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Re: "defmacro" and "local variables" and "let" and "nconc" strange behav

From: Oleksandr Gavenko
Subject: Re: "defmacro" and "local variables" and "let" and "nconc" strange behaviour...
Date: Sun, 13 Jan 2013 19:25:04 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux)

On 2013-01-13, Le Wang wrote:

> On Sun, Jan 13, 2013 at 11:00 PM, Oleksandr Gavenko <> 
> wrote:
>> On 2013-01-13, Barry Margolin wrote:
>>> In article <>,
>>>  Oleksandr Gavenko <> 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
>> `(...)?
> In lisp, code is data.  See my enlightenment process:

I follow you link and found different explanation from Drew Adams:

> Note that different Lisps (and different implementations of the same Lisp) can
> treat a sexp such as '(a b) differently - they might or might not create a new
> list each time it is read or eval'd.

> To be sure to get what you expect in situations like this, do not use '(...).
> Use `cons' or `list' or equivalent backquote syntax.  Do not expect '(...) to
> create new list structure each time it is read/eval'd.

That's why my code fail with '(nil)...

Best regards!

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