help.el 55.8 KB
Newer Older
Eric S. Raymond's avatar
Eric S. Raymond committed
1 2
;;; help.el --- help commands for Emacs

3
;; Copyright (C) 1985, 1986, 1993, 1994, 1998, 1999, 2000 Free Software Foundation, Inc.
Eric S. Raymond's avatar
Eric S. Raymond committed
4

Eric S. Raymond's avatar
Eric S. Raymond committed
5
;; Maintainer: FSF
Eric S. Raymond's avatar
Eric S. Raymond committed
6
;; Keywords: help, internal
Eric S. Raymond's avatar
Eric S. Raymond committed
7

Richard M. Stallman's avatar
Richard M. Stallman committed
8 9 10 11
;; 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
Eric S. Raymond's avatar
Eric S. Raymond committed
12
;; the Free Software Foundation; either version 2, or (at your option)
Richard M. Stallman's avatar
Richard M. Stallman committed
13 14 15 16 17 18 19 20
;; 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
Erik Naggum's avatar
Erik Naggum committed
21 22 23
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.
Richard M. Stallman's avatar
Richard M. Stallman committed
24

25 26
;;; Commentary:

27
;; This code implements GNU Emacs' on-line help system, the one invoked by
28
;; `M-x help-for-help'.
29

Eric S. Raymond's avatar
Eric S. Raymond committed
30 31
;;; Code:

32 33
;; Get the macro make-help-screen when this is compiled,
;; or run interpreted, but not when the compiled code is loaded.
34
(eval-when-compile (require 'help-macro))
Dave Love's avatar
Dave Love committed
35
(eval-when-compile (require 'view))
36

Richard M. Stallman's avatar
Richard M. Stallman committed
37 38 39
(defvar help-map (make-sparse-keymap)
  "Keymap for characters following the Help key.")

Karl Heuer's avatar
Karl Heuer committed
40 41 42
(defvar help-mode-map (make-sparse-keymap)
  "Keymap for help mode.")

Eric S. Raymond's avatar
Eric S. Raymond committed
43
(define-key global-map (char-to-string help-char) 'help-command)
44 45
(define-key global-map [help] 'help-command)
(define-key global-map [f1] 'help-command)
Richard M. Stallman's avatar
Richard M. Stallman committed
46 47
(fset 'help-command help-map)

Eric S. Raymond's avatar
Eric S. Raymond committed
48
(define-key help-map (char-to-string help-char) 'help-for-help)
49 50
(define-key help-map [help] 'help-for-help)
(define-key help-map [f1] 'help-for-help)
Richard M. Stallman's avatar
Richard M. Stallman committed
51 52 53 54 55
(define-key help-map "?" 'help-for-help)

(define-key help-map "\C-c" 'describe-copying)
(define-key help-map "\C-d" 'describe-distribution)
(define-key help-map "\C-w" 'describe-no-warranty)
56
(define-key help-map "\C-p" 'describe-project)
57
(define-key help-map "a" 'apropos-command)
Richard M. Stallman's avatar
Richard M. Stallman committed
58 59 60 61 62 63 64 65 66

(define-key help-map "b" 'describe-bindings)

(define-key help-map "c" 'describe-key-briefly)
(define-key help-map "k" 'describe-key)

(define-key help-map "d" 'describe-function)
(define-key help-map "f" 'describe-function)

67 68
(define-key help-map "F" 'view-emacs-FAQ)

Richard M. Stallman's avatar
Richard M. Stallman committed
69
(define-key help-map "i" 'info)
70
(define-key help-map "4i" 'info-other-window)
Jim Blandy's avatar
Jim Blandy committed
71 72
(define-key help-map "\C-f" 'Info-goto-emacs-command-node)
(define-key help-map "\C-k" 'Info-goto-emacs-key-command-node)
73
(define-key help-map "\C-i" 'info-lookup-symbol)
Richard M. Stallman's avatar
Richard M. Stallman committed
74 75 76 77 78 79 80 81

(define-key help-map "l" 'view-lossage)

(define-key help-map "m" 'describe-mode)

(define-key help-map "\C-n" 'view-emacs-news)
(define-key help-map "n" 'view-emacs-news)

