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cl-delete-if vs list of overlays
From: |
Omar Polo |
Subject: |
cl-delete-if vs list of overlays |
Date: |
Wed, 09 Dec 2020 10:53:26 +0100 |
User-agent: |
mu4e 1.4.13; emacs 27.1 |
Hello,
I was playing with overlays and the fringe, when I found something I
couldn't really understand. Hence this help request :)
Let me say beforehand that on a second thought what I was doing isn't
probably the best way to manage the overlays, that I don't really need
to maintain a list, but I'm still asking because I'm curious.
I'm adding and removing markers on the fringe, and saving the respective
overlays in a list. When I have to remove a marker, I use cl-delete-if
to remove the correct overlay from the list (while also doing the
side-effect of `delete-overlay'). The thing is, the removed overlays
are still present in the list.
The following is a proof-of-code to better explain what I mean:
(require 'cl-lib)
(defvar foo--overlays nil
"List of active overlays.")
(defun foo--delete-overlay-at (point)
"t if an overlay was deleted."
(let (delp)
(cl-delete-if (lambda (overlay)
(when (= point (overlay-start overlay))
(delete-overlay overlay)
(setq delp t)))
foo--overlays)
delp))
(defun foo-toggle-this-line ()
"Add/remove a marker on the current line."
(interactive)
(save-excursion
(beginning-of-line)
(unless (foo--delete-overlay-at (point))
(let ((overlay (make-overlay (point) (point))))
(overlay-put overlay 'before-string
(propertize "A"
'display '(left-fringe
right-triangle)))
(push overlay foo--overlays)))))
(provide 'foo)
;;; foo.el ends here
Try to add a marker on a line with (foo-toggle-this-line) and then
remove it: you'll find that `foo--overlays' is not nil.
(You'll actually get an error if you play with it too much, because for
deleted overlays `overlay-start' returns nil and that makes = raise an
error)
Why is cl-delete-if not removing items from the list? (setq delp t)
evaluates to t, so it should delete. cl-delete-if a destructive
function. This is what I'm not getting.
Thanks,
Omar Polo
- cl-delete-if vs list of overlays,
Omar Polo <=