GNU bug report logs - #14859
my-or macro in documentation

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Josep Portella Florit <jpf <at> primfilat.com>
To: bug-guile <at> gnu.org
Subject: my-or macro in documentation
Date: Sat, 13 Jul 2013 20:52:45 +0200

Hi!

The my-or example macro in the documentation has a subtle bug.

          (letrec-syntax ((my-or
                           (syntax-rules ()
                             ((my-or)
                              #t)
                             ((my-or exp)
                              exp)
                             ((my-or exp rest ...)
                              (let ((t exp))
                                (if exp
                                    exp
                                    (my-or rest ...)))))))
            (my-or #f "rockaway beach"))

In the third rule t is bound to exp, presumably to use t instead of exp
to prevent evaluating exp multiple times, but t is never used and exp is
used again.  This is the bug in action:

scheme@(guile-user)> (letrec-syntax ((my-or
                           (syntax-rules ()
                             ((my-or)
                              #t)
                             ((my-or exp)
                              exp)
                             ((my-or exp rest ...)
                              (let ((t exp))
                                (if exp
                                    exp
                                    (my-or rest ...)))))))
            (my-or (display "*") 1))
***scheme@(guile-user)>

I'm sure you already know how to fix it :-)

          (letrec-syntax ((my-or
                           (syntax-rules ()
                             ((my-or)
                              #t)
                             ((my-or exp)
                              exp)
                             ((my-or exp rest ...)
                              (let ((t exp))
                                (if t
                                    t
                                    (my-or rest ...)))))))
            (my-or #f "rockaway beach"))

Josep

Message #10 received at 14859-done <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Josep Portella Florit <jpf <at> primfilat.com>
Cc: 14859-done <at> debbugs.gnu.org
Subject: Re: bug#14859: my-or macro in documentation
Date: Sat, 13 Jul 2013 20:41:24 -0400

Hi!

Josep Portella Florit <jpf <at> primfilat.com> writes:

> The my-or example macro in the documentation has a subtle bug.
>
>           (letrec-syntax ((my-or
>                            (syntax-rules ()
>                              ((my-or)
>                               #t)
>                              ((my-or exp)
>                               exp)
>                              ((my-or exp rest ...)
>                               (let ((t exp))
>                                 (if exp
>                                     exp
>                                     (my-or rest ...)))))))
>             (my-or #f "rockaway beach"))
>
> In the third rule t is bound to exp, presumably to use t instead of exp
> to prevent evaluating exp multiple times, but t is never used and exp is
> used again.

Indeed!  This was reported shortly after the 2.0.9 release, and has been
fixed in the stable-2.0 branch.

http://bugs.gnu.org/14203
http://git.savannah.gnu.org/gitweb/?p=guile.git;a=commitdiff;h=e006d87ba5942b6e49b39b951413dfe63785a398

    Thanks!
      Mark

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