82
(define-key help-map "p" 'finder-by-keyword)
83 84
(autoload 'finder-by-keyword "finder"
  "Find packages matching a given keyword." t)
85

86 87
(define-key help-map "P" 'view-emacs-problems)

Richard M. Stallman's avatar
Richard M. Stallman committed
88 89 90 91 92 93 94 95
(define-key help-map "s" 'describe-syntax)

(define-key help-map "t" 'help-with-tutorial)

(define-key help-map "w" 'where-is)

(define-key help-map "v" 'describe-variable)

Richard M. Stallman's avatar
Richard M. Stallman committed
96 97
(define-key help-map "q" 'help-quit)

98 99 100 101 102
(define-key help-mode-map [mouse-2] 'help-follow-mouse)
(define-key help-mode-map "\C-c\C-b" 'help-go-back)
(define-key help-mode-map "\C-c\C-c" 'help-follow)
(define-key help-mode-map "\t" 'help-next-ref)
(define-key help-mode-map [backtab] 'help-previous-ref)
103
(define-key help-mode-map [(shift tab)] 'help-previous-ref)
104 105 106 107 108
;; Documentation only, since we use minor-mode-overriding-map-alist.
(define-key help-mode-map "\r" 'help-follow)

(defvar help-xref-stack nil
  "A stack of ways by which to return to help buffers after following xrefs.
109
Used by `help-follow' and `help-xref-go-back'.
Dave Love's avatar
Dave Love committed
110 111 112 113
An element looks like (POSITION FUNCTION ARGS...), where POSITION is
`(POINT . BUFFER-NAME)'.
To use the element, do (apply FUNCTION ARGS) then goto the point in
the named buffer.")
114 115 116
(put 'help-xref-stack 'permanent-local t)

(defvar help-xref-stack-item nil
117 118
  "An item for `help-follow' in this buffer to push onto `help-xref-stack'.
The format is (FUNCTION ARGS...).")
119 120 121
(put 'help-xref-stack-item 'permanent-local t)

(setq-default help-xref-stack nil help-xref-stack-item nil)
Simon Marshall's avatar
Simon Marshall committed
122

Dave Love's avatar
Dave Love committed
123 124 125 126 127
(defcustom help-mode-hook nil
  "Hook run by `help-mode'."
  :type 'hook
  :group 'help)

Karl Heuer's avatar
Karl Heuer committed
128
(defun help-mode ()
129
  "Major mode for viewing help text and navigating references in it.
Karl Heuer's avatar
Karl Heuer committed
130 131 132 133 134 135 136 137
Entry to this mode runs the normal hook `help-mode-hook'.
Commands:
\\{help-mode-map}"
  (interactive)
  (kill-all-local-variables)
  (use-local-map help-mode-map)
  (setq mode-name "Help")
  (setq major-mode 'help-mode)
Simon Marshall's avatar
Simon Marshall committed
138
  (make-local-variable 'font-lock-defaults)
139
  (setq font-lock-defaults nil)         ; font-lock would defeat xref
140
  (view-mode)
141 142
  (make-local-variable 'view-no-disable-on-exit)
  (setq view-no-disable-on-exit t)
143 144
  ;; `help-make-xrefs' would be run here if not invoked from
  ;; `help-mode-maybe'.
Karl Heuer's avatar
Karl Heuer committed
145 146
  (run-hooks 'help-mode-hook))

147 148 149 150
(defun help-mode-setup ()
  (help-mode)
  (setq buffer-read-only nil))

151
(add-hook 'temp-buffer-setup-hook 'help-mode-setup)
152 153

(defun help-mode-finish ()
154 155 156 157 158
  (when (eq major-mode 'help-mode) 
    ;; View mode's read-only status of existing *Help* buffer is lost
    ;; by with-output-to-temp-buffer.
    (toggle-read-only 1)
    (help-make-xrefs (current-buffer)))
159 160
  (setq view-return-to-alist
	(list (cons (selected-window) help-return-method))))
161

162
(add-hook 'temp-buffer-show-hook 'help-mode-finish)
163

Richard M. Stallman's avatar
Richard M. Stallman committed
164
(defun help-quit ()
Richard M. Stallman's avatar
Richard M. Stallman committed
165
  "Just exit from the Help command's command loop."
Richard M. Stallman's avatar
Richard M. Stallman committed
166 167 168
  (interactive)
  nil)

