Strange behavior of macro

Strange behavior of macro

[Herbert Euler]

Hello,

Is the following in Emacs a expected behavior of macro?

    (defmacro m1 (v)
      (let ((len (length (eval v))))
        `(quote ,len)))
         => m1

    (m1 '(1 2 3))
         => 3

    (setq v1 '(1 2 3))
         => (1 2 3)

    (m1 v1)
         => 3

    (setq v v1)
         => (1 2 3)

    (m1 v)
         => error: (wrong-type-argument sequencep v)

Thanks.

Regards,
Guanpeng Xu

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Re: Strange behavior of macro

[David Kastrup]

"Herbert Euler" <herberteuler@hotmail.com> writes:

> Is the following in Emacs a expected behavior of macro?
>
>    (defmacro m1 (v)
>      (let ((len (length (eval v))))
>        `(quote ,len)))
>         => m1
>
>    (m1 '(1 2 3))
>         => 3
>
>    (setq v1 '(1 2 3))
>         => (1 2 3)
>
>    (m1 v1)
>         => 3
>
>    (setq v v1)
>         => (1 2 3)
>
>    (m1 v)
>         => error: (wrong-type-argument sequencep v)

In combination with dynamic scope, yes.  m1 gets passed 'v as v, so
now v is bound to 'v and (eval v) again delivers 'v.

Evaluating a macro argument in the macro itself is often asking for
trouble.  And Elisp's scoping rules don't exactly help.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum

Re: Strange behavior of macro

[Andreas Schwab]

"Herbert Euler" <herberteuler@hotmail.com> writes:

> Is the following in Emacs a expected behavior of macro?
>
>    (defmacro m1 (v)
>      (let ((len (length (eval v))))
>        `(quote ,len)))
>         => m1
[...]
>    (m1 v)
>         => error: (wrong-type-argument sequencep v)

See (elisp)Eval During Expansion.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: Strange behavior of macro

[Herbert Euler]

>From: Andreas Schwab <schwab@suse.de>
>To: "Herbert Euler" <herberteuler@hotmail.com>
>CC: emacs-devel@gnu.org
>Subject: Re: Strange behavior of macro
>Date: Tue, 30 May 2006 12:23:49 +0200
>
>"Herbert Euler" <herberteuler@hotmail.com> writes:
>
> > Is the following in Emacs a expected behavior of macro?
> >
> >    (defmacro m1 (v)
> >      (let ((len (length (eval v))))
> >        `(quote ,len)))
> >         => m1
>[...]
> >    (m1 v)
> >         => error: (wrong-type-argument sequencep v)
>
>See (elisp)Eval During Expansion.

OK, so it's an accepted behavior.  Btw, in the test of GNU clisp,
it's safe to write such a macro:

[1]> (defmacro m1 (v)
       (let ((len (length (eval v))))
         `(quote ,len)))
M1
[2]> (m1 '(1 2 3))
3
[3]> (setq v1 '(1 2 3))
(1 2 3)
[4]> (m1 v1)
3
[5]> (setq v v1)
(1 2 3)
[6]> (m1 v)
3
[7]>

Regards,
Guanpeng Xu

_________________________________________________________________
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Re: Strange behavior of macro

[Andreas Schwab]

"Herbert Euler" <herberteuler@hotmail.com> writes:

>>From: Andreas Schwab <schwab@suse.de>
>>To: "Herbert Euler" <herberteuler@hotmail.com>
>>CC: emacs-devel@gnu.org
>>Subject: Re: Strange behavior of macro
>>Date: Tue, 30 May 2006 12:23:49 +0200
>>
>>"Herbert Euler" <herberteuler@hotmail.com> writes:
>>
>> > Is the following in Emacs a expected behavior of macro?
>> >
>> >    (defmacro m1 (v)
>> >      (let ((len (length (eval v))))
>> >        `(quote ,len)))
>> >         => m1
>>[...]
>> >    (m1 v)
>> >         => error: (wrong-type-argument sequencep v)
>>
>>See (elisp)Eval During Expansion.
>
> OK, so it's an accepted behavior.  Btw, in the test of GNU clisp,
> it's safe to write such a macro:

Emacs Lisp is using dynamic binding, not lexical binding.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."