GNU bug report logs - #14203
Manual: 'my-or'; 'let' inside macros

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Message #10 received at 14203-done <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Nikita Karetnikov <nikita <at> karetnikov.org>
Cc: 14203-done <at> debbugs.gnu.org
Subject: Re: bug#14203: Manual: 'my-or'; 'let' inside macros
Date: Sun, 14 Apr 2013 13:53:38 -0400

Nikita Karetnikov <nikita <at> karetnikov.org> writes:

> I think this example [1,2]:
>
> (define-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 ...))))))
>
> should look like this:
>
> (define-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 ...))))))

Indeed, thanks!  I've pushed this fix to the stable-2.0 branch, and am
closing this bug.

Answers to your other questions follow.

> AFAICT, it's described here [3], but Guile is not affected, right?
> [3] http://stackoverflow.com/a/3215238

That post gives an example that looks superficially similar, but is
actually entirely different:

  (define remove!
    (let ((null? null?)
          (cdr cdr)
          (eq? eq?))
      (lambda ... function that uses null?, cdr, eq? ...)

Indeed, this is not necessary in Guile due to its module system.

> So the following works as well:
>
> (define-syntax my-or
>   (syntax-rules ()
>     ((my-or)
>      #t)
>     ((my-or exp)
>      exp)
>     ((my-or exp rest ...)
>      (if exp
>          exp
>          (my-or rest ...)))))

The above definition has a problem: it would result in 'exp' being
evaluated more than once, unless it returns false.

For example, if you used your proposed definition above,
(my-or (read) 5) would expand to:

  (if (read)
      (read)
      5)

Which would obviously not do what you expect from 'or'.  Instead, we
want:

  (let ((t (read)))
    (if t
        t
        5))

> Note that 'my-or' is used in several places (e.g., [4]) and it's
> necessary to change them all.
> [4] https://gnu.org/software/guile/manual/guile.html#Syntax-Case

Unless I'm mistaken, the Syntax-case section uses 'my-or', but does not
define it, so I don't think anything needs to be fixed there.  Right?

     Thanks,
       Mark

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