bug#71865: [PATCH] New user option `fill-sentence-end-double-space`

[Stefan Kangas <stefankangas@gmail.com>]

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Severity: wishlist

Consider doing this in emacs -Q

    M-: (setq sentence-end-double-space nil) RET

Now put point at a paragraph like this

    Foo bar baz.  Hello hello.

Press M-q and end up with:

    Foo bar baz. Hello hello.

However, in some cases the user will _not_ want this be reflowed, while
still wanting to make the sentence commands navigate correctly even if
there is only one space.

For that purpose, I suggest adding a new user option
`fill-sentence-end-double-space', which controls _only_ the behavior
when refilling.  That way you can customize sentence commands separately
from fill commands.

See the attached patch (still lacking NEWS and documentation changes).

Thoughts?

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From fb31e14562f2a455659d122e8ff921c49b140957 Mon Sep 17 00:00:00 2001
From: Stefan Kangas <stefankangas@gmail.com>
Date: Mon, 1 Jul 2024 02:53:25 +0200
Subject: [PATCH] New user option `fill-sentence-end-double-space`

* lisp/textmodes/fill.el
(fill-sentence-end-double-space): New user option.
(fill-sentence-end-double-space-safe-p): New function.
(fill-paragraph): Use the above new user option to decide the value of
'sentence-end-double-space' for the duration of this command.
* lisp/textmodes/paragraphs.el (sentence-end-double-space): Move user
option to :group 'sentence'.
---
 .dir-locals.el               |  2 ++
 lisp/textmodes/fill.el       | 27 ++++++++++++++++++++++++++-
 lisp/textmodes/paragraphs.el |  3 +--
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/.dir-locals.el b/.dir-locals.el
index c74da88a811..d6d82ddc8fb 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -3,6 +3,8 @@
 
 ((nil . ((tab-width . 8)
          (sentence-end-double-space . t)
+         ;; TODO: Enable this once more people are on Emacs 31.
+         ;;(fill-sentence-end-double-space . t)
          (fill-column . 72)
 	 (emacs-lisp-docstring-fill-column . 72)
          (vc-git-annotate-switches . "-w")
diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index 29c56f8feaf..1aa33b6a8d1 100644
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -49,6 +49,27 @@ colon-double-space
   :type 'boolean
   :safe #'booleanp)
 
+(defun fill-sentence-end-double-space-safe-p (val)
+  "Return non-nil if VAL is one of `default', nil or t."
+  (memq val '(default t nil)))
+
+(defcustom fill-sentence-end-double-space 'default
+  "Controls if fill commands should consider two spaces to end a sentence.
+The default value, `default' means that this should be controlled by the
+user option `sentence-end-double-space'.
+
+Setting this to nil or t will override `sentence-end-double-space'
+during fill commands, and have the same meaning as if you customized
+that user option to that value.
+
+The purpose of this is to be able to customize fill commands separately
+from sentence commands."
+  :type '(choice (const :tag "Use `sentence-end-double-space'" default)
+                 (const :tag "Two spaces ends sentence" t)
+                 (const :tag "One space ends sentence" nil))
+  :safe #'fill-sentence-end-double-space-safe-p
+  :version "31.1")
+
 (defcustom fill-separate-heterogeneous-words-with-space nil
   "Non-nil means to use a space to separate words of a different kind.
 For example, when an English word at the end of a line and a CJK word
@@ -841,6 +862,10 @@ fill-paragraph
   (interactive (progn
 		 (barf-if-buffer-read-only)
 		 (list (if current-prefix-arg 'full) t)))
+  (let ((sentence-end-double-space
+         (if (eq fill-sentence-end-double-space 'default)
+             sentence-end-double-space
+           fill-sentence-end-double-space)))
   (with-buffer-unmodified-if-unchanged
     (or
      ;; 1. Fill the region if it is active when called interactively.
@@ -901,7 +926,7 @@ fill-paragraph
                        ;; fill-region.
                        (fill-region beg end justify)
                      (fill-region-as-paragraph beg end justify))))))
-       fill-pfx))))
+       fill-pfx)))))
 
 (declare-function comment-search-forward "newcomment" (limit &optional noerror))
 (declare-function comment-string-strip "newcomment" (str beforep afterp))
diff --git a/lisp/textmodes/paragraphs.el b/lisp/textmodes/paragraphs.el
index af99a96e045..5792adc0d75 100644
--- a/lisp/textmodes/paragraphs.el
+++ b/lisp/textmodes/paragraphs.el
@@ -125,8 +125,7 @@ sentence-end-double-space
 regexp describing the end of a sentence, when the value of the variable
 `sentence-end' is nil.  See Info node `(elisp)Standard Regexps'."
   :type 'boolean
-  :safe #'booleanp
-  :group 'fill)
+  :safe #'booleanp)
 
 (defcustom sentence-end-without-period nil
   "Non-nil means a sentence will end without a period.
-- 
2.45.2