From: Kelly Dean <kellydeanch@yahoo.com>
To: help-gnu-emacs@gnu.org
Subject: Unreferenced symbols in closures, and a problem with closures in minor modes
Date: Tue, 28 May 2013 15:35:58 -0700 (PDT) [thread overview]
Message-ID: <1369780558.54419.YahooMailClassic@web141103.mail.bf1.yahoo.com> (raw)
(setq lexical-binding t)
As expected, (let ((x 0)) (lambda () x)) -> (closure ((x . 0) t) nil x)
But, (let ((x 0)) (lambda () 0)) -> (closure ((x . 0) t) nil 0)
And, (lambda (x) x) -> (closure (t) (x) x)
Why do closures' lexical environments include unreferenced symbols? And why produce a closure with an empty environment, instead of just a lambda form?
And I have a problem with unwanted closures in minor modes:
(defvar foo-hook nil)
(define-minor-mode bar "Bar" nil " Bar" nil
(if bar (add-hook 'foo-hook (lambda () (bar 0)) nil t)
(remove-hook 'foo-hook (lambda () (bar 0)) t)))
M-x bar RET C-h v foo-hook RET, and I get:
foo-hook's value is ((closure
((last-message)
(arg . toggle)
t)
nil
(bar 0))
t)
Which won't be removed when foo-hook runs, because arg will be 0, not 'toggle. I tried to thwart the closure like this:
(defvar foo2-hook nil)
(define-minor-mode bar2 "Bar2" nil " Bar2" nil
(if bar2 (add-hook 'foo2-hook (let ((arg 0)) (lambda () (bar2 0))) nil t)
(remove-hook 'foo2-hook (let ((arg 0)) (lambda () (bar2 0))) t)))
M-x bar2 RET C-h v foo2-hook RET, and I get:
foo2-hook's value is ((closure
((arg . 0)
(last-message)
(arg . toggle)
t)
nil
(bar2 0))
t)
Argh! Surely this is a misfeature?
Also I don't know why last-message is there, but at least it doesn't interfere.
On a previous experiment that I can't reproduce, involving several additions and removals, I got:
Value: ((closure
((last-message)
(arg . 1)
i t)
nil
(bar 0))
(closure
((last-message)
(arg . 1)
i t)
nil
(bar 0))
t)
I have no idea what the symbol i means or where it came from. If it were setting i to nil in the lexical environment, it would be (i), not i.
Also, the two closures are identical, yet add-hook isn't supposed to add duplicates.
Help?
next reply other threads:[~2013-05-28 22:35 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-28 22:35 Kelly Dean [this message]
2013-05-29 1:28 ` Unreferenced symbols in closures, and a problem with closures in minor modes Stefan Monnier
-- strict thread matches above, loose matches on Subject: below --
2013-05-29 4:44 Kelly Dean
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