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Re: "defmacro" and "local variables" and "let" and "nconc" strange behav
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From: |
Oleksandr Gavenko |
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Subject: |
Re: "defmacro" and "local variables" and "let" and "nconc" strange behaviour... |
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Date: |
Sun, 13 Jan 2013 17:00:40 +0200 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux) |
On 2013-01-13, Barry Margolin wrote:
> In article <mailman.17290.1358086558.855.help-gnu-emacs@gnu.org>,
> Oleksandr Gavenko <gavenkoa@gmail.com> wrote:
>
>> I wrote simple macro:
>>
>> (defmacro my-filter (pred list)
>> "Construct list with elements from LIST which satisfy PRED."
>> (let ( (r (make-symbol "r_")) )
>> `(let ( (,r '(nil)) )
>> (mapc (lambda (item)
>> (when (,pred item)
>> (nconc ,r (cons item nil))))
>> ,list)
>> (cdr ,r))))
>> [...]
>> Why instead of '(nil) I get something else?
>
> Because nconc is destructively modifying its own source code.
I know that nconc destructive. But how this effect '(nil) in defmacro inside
`(...)?
--
Best regards!