emacs/lisp/font-core.el

;;; font-core.el --- Core interface to font-lock  -*- lexical-binding: t; -*-

;; Copyright (C) 1992-2022 Free Software Foundation, Inc.

;; Maintainer: emacs-devel@gnu.org
;; Keywords: languages, faces
;; Package: emacs

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;;; Code:

;; This variable is used by mode packages that support Font Lock mode by
;; defining their own keywords to use for `font-lock-keywords'.  (The mode
;; command should make it buffer-local and set it to provide the set up.)
(defvar-local font-lock-defaults nil
  "Defaults for Font Lock mode specified by the major mode.
Defaults should be of the form:

 (KEYWORDS [KEYWORDS-ONLY [CASE-FOLD [SYNTAX-ALIST ...]]])

KEYWORDS may be a symbol (a variable or function whose value is the keywords
to use for fontification) or a list of symbols (specifying different levels
of fontification).

If KEYWORDS-ONLY is non-nil, syntactic fontification (strings and
comments) is not performed.

If CASE-FOLD is non-nil, the case of the keywords is ignored when fontifying.

If SYNTAX-ALIST is non-nil, it should be a list of cons pairs of the form
\(CHAR-OR-STRING . STRING) used to set the local Font Lock syntax table, for
keyword and syntactic fontification (see `modify-syntax-entry').

These item elements are used by Font Lock mode to set the variables
`font-lock-keywords', `font-lock-keywords-only',
`font-lock-keywords-case-fold-search', `font-lock-syntax-table'.

Further item elements are alists of the form (VARIABLE . VALUE) and are in no
particular order.  Each VARIABLE is made buffer-local before set to VALUE.

Currently, appropriate variables include `font-lock-mark-block-function'.
If this is non-nil, it should be a function with no args used to mark any
enclosing block of text, for fontification via \\[font-lock-fontify-block].
Typical values are `mark-defun' for programming modes or `mark-paragraph' for
textual modes (i.e., the mode-dependent function is known to put point and mark
around a text block relevant to that mode).

Other variables include that for syntactic keyword fontification,
`font-lock-syntactic-keywords' and those for buffer-specialized fontification
functions, `font-lock-fontify-buffer-function',
`font-lock-unfontify-buffer-function', `font-lock-fontify-region-function',
`font-lock-unfontify-region-function', and `font-lock-inhibit-thing-lock'.")
(put 'font-lock-defaults 'risky-local-variable t)

(defvar font-lock-function 'font-lock-default-function
  "A function which is called when `font-lock-mode' is toggled.
It will be passed one argument, which is the current value of
`font-lock-mode'.")

;; The mode for which font-lock was initialized, or nil if none.
(defvar font-lock-major-mode)

(define-minor-mode font-lock-mode
  "Toggle syntax highlighting in this buffer (Font Lock mode).

When Font Lock mode is enabled, text is fontified as you type it:

 - Comments are displayed in `font-lock-comment-face';
 - Strings are displayed in `font-lock-string-face';
 - Certain other expressions are displayed in other faces
   according to the value of the variable `font-lock-keywords'.

To customize the faces (colors, fonts, etc.) used by Font Lock for
fontifying different parts of buffer text, use \\[customize-face].

You can enable Font Lock mode in any major mode automatically by
turning on in the major mode's hook.  For example, put in your
~/.emacs:

 (add-hook \\='c-mode-hook \\='turn-on-font-lock)

Alternatively, you can use Global Font Lock mode to automagically
turn on Font Lock mode in buffers whose major mode supports it
and whose major mode is one of `font-lock-global-modes'.  For
example, put in your ~/.emacs:

 (global-font-lock-mode t)

Where major modes support different levels of fontification, you
can use the variable `font-lock-maximum-decoration' to specify
which level you generally prefer.  When you turn Font Lock mode
on/off the buffer is fontified/defontified.

To add your own highlighting for some major mode, and modify the
highlighting selected automatically via the variable
`font-lock-maximum-decoration', you can use
`font-lock-add-keywords'.

To fontify a buffer, without turning on Font Lock mode and
regardless of buffer size, you can use \\[font-lock-fontify-buffer].

To fontify a block (the function or paragraph containing point,
or a number of lines around point), perhaps because modification
on the current line caused syntactic change on other lines, you
can use \\[font-lock-fontify-block].

You can set your own default settings for some mode, by setting a
buffer local value for `font-lock-defaults', via its mode hook.

The above is the default behavior of `font-lock-mode'; you may
specify your own function which is called when `font-lock-mode'
is toggled via `font-lock-function'."
  :after-hook (font-lock-initial-fontify)
  ;; Don't turn on Font Lock mode if we don't have a display (we're running a
  ;; batch job) or if the buffer is invisible (the name starts with a space).
  (when (or noninteractive (eq (aref (buffer-name) 0) ?\s))
    (setq font-lock-mode nil))
  (funcall font-lock-function font-lock-mode)
  ;; Arrange to unfontify this buffer if we change major mode later.
  (if font-lock-mode
      (add-hook 'change-major-mode-hook 'font-lock-change-mode nil t)
    (remove-hook 'change-major-mode-hook 'font-lock-change-mode t)))

;; Get rid of fontification for the old major mode.
;; We do this when changing major modes.
(defun font-lock-change-mode ()
  (font-lock-mode -1))

(defun font-lock-defontify ()
  "Clear out all `font-lock-face' properties in current buffer.
A major mode that uses `font-lock-face' properties might want to put
this function onto `change-major-mode-hook'."
  (let ((modp (buffer-modified-p))
	(inhibit-read-only t))
    (save-restriction
      (widen)
      (remove-list-of-text-properties (point-min) (point-max)
				      '(font-lock-face)))
    (restore-buffer-modified-p modp)))

