bug#55692: 29.0.50; delete-selection-mode: Replace region only if set using the mouse

[Visuwesh <visuweshm@gmail.com>]

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[ஞாயிறு மே 29, 2022] Eli Zaretskii wrote:

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: 55692@debbugs.gnu.org
>> Date: Sun, 29 May 2022 13:40:20 +0530
>> 
>> >>    By default, text insertion occurs normally even if the mark is
>> >>  active---for example, typing @kbd{a} inserts the character @samp{a},
>> >>  then deactivates the mark.  Delete Selection mode, a minor mode,
>> >>  modifies this behavior: if you enable that mode, then inserting text
>> >>  while the mark is active causes the text in the region to be deleted
>> >> -first.  To toggle Delete Selection mode on or off, type @kbd{M-x
>> >> -delete-selection-mode}.
>> >> +first.  If you want to replace only temporary regions, set by
>> >> +mouse-dragging or shift-selection, then change the variable
>> >
>> > There should be cross-references here to where these methods are
>> > described in the manual.
>> >
>> 
>> These methods as in "mouse-dragging" or "shift-selection".  I'm afraid I
>> don't fully understand what you mean.
>
> I mean to add @xref or @pxref to the places in the manual where mouse
> selection and shift-selection are described.  Those places are,
> respectively, the nodes "Setting Mark" and "Shift Selection".
>

Done.

>> >> @@ -251,6 +259,10 @@ delete-selection-pre-hook
>> >>  have this property won't delete the selection.
>> >>  See `delete-selection-helper'."
>> >>    (when (and delete-selection-mode (use-region-p)
>> >> +             (if delete-selection-temporary-regions-only
>> >> +                 (and (consp transient-mark-mode)
>> >> +                      (eq (car transient-mark-mode) 'only))
>> >> +               t)
>> >
>> > I wonder whether we should also optionally allow replacing text in
>> > regions activated by "C-u C-x C-x" when transient-mark-mode is off.
>> > Those cause transient-mark-mode to have the value 'lambda', not
>> > '(only...)'.  So maybe the new defcustom should be a tristate, not a
>> > boolean?
>> >
>> 
>> Sure, that could be useful.  But I have transient-mark-mode turned on
>> always so I can't comment much on it.  However, what would the third
>> state be?
>
> Some symbol.
>
>> When the defcustom is t, we could have it check for '(only . ...)'
>> and the value 'lambda', no?
>
> Yes, we could have:
>
>   . nil - always replace the region
>   . t   - replace region from mouse, shift-selection, and "C-u C-x C-x"
>   . selection - only replace  region from mouse and shift-selection
>

Done.

>> > Also, can we have the above condition in a more elegant form?  In
>> > general, whenever you have something like
>> >
>> >    (if SOMETHING foo t)
>> >
>> > it begs to be rewritten so the "t" part is not needed, because 'if'
>> > itself already returns a boolean value.
>> 
>> Sure.  I will factor out the condition into a separate function.  Is
>> that more elegant?
>
> No, that's overkill.  I meant that if you need an 'if' to return t or
> something else, you could instead make it return nil or something
> else, as in
>
>    (if SOMETHING foo)
>
> and reverse the condition.
>
> But if you still don't understand, just forget this comment.

I don't think I fully understood what you meant but I took a shot at it
anyway.  Please review updated patch.


