Re: [Didier Verna] multiple string reads/writes of SB-IMPL::BAKQ-COMMA

You'll get correct results doing it this way -

(with-input-from-string (s "(defmacro test (&body body)

           (let ((obj (gensym)))

             `(let (,obj)

                (setq ,obj (list ,@body))

                ,obj)))")
           (eval (with-input-from-string
                   (s (write-to-string (read s) :pretty nil :readably t))
                   (read s))))




On Fri, Oct 11, 2013 at 4:27 AM, Didier Verna <[email protected]> wrote:

>
>   Hi,
>
> on second thought, I'm forwarding this to sbcl-devel because I think
> this might be a bug. The same scenario works fine with CCL. CMUCL gives
> a note and a warning but still works:
>
> CL-USER> (test 'a 'b)
> ; In: TEST 'A
>
> ;   (TEST 'A 'B)
> ; ==>
> ;   (LET (#)
> ;     (SETQ #:G0 #)
> ;     #:G0)
> ; Note: Variable LISP::BACKQ-COMMA defined but never used.
> ; ;
>
> ; Warning: These variables are undefined:
> ;   #:G0 OBJ
> ; (A B)
>
> The macroepansion is however similar to that of SBCL:
>
> CL-USER> (macroexpand '(test 'a 'b))
> (LET ((LISP::BACKQ-COMMA OBJ))
>   (SETQ #:G4368 (LIST 'A 'B))
>   #:G4368)
> T
>
>
>
>
>
>
> ---------- Forwarded message ----------
> From: Didier Verna <[email protected]>
> To: SBCL Help <[email protected]>
> Cc:
> Date: Fri, 11 Oct 2013 09:14:06 +0200
> Subject: multiple string reads/writes of SB-IMPL::BAKQ-COMMA
>
>   Hello,
>
> the backquote syntax gives me a problem when doing multiple (in fact 2)
> reads and writes of an expression. Consider this:
>
> CL-USER> (defmacro test (&body body)
>            (let ((obj (gensym)))
>              `(let (,obj)
>                 (setq ,obj (list ,@body))
>                 ,obj)))
> TEST
> CL-USER> (test 'foo 'bar)
> (FOO BAR)
>
>
> CL-USER> (with-input-from-string (s "(defmacro test (&body body)
>            (let ((obj (gensym)))
>              `(let (,obj)
>                 (setq ,obj (list ,@body))
>                 ,obj)))")
>            (eval (read s)))
> STYLE-WARNING: redefining COMMON-LISP-USER::TEST in DEFMACRO
> TEST
> CL-USER> (test 'foo 'bar)
> (FOO BAR)
>
> ;; Okay, but then...
>
>
> CL-USER> (with-input-from-string (s "(defmacro test (&body body)
>            (let ((obj (gensym)))
>              `(let (,obj)
>                 (setq ,obj (list ,@body))
>                 ,obj)))")
>            (eval (with-input-from-string (s (prin1-to-string (read s)))
>                    (read s))))
> STYLE-WARNING: redefining COMMON-LISP-USER::TEST in DEFMACRO
> TEST
> CL-USER> (test 'foo 'bar)
> ; in: TEST 'FOO
> ;     (LET ((SB-IMPL::BACKQ-COMMA OBJ))
> ;       (SETQ #:G2191 (LIST 'FOO 'BAR))
> ;       #:G2191)
> ;
> ; caught STYLE-WARNING:
> ;   The variable SB-IMPL::BACKQ-COMMA is defined but never used.
>
> ;     (SETQ #:G2191 (LIST 'FOO 'BAR))
> ;
> ; caught WARNING:
> ;   undefined variable: #:G2191
>
>
> and it breaks. What happens is that in the first case, SBCL understands
> that (SB-IMPL::BACKQ-COMMA OBJ) really means ,obj in the let form, but
> if I read this expression, write it to a string and read it again for
> evaluation, then it thinks that I'm trying to bind SB-IMPL::BACKQ-COMMA
> to OBJ...
>
>
> How can I avoid this behavior ?
>
>
> Thanks !
>
> --
> Resistance is futile. You will be jazzimilated.
>
> Lisp, Jazz, Aïkido: http://www.didierverna.info
>
>
>
> --
> Resistance is futile. You will be jazzimilated.
>
> Lisp, Jazz, Aïkido: http://www.didierverna.info
>
>
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