From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Helmut Eller <eller.helmut@gmail.com>
Cc: 8711@debbugs.gnu.org
Subject: bug#8711: 24.0.50; binding _ to unused values with lexical-binding
Date: Tue, 24 May 2011 09:42:31 -0300 [thread overview]
Message-ID: <jwvfwo4gu2z.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <m28vtwvdpz.fsf@gmail.com> (Helmut Eller's message of "Tue, 24 May 2011 08:01:44 +0200")
> ;; -*- lexical-binding: t -*-
> (defun foo (x) (destructuring-bind (y &rest _) x y))
> x.el:2:1:Warning: variable `_' not left unused
Ah, yes, this one. This is because CL's destructuring-bind expands the
above to something like
(let ((_ x)
(y (prog1 (car _) (setq _ (cdr _)))))
y)
where _ is indeed not left unused. It also means that a closure that
for (destructuring-bind (y &rest z) x (lambda () z)) the closure
captures a mutated var which is something more costly (the var needs to
be put inside a cons cell, leading to code along the lines of:
(let ((z (list x))
(y (prog1 (car (car z)) (setcar z (cdr (car z))))))
(make-closure () [z] (car z)))
Again this is a side-effect of `let' being costly in dynbind code.
For lexbind code it would be better for destructuring-bind to use an
additional internal var:
(let ((<internal> x)
(y (prog1 (car <internal>) (setq <internal> (cdr <internal>))))
(_ <internal>))
y)
Which would not generate any warning and would lead to more efficient
code when the &rest var is captured by a closure.
Stefan
next prev parent reply other threads:[~2011-05-24 12:42 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-05-21 18:44 bug#8711: 24.0.50; binding _ to unused values with lexical-binding Helmut Eller
2011-05-23 9:01 ` Lawrence Mitchell
2011-05-23 14:24 ` Stefan Monnier
2011-05-23 18:23 ` Helmut Eller
2011-05-23 19:29 ` Stefan Monnier
2011-05-23 20:16 ` Helmut Eller
2011-05-24 0:56 ` Stefan Monnier
2011-05-24 6:01 ` Helmut Eller
2011-05-24 12:42 ` Stefan Monnier [this message]
2011-06-02 11:17 ` Juanma Barranquero
2011-06-02 12:45 ` Stefan Monnier
2011-06-02 13:41 ` Juanma Barranquero
2011-06-02 14:00 ` Stefan Monnier
2011-06-02 17:10 ` Juanma Barranquero
2011-05-23 20:32 ` Helmut Eller
2011-05-24 0:51 ` Stefan Monnier
2022-05-08 12:33 ` bug#8711: bug#26960: 26.0.50; Complaints about unused variable in cl-destructuring-bind Lars Ingebrigtsen
2022-05-08 13:32 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-09 9:25 ` bug#8711: " Lars Ingebrigtsen
2022-05-09 12:26 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
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