Proposed changes to text-mode.el

["Basil L. Contovounesios" <contovob@tcd.ie>]

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From 36a7cded27791a26cc4979a1a27a6fa59d9a46f5 Mon Sep 17 00:00:00 2001
From: "Basil L. Contovounesios" <contovob@tcd.ie>
Date: Mon, 18 Mar 2019 23:45:55 +0000
Subject: [PATCH 1/3] Use lexical-binding in text-mode.el

* lisp/textmodes/text-mode.el: Use lexical-binding.
(text-mode, paragraph-indent-minor-mode, text-mode-hook-identify):
Use setq-local.
(toggle-text-mode-auto-fill): Quote function symbols as such.
(center-line): Minor simplification.

* doc/lispref/modes.texi (Example Major Modes): Update code example
for these changes to text-mode.
---
 doc/lispref/modes.texi      |  7 +++----
 lisp/textmodes/text-mode.el | 34 +++++++++++++++-------------------
 2 files changed, 18 insertions(+), 23 deletions(-)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 1afbc5a5ce..7b64a56b19 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -1291,10 +1291,9 @@ Example Major Modes
 Turning on Text mode runs the normal hook `text-mode-hook'."
 @end group
 @group
-  (set (make-local-variable 'text-mode-variant) t)
-  (set (make-local-variable 'require-final-newline)
-       mode-require-final-newline)
-  (set (make-local-variable 'indent-line-function) 'indent-relative))
+  (setq-local text-mode-variant t)
+  (setq-local require-final-newline mode-require-final-newline)
+  (setq-local indent-line-function #'indent-relative))
 @end group
 @end smallexample
 
diff --git a/lisp/textmodes/text-mode.el b/lisp/textmodes/text-mode.el
index 931faadb5b..90bb3eb3b5 100644
--- a/lisp/textmodes/text-mode.el
+++ b/lisp/textmodes/text-mode.el
@@ -1,4 +1,4 @@
-;;; text-mode.el --- text mode, and its idiosyncratic commands
+;;; text-mode.el --- text mode, and its idiosyncratic commands  -*- lexical-binding: t -*-
 
 ;; Copyright (C) 1985, 1992, 1994, 2001-2019 Free Software Foundation,
 ;; Inc.
@@ -104,10 +104,9 @@ text-mode
  (see the variable `adaptive-fill-mode').
 \\{text-mode-map}
 Turning on Text mode runs the normal hook `text-mode-hook'."
-  (set (make-local-variable 'text-mode-variant) t)
-  (set (make-local-variable 'require-final-newline)
-       mode-require-final-newline)
-  (set (make-local-variable 'indent-line-function) 'indent-relative))
+  (setq-local text-mode-variant t)
+  (setq-local require-final-newline mode-require-final-newline)
+  (setq-local indent-line-function #'indent-relative))
 
 (define-derived-mode paragraph-indent-text-mode text-mode "Parindent"
   "Major mode for editing text, with leading spaces starting a paragraph.
@@ -131,14 +130,12 @@ paragraph-indent-minor-mode
   :initial-value nil
   ;; Change the definition of a paragraph start.
   (let ((ps-re "[ \t\n\f]\\|"))
-    (if (eq t (compare-strings ps-re nil nil
-                               paragraph-start nil (length ps-re)))
+    (if (string-prefix-p ps-re paragraph-start)
         (if (not paragraph-indent-minor-mode)
-            (set (make-local-variable 'paragraph-start)
-                 (substring paragraph-start (length ps-re))))
+            (setq-local paragraph-start
+                        (substring paragraph-start (length ps-re))))
       (if paragraph-indent-minor-mode
-          (set (make-local-variable 'paragraph-start)
-               (concat ps-re paragraph-start)))))
+          (setq-local paragraph-start (concat ps-re paragraph-start)))))
   ;; Change the indentation function.
   (if paragraph-indent-minor-mode
       (add-function :override (local 'indent-line-function)
@@ -154,7 +151,7 @@ 'indented-text-mode
 (defun text-mode-hook-identify ()
   "Mark that this mode has run `text-mode-hook'.
 This is how `toggle-text-mode-auto-fill' knows which buffers to operate on."
-  (set (make-local-variable 'text-mode-variant) t))
+  (setq-local text-mode-variant t))
 
 (defun toggle-text-mode-auto-fill ()
   "Toggle whether to use Auto Fill in Text mode and related modes.
@@ -163,8 +160,8 @@ toggle-text-mode-auto-fill
   (interactive)
   (let ((enable-mode (not (memq 'turn-on-auto-fill text-mode-hook))))
     (if enable-mode
-	(add-hook 'text-mode-hook 'turn-on-auto-fill)
-      (remove-hook 'text-mode-hook 'turn-on-auto-fill))
+        (add-hook 'text-mode-hook #'turn-on-auto-fill)
+      (remove-hook 'text-mode-hook #'turn-on-auto-fill))
     (dolist (buffer (buffer-list))
       (with-current-buffer buffer
 	(if (or (derived-mode-p 'text-mode) text-mode-variant)
@@ -214,15 +211,14 @@ center-line
   (while (not (eq nlines 0))
     (save-excursion
       (let ((lm (current-left-margin))
-	    line-length)
+            space)
 	(beginning-of-line)
 	(delete-horizontal-space)
 	(end-of-line)
 	(delete-horizontal-space)
-	(setq line-length (current-column))
-	(if (> (- fill-column lm line-length) 0)
-	    (indent-line-to
-	     (+ lm (/ (- fill-column lm line-length) 2))))))
+        (setq space (- fill-column lm (current-column)))
+        (if (> space 0)
+            (indent-line-to (+ lm (/ space 2))))))
     (cond ((null nlines)
 	   (setq nlines 0))
 	  ((> nlines 0)
-- 
2.20.1