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From 4649ff74c384fa22edb19234928df6322efd6099 Mon Sep 17 00:00:00 2001
From: Visuwesh <visuweshm@gmail.com>
Date: Sun, 29 May 2022 11:45:19 +0530
Subject: [PATCH] delete-selection-mode: Add user option to delete temporary
 regions only

* lisp/delsel.el (delete-selection-temporary-region): Add new
user option.
(delete-selection-pre-hook): Respect it.
* doc/emacs/mark.texi (Using Region): Document the new user option.
* etc/NEWS: Announce the new user option. (bug#55692)
---
 doc/emacs/mark.texi | 11 +++++++++--
 etc/NEWS            |  5 +++++
 lisp/delsel.el      | 18 ++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/doc/emacs/mark.texi b/doc/emacs/mark.texi
index 91c44d527b..397f941b2d 100644
--- a/doc/emacs/mark.texi
+++ b/doc/emacs/mark.texi
@@ -291,13 +291,20 @@ Using Region
 @cindex Delete Selection mode
 @cindex mode, Delete Selection
 @findex delete-selection-mode
+@vindex delete-selection-temporary-region
   By default, text insertion occurs normally even if the mark is
 active---for example, typing @kbd{a} inserts the character @samp{a},
 then deactivates the mark.  Delete Selection mode, a minor mode,
 modifies this behavior: if you enable that mode, then inserting text
 while the mark is active causes the text in the region to be deleted
-first.  To toggle Delete Selection mode on or off, type @kbd{M-x
-delete-selection-mode}.
+first.  If you want to replace only temporary regions, set by
+mouse-dragging or shift-selection (@pxref{Setting Mark}) or @kbd{C-u
+C-x C-x}, then change the variable
+@code{delete-selection-temporary-region} to @code{t}.  But if you do
+not want to consider the region made active by @kbd{C-u C-x C-x}
+(@pxref{Disabled Transient Mark}), then set the variable to
+@code{selection}.  To toggle Delete Selection mode on or off, type
+@kbd{M-x delete-selection-mode}.
 
 @node Mark Ring
 @section The Mark Ring
diff --git a/etc/NEWS b/etc/NEWS
index 85a0ee44b9..43a0e1aaab 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -271,6 +271,11 @@ startup.  Previously, these functions ignored
 \f
 * Changes in Emacs 29.1
 
++++
+** New user option 'delete-selection-temporary-region'.
+When non-nil, 'delete-selection-mode' will only delete the temporary
+regions (usually set by mouse-dragging or shift-selection).
+
 +++
 ** New user option 'switch-to-prev-buffer-skip-regexp'.
 This should be a regexp or a list of regexps; buffers whose names
diff --git a/lisp/delsel.el b/lisp/delsel.el
index f5fe7cf793..8691f4720b 100644
--- a/lisp/delsel.el
+++ b/lisp/delsel.el
@@ -64,6 +64,19 @@ delete-selection-save-to-register
   "If non-nil, deleted region text is stored in this register.
 Value must be the register (key) to use.")
 
+(defcustom delete-selection-temporary-region nil
+  "Whether to delete only temporary regions.
+When non-nil, typed text replaces only the regions set by
+mouse-dragging, shift-selection, and \"\\[universal-argument] \\[exchange-point-and-mark]\" when
+`transient-mark-mode' is turned off.  If the value is the symbol
+`selection', then replace only the regions set by mouse-dragging
+and shift-selection."
+  :version "29.1"
+  :group 'editing-basics
+  :type '(choice (const :tag "Replace all regions" nil)
+                 (const :tag "Replace region from mouse, shift-selection, and \"C-u C-x C-x\"" t)
+                 (const :tag "Replace region from mouse and shift-selection" selection)))
+
 ;;;###autoload
 (defalias 'pending-delete-mode 'delete-selection-mode)
 
@@ -251,6 +264,11 @@ delete-selection-pre-hook
 have this property won't delete the selection.
 See `delete-selection-helper'."
   (when (and delete-selection-mode (use-region-p)
+             (not (when delete-selection-temporary-region
+                    (not (or (and (consp transient-mark-mode)
+                                  (eq (car transient-mark-mode) 'only))
+                             (and (not (eq delete-selection-temporary-region 'selection))
+                                  (eq transient-mark-mode 'lambda))))))
 	     (not buffer-read-only))
     (delete-selection-helper (and (symbolp this-command)
                                   (get this-command 'delete-selection)))))
-- 
2.33.1