From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Christian Ohler <ohler@gnu.org>
Cc: Juanma Barranquero <lekktu@gmail.com>, emacs-devel@gnu.org
Subject: Re: Lexical binding
Date: Mon, 04 Apr 2011 12:22:14 -0400 [thread overview]
Message-ID: <jwv7hba2e50.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <4D99038B.7060702@gnu.org> (Christian Ohler's message of "Mon, 04 Apr 2011 09:32:27 +1000")
>>> (loop for (nil width . nil) in bs-attributes-list
>>> if (numberp width) sum width)
>> Yes, but it is less informative.
BTW, I believe part of the problem is that the loop macro will use
something of the form
(let (var)
(while ...
(setq var ...)
...))
instead of
(while ...
(let ((var ...))
...))
The former is likely to bump into the current problem, while the second
isn't. The former is popular in Elisp because it's slightly faster, but
with lexical scoping the second is actually more efficient. This is in
part because a lexical `let' is cheaper so it can be moved into the loop
without problems, and in other part because the second form avoids some
more costly cases:
let's take `dotimes' as an example. What should
(let ((res ()))
(dotimes (i 3) (push (lambda () i) res))
res)
return? Clearly it will be a list of 3 functions, each one taking
0 args and returning a number, but what should the exact return value
be? With the let-while-setq form of the loop, you'd get three times the
same function returning 3, whereas with the while-let form you'd get
functions returning 2, 1, and 0.
To get the first result, you need to make sure all three functions refer
to some memory cell holding the value of the variable `i', so you get an
extra indirection. I also find the second behavior more desirable.
So I've changed the dotimes (and dolist) macros to use the while-let
instead of let-while-setq form when used with lexical scoping.
Maybe we should do the same for loop, but I'm definitely not going to
take on this challenge.
Stefan
prev parent reply other threads:[~2011-04-04 16:22 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-01 17:32 Lexical binding Stefan Monnier
2011-04-01 19:12 ` David De La Harpe Golden
2011-04-01 19:46 ` Eli Zaretskii
2011-04-04 16:05 ` Stefan Monnier
2011-04-01 20:04 ` Daniel Colascione
2011-04-01 20:39 ` cedet-bzr build failure (was : Lexical binding) Darren Hoo
2011-04-01 21:21 ` David Engster
2011-04-01 22:26 ` Darren Hoo
2011-04-02 12:40 ` David Engster
2011-04-02 13:42 ` Eric M. Ludlam
2011-04-02 18:22 ` cedet-bzr build failure David Engster
2011-04-02 18:29 ` cedet-bzr build failure (was : Lexical binding) Darren Hoo
2011-04-01 20:42 ` Lexical binding Eli Zaretskii
2011-04-01 22:13 ` Juanma Barranquero
2011-04-02 2:33 ` Juanma Barranquero
2011-04-02 3:36 ` Juanma Barranquero
2011-04-02 18:38 ` Stefan Monnier
2011-04-04 16:04 ` Juanma Barranquero
2011-04-04 21:44 ` Stefan Monnier
2011-04-04 21:56 ` Juanma Barranquero
2011-04-04 22:03 ` David Kastrup
2011-04-04 22:34 ` Stefan Monnier
2011-04-02 18:38 ` Stefan Monnier
2011-04-02 18:50 ` Juanma Barranquero
2011-04-02 18:57 ` Juanma Barranquero
2011-04-03 12:05 ` Christian Ohler
2011-04-03 12:26 ` Juanma Barranquero
2011-04-03 23:32 ` Christian Ohler
2011-04-04 0:12 ` Juanma Barranquero
2011-04-04 16:22 ` Stefan Monnier [this message]
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