From: Oleksandr Gavenko <gavenkoa@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: "defmacro" and "local variables" and "let" and "nconc" strange behaviour...
Date: Sun, 13 Jan 2013 16:15:32 +0200 [thread overview]
Message-ID: <87k3rh6uez.fsf@gavenkoa.example.com> (raw)
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?
--
Best regards!
next reply other threads:[~2013-01-13 14:15 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-13 14:15 Oleksandr Gavenko [this message]
2013-01-13 14:18 ` "defmacro" and "local variables" and "let" and "nconc" strange behaviour Oleksandr Gavenko
[not found] ` <mailman.17291.1358086805.855.help-gnu-emacs@gnu.org>
2013-01-14 9:04 ` jack-mac
[not found] <mailman.17290.1358086558.855.help-gnu-emacs@gnu.org>
2013-01-13 14:21 ` Barry Margolin
2013-01-13 15:00 ` Oleksandr Gavenko
2013-01-13 16:45 ` Le Wang
2013-01-13 17:25 ` Oleksandr Gavenko
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