectorp object)
	             (not (stringp object))
		     (not (bit-vector-p object)))
	        (listp object))
      (when *unquote-occured*
	(error-of-type 'error 
	  (TEXT "~S: unquotes may occur only in (...) or #(...) forms")
	  'read)))

    ;; Read-time check for ,@ or ,. not in list.
    ;; Also tested at macro-expansion time for
    ;; backquotes synthesized by means other than the reader
    (when (consp object)
      (case (car object)
	((SPLICE)
	   (error-of-type 'error 
	     (TEXT "the syntax `,@form is undefined behavior")))
	((NSPLICE) 
	   (error-of-type 'error 
	     (TEXT "the syntax `,.form is undefined behavior")))))
    (list 'BACKQUOTE object)))

> I am not really sure on this, so maybe you could raise this issue on
> c.l.l?

What for; it's clear you want early detection. On c.l.l, people will
probably just reiterate that it's undefined behavior, but that catching
errors early is nice. ;)