(defvar font-lock-set-defaults)
(defun font-lock-default-function (mode)
  ;; Turn on Font Lock mode.
  (when mode
    (setq-local char-property-alias-alist
                (copy-tree char-property-alias-alist))
    ;; Add `font-lock-face' as an alias for the `face' property.
    (let ((elt (assq 'face char-property-alias-alist)))
      (if elt
	  (unless (memq 'font-lock-face (cdr elt))
	    (setcdr elt (nconc (cdr elt) (list 'font-lock-face))))
	(push (list 'face 'font-lock-face) char-property-alias-alist))))
  ;; Turn off Font Lock mode.
  (unless mode
    ;; Remove `font-lock-face' as an alias for the `face' property.
    (setq-local char-property-alias-alist
                (copy-tree char-property-alias-alist))
    (let ((elt (assq 'face char-property-alias-alist)))
      (when elt
	(setcdr elt (remq 'font-lock-face (cdr elt)))
	(when (null (cdr elt))
	  (setq char-property-alias-alist
		(delq elt char-property-alias-alist))))))

  ;; Only do hard work if the mode has specified stuff in
  ;; `font-lock-defaults'.
  (when (font-lock-specified-p mode)
    (font-lock-mode-internal mode)))

(defun turn-on-font-lock ()
  "Turn on Font Lock mode (only if the terminal can display it)."
  (unless font-lock-mode
    (font-lock-mode)))

;;; Global Font Lock mode.

;; A few people have hassled in the past for a way to make it easier to turn on
;; Font Lock mode, without the user needing to know for which modes s/he has to
;; turn it on, perhaps the same way hilit19.el/hl319.el does.  I've always
;; balked at that way, as I see it as just re-molding the same problem in
;; another form.  That is; some person would still have to keep track of which
;; modes (which may not even be distributed with Emacs) support Font Lock mode.
;; The list would always be out of date.  And that person might have to be me.

;; Implementation.
;;
;; In a previous discussion the following hack came to mind.  It is a gross
;; hack, but it generally works.  We use the convention that major modes start
;; by calling the function `kill-all-local-variables', which in turn runs
;; functions on the hook variable `change-major-mode-hook'.  We attach our
;; function `font-lock-change-major-mode' to that hook.  Of course, when this
;; hook is run, the major mode is in the process of being changed and we do not
;; know what the final major mode will be.  So, `font-lock-change-major-mode'
;; only (a) notes the name of the current buffer, and (b) adds our function
;; `turn-on-font-lock-if-desired' to the hook variables
;; `after-change-major-mode-hook' and `post-command-hook' (for modes
;; that do not yet run `after-change-major-mode-hook').  By the time
;; the functions on the first of these hooks to be run are run, the new major
;; mode is assumed to be in place.  This way we get a Font Lock function run
;; when a major mode is turned on, without knowing major modes or their hooks.
;;
;; Naturally this requires that major modes run `kill-all-local-variables'
;; and `after-change-major-mode-hook', as they are supposed to.  For modes
;; that do not run `after-change-major-mode-hook' yet, `post-command-hook'
;; takes care of things if the mode is set directly or indirectly by
;; an interactive command; however, problems can occur if the mode is
;; set by a timer or process: in that case, proper handling of Font Lock mode
;; may be delayed until the next interactive command.

;; User interface.
;;
;; Although Global Font Lock mode is a pseudo-mode, I think that the user
;; interface should conform to the usual Emacs convention for modes, i.e., a
;; command to toggle the feature (`global-font-lock-mode') with a variable for
;; finer control of the mode's behavior (`font-lock-global-modes').
;;
;; The feature should not be enabled by loading font-lock.el, since other
;; mechanisms for turning on Font Lock mode, such as M-x font-lock-mode RET or
;; (add-hook 'c-mode-hook 'turn-on-font-lock), would cause Font Lock mode to be
;; turned on everywhere.  That would not be intuitive or informative because
;; loading a file tells you nothing about the feature or how to control it.  It
;; would also be contrary to the Principle of Least Surprise.  sm.

(defcustom font-lock-global-modes t
  "Modes for which Font Lock mode is automagically turned on.
Global Font Lock mode is controlled by the command `global-font-lock-mode'.
If nil, means no modes have Font Lock mode automatically turned on.
If t, all modes that support Font Lock mode have it automatically turned on.
If a list, it should be a list of `major-mode' symbol names for which Font Lock
mode should be automatically turned on.  The sense of the list is negated if it
begins with `not'.  For example:
 (c-mode c++-mode)
means that Font Lock mode is turned on for buffers in C and C++ modes only."
  :type '(choice (const :tag "none" nil)
		 (const :tag "all" t)
		 (set :menu-tag "mode specific" :tag "modes"
		      :value (not)
		      (const :tag "Except" not)
		      (repeat :inline t (symbol :tag "mode"))))
  :group 'font-lock)

(defun turn-on-font-lock-if-desired ()
  (when (cond ((eq font-lock-global-modes t)
	       t)
	      ((eq (car-safe font-lock-global-modes) 'not)
	       (not (memq major-mode (cdr font-lock-global-modes))))
	      (t (memq major-mode font-lock-global-modes)))
    (let (inhibit-quit)
      (turn-on-font-lock))))

(define-globalized-minor-mode global-font-lock-mode
  font-lock-mode turn-on-font-lock-if-desired
  ;; What was this :extra-args thingy for?  --Stef
  ;; :extra-args (dummy)
  :initialize 'custom-initialize-delay
  :init-value (not (or noninteractive emacs-basic-display))
  :group 'font-lock
  :version "22.1")

;;; End of Global Font Lock mode.

(provide 'font-core)

;;; font-core.el ends here