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From fa3f26d44c746fdf52520d18bded7e8b7987852a Mon Sep 17 00:00:00 2001
From: "Basil L. Contovounesios" <contovob@tcd.ie>
Date: Tue, 26 Feb 2019 16:13:23 +0000
Subject: [PATCH 2/3] Do not set indent-line-function in text-mode

* lisp/textmodes/text-mode.el (text-mode): Do not reset
indent-line-function to its global default value of indent-relative.
* doc/lispref/modes.texi (Example Major Modes):
* etc/NEWS: Document change accordingly.
---
 doc/lispref/modes.texi      |  7 +------
 etc/NEWS                    | 11 +++++++++++
 lisp/textmodes/text-mode.el |  3 +--
 3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 7b64a56b19..4315b70ed7 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -1292,15 +1292,10 @@ Example Major Modes
 @end group
 @group
   (setq-local text-mode-variant t)
-  (setq-local require-final-newline mode-require-final-newline)
-  (setq-local indent-line-function #'indent-relative))
+  (setq-local require-final-newline mode-require-final-newline))
 @end group
 @end smallexample
 
-@noindent
-(The last line is redundant nowadays, since @code{indent-relative} is
-the default value, and we'll delete it in a future version.)
-
 @cindex @file{lisp-mode.el}
   The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction
 mode) have more features than Text mode and the code is correspondingly
diff --git a/etc/NEWS b/etc/NEWS
index afee1e1dca..69af0447b2 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1233,6 +1233,17 @@ near the current column in Tabulated Lists (see variables
 'tabulated-list-tty-sort-indicator-asc', and
 'tabulated-list-tty-sort-indicator-desc').
 
+** Text mode
+
++++
+*** 'text-mode' no longer sets the value of 'indent-line-function'.
+The global value of 'indent-line-function', which defaults to
+'indent-relative', will no longer be reset locally when turning on
+'text-mode'.
+
+To get back the old behavior, add a function to 'text-mode-hook' which
+performs (setq-local indent-line-function #'indent-relative).
+
 \f
 * New Modes and Packages in Emacs 27.1
 
diff --git a/lisp/textmodes/text-mode.el b/lisp/textmodes/text-mode.el
index 90bb3eb3b5..6114490208 100644
--- a/lisp/textmodes/text-mode.el
+++ b/lisp/textmodes/text-mode.el
@@ -105,8 +105,7 @@ text-mode
 \\{text-mode-map}
 Turning on Text mode runs the normal hook `text-mode-hook'."
   (setq-local text-mode-variant t)
-  (setq-local require-final-newline mode-require-final-newline)
-  (setq-local indent-line-function #'indent-relative))
+  (setq-local require-final-newline mode-require-final-newline))
 
 (define-derived-mode paragraph-indent-text-mode text-mode "Parindent"
   "Major mode for editing text, with leading spaces starting a paragraph.
-- 
2.20.1


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From 1bc70af0bca2d1cb9e6d8799748bf5c262b9a5eb Mon Sep 17 00:00:00 2001
From: "Basil L. Contovounesios" <contovob@tcd.ie>
Date: Mon, 25 Mar 2019 23:10:59 +0000
Subject: [PATCH 3/3] Make text-mode-variant obsolete

* lisp/textmodes/text-mode.el (text-mode-variant): Make obsolete,
suggesting derived-mode-p as a better alternative.
* etc/NEWS: Announce obsolescence.
---
 etc/NEWS                    | 3 +++
 lisp/textmodes/text-mode.el | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 69af0447b2..f3d09bc3dc 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1244,6 +1244,9 @@ The global value of 'indent-line-function', which defaults to
 To get back the old behavior, add a function to 'text-mode-hook' which
 performs (setq-local indent-line-function #'indent-relative).
 
++++
+*** 'text-mode-variant' is now obsolete, use 'derived-mode-p' instead.
+
 \f
 * New Modes and Packages in Emacs 27.1
 
diff --git a/lisp/textmodes/text-mode.el b/lisp/textmodes/text-mode.el
index 6114490208..e676a5dae2 100644
--- a/lisp/textmodes/text-mode.el
+++ b/lisp/textmodes/text-mode.el
@@ -38,8 +38,8 @@ text-mode-hook
   :group 'text)
 
 (defvar text-mode-variant nil
-  "Non-nil if this buffer's major mode is a variant of Text mode.
-Use (derived-mode-p \\='text-mode) instead.")
+  "Non-nil if this buffer's major mode is a variant of Text mode.")
+(make-obsolete-variable 'text-mode-variant 'derived-mode-p "27.1")
 
 (defvar text-mode-syntax-table
   (let ((st (make-syntax-table)))
-- 
2.20.1


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I attach three patches for lisp/textmodes/text-mode.el arising from the
discussion in bug#34671.

The first enables lexical-binding and makes some minor simplifications.

The second makes text-mode no longer reset indent-line-function to
indent-relative locally, a change (info "(elisp) Example Major Modes")
has been promising for many moons.  AFAICT, text-mode works even when
indent-line-function is not set to indent-relative (as I'd hope).  I've
had a quick look through all the modes derived from text-mode in
emacs.git and nothing strikes me as an obvious incompatibility with the
proposed change.  Are there any significant reasons to keep the current
setting of indent-line-function in text-mode?

The third obsoletes the variable text-mode-variant, which is unused in
both emacs.git and elpa.git, and has been recommending derived-mode-p as
a better alternative in its docstring for almost two decades.

WDYT?

Thanks,

-- 
Basil