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

Discussion:

Didier Verna

2013-10-11 08:27:43 UTC

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

Douglas Katzman

2013-10-11 13:09:38 UTC

Permalink

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))))

Post by Didier Verna
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
CL-USER> (test 'a 'b)
; In: TEST 'A
; (TEST 'A 'B)
; ==>
; (LET (#)
; (SETQ #:G0 #)
; #:G0)
; Note: Variable LISP::BACKQ-COMMA defined but never used.
; ;
; #:G0 OBJ
; (A B)
CL-USER> (macroexpand '(test 'a 'b))
(LET ((LISP::BACKQ-COMMA OBJ))
(SETQ #:G4368 (LIST 'A 'B))
#:G4368)
T
---------- Forwarded message ----------
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)
CL-USER> (defmacro test (&body body)
(let ((obj (gensym)))
`(let (,obj)
,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)
,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)
,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)
;
; The variable SB-IMPL::BACKQ-COMMA is defined but never used.
; (SETQ #:G2191 (LIST 'FOO 'BAR))
;
; 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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Faré

2013-10-11 22:46:25 UTC

Permalink

The heart of the issue is that the pretty-printer for LET forms is
confused by the comma.
A workaround would be to use (let ((,obj nil)) ...)
A fix would be to fix the pretty-printer of LET recognize SB-IMPL::BACKQ-COMMA
(and while we're at it FARE-QUASIQUOTE::UNQUOTE, etc.?) or better,
to be less clever with the printing of bindings, just setting the start column?

—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org
A successful [software] tool is one that was used to do something
undreamed of by its author.
— S. C. Johnson

Post by Douglas Katzman
You'll get correct results doing it this way -
(with-input-from-string (s "(defmacro test (&body body)
(let ((obj (gensym)))
`(let (,obj)
,obj)))")
(eval (with-input-from-string
(s (write-to-string (read s) :pretty nil :readably t))
(read s))))

------------------------------------------------------------------------------
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Explore tips for MPI, OpenMP, advanced profiling, and more. Get the most
from
the latest Intel processors and coprocessors. See abstracts and register >
http://pubads.g.doubleclick.net/gampad/clk?id=60134071&iu=/4140/ostg.clktrk
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Sbcl-devel mailing list
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Didier Verna

2013-10-14 12:55:08 UTC

Permalink

Post by Douglas Katzman
You'll get correct results doing it this way -
(s (write-to-string (read s) :pretty nil :readably t))

Yes. In fact, I was fooled by the fact that PRIN1 and friends are
described to produce output suitable "for input to READ", but in fact
don't bind *PRINT-READABLY* (this is even mentioned explicitly in the
CLHS...).

--
Resistance is futile. You will be jazzimilated.

Lisp, Jazz, Aïkido: http://www.didierverna.info