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Subject: |
Manual: 'my-or'; 'let' inside macros |
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Date: |
Sun, 14 Apr 2013 20:33:12 +0400 |
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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--- Begin Message ---
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Subject: |
Re: bug#14203: Manual: 'my-or'; 'let' inside macros |
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Date: |
Sun, 14 Apr 2013 13:53:38 -0400 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Nikita Karetnikov <address@hidden> 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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