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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 16:18:46 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux) |
On 2013-01-13, 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))))
>
> When I evaluate several times:
>
> (my-filter (lambda (x) x) '(1 nil "a"))
>
> I get sequentially:
>
> (1 "a")
> (1 "a" 1 "a")
> (1 "a" 1 "a" 1 "a")
> (1 "a" 1 "a" 1 "a" 1 "a")
>
> When I eval:
>
> (pp (macroexpand '(my-filter (lambda (x) x) '(1 nil "a"))))
>
> I get:
>
> (let
> ((r_
> '(nil 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a")))
> (mapc
> (lambda
> (item)
> (when
> ((lambda
> (x)
> x)
> item)
> (nconc r_
> (cons item nil))))
> '(1 nil "a"))
> (cdr r_))
>
> Why instead of '(nil) I get something else?
I found fix by changing '(nil) to (list nil). Please explain why first variant
fail?
--
Best regards!