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#14203
Manual: 'my-or'; 'let' inside macros
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Your bug report
#14203: Manual: 'my-or'; 'let' inside macros
which was filed against the guile package, has been closed.
The explanation is attached below, along with your original report.
If you require more details, please reply to 14203 <at> debbugs.gnu.org.
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14203: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=14203
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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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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 ...))))))
Otherwise, what's the rationale behind 'let'?
AFAICT, it's described here [3], but Guile is not affected, right? 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 ...)))))
Note that 'my-or' is used in several places (e.g., [4]) and it's
necessary to change them all. Also, there are other macros that use
'let' (e.g., 'cond1').
[1] https://gnu.org/software/guile/manual/guile.html#Defining-Macros
[2] https://gnu.org/software/guile/manual/guile.html#Hygiene
[3] http://stackoverflow.com/a/3215238
[4] https://gnu.org/software/guile/manual/guile.html#Syntax-Case
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This bug report was last modified 12 years and 92 days ago.
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