169 170
(defun help-with-tutorial (&optional arg)
  "Select the Emacs learn-by-doing tutorial.
171
If there is a tutorial version written in the language
172
of the selected language environment, that version is used.
173
If there's no tutorial in that language, `TUTORIAL' is selected.
174
With arg, you are asked to choose which language."
175
  (interactive "P")
176 177 178 179
  (let ((lang (if arg
		  (read-language-name 'tutorial "Language: " "English")
		(if (get-language-info current-language-environment 'tutorial)
		    current-language-environment
180 181
		  "English")))
	file filename)
182
    (setq filename (get-language-info lang 'tutorial))
183
    (setq file (expand-file-name (concat "~/" filename)))
Richard M. Stallman's avatar
Richard M. Stallman committed
184 185 186 187 188 189
    (delete-other-windows)
    (if (get-file-buffer file)
	(switch-to-buffer (get-file-buffer file))
      (switch-to-buffer (create-file-buffer file))
      (setq buffer-file-name file)
      (setq default-directory (expand-file-name "~/"))
Jim Blandy's avatar
Jim Blandy committed
190
      (setq buffer-auto-save-file-name nil)
191
      (insert-file-contents (expand-file-name filename data-directory))
Richard M. Stallman's avatar
Richard M. Stallman committed
192 193 194 195
      (goto-char (point-min))
      (search-forward "\n<<")
      (beginning-of-line)
      (delete-region (point) (progn (end-of-line) (point)))
196
      (let ((n (- (window-height (selected-window))
Richard M. Stallman's avatar
Richard M. Stallman committed
197
		  (count-lines (point-min) (point))
198
		  6)))
199
	(if (< n 12)
200 201 202 203 204 205
	    (newline n)
	  ;; Some people get confused by the large gap.
	  (newline (/ n 2))
	  (insert "[Middle of page left blank for didactic purposes.  "
		  "Text continues below]")
	  (newline (- n (/ n 2)))))
Richard M. Stallman's avatar
Richard M. Stallman committed
206 207 208
      (goto-char (point-min))
      (set-buffer-modified-p nil))))

209 210 211 212 213 214 215 216 217 218 219 220 221
(defun mode-line-key-binding (key)
  "Value is the binding of KEY in the mode line or nil if none."
  (let (string-info defn)
    (when (and (eq 'mode-line (aref key 0))
	       (consp (setq string-info (nth 4 (event-start (aref key 1))))))
    (let* ((string (car string-info))
	   (pos (cdr string-info))
	   (local-map (and (> pos 0)
			   (< pos (length string))
			   (get-text-property pos 'local-map string))))
      (setq defn (and local-map (lookup-key local-map key)))))
    defn))

222 223 224 225
(defun describe-key-briefly (key &optional insert)
  "Print the name of the function KEY invokes.  KEY is a string.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
  (interactive "kDescribe key briefly: \nP")
226 227
  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
228
	  (standard-output (if insert (current-buffer) t))
229 230 231 232 233 234 235 236 237 238 239 240 241
	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
      ;; Ok, now look up the key and name the command.
242 243
      (let ((defn (or (mode-line-key-binding key)
		      (key-binding key)))
244
	    (key-desc (key-description key)))
245
	(if (or (null defn) (integerp defn))
246 247
	    (princ (format "%s is undefined" key-desc))
	  (princ (format (if insert
248
			     "`%s' (`%s')"
249 250 251 252 253
			   (if (windowp window)
			       "%s at that spot runs the command %s"
			     "%s runs the command %s"))
			 key-desc
			 (if (symbolp defn) defn (prin1-to-string defn)))))))))
Richard M. Stallman's avatar
Richard M. Stallman committed
254

255 256 257 258 259 260 261
(defvar help-return-method nil
  "What to do to \"exit\" the help buffer.
This is a list
 (WINDOW . t)              delete the selected window, go to WINDOW.
 (WINDOW . quit-window)    do quit-window, then select WINDOW.
 (WINDOW BUF START POINT)  display BUF at START, POINT, then select WINDOW.")

Richard M. Stallman's avatar
Richard M. Stallman committed
262 263 264 265 266
(defun print-help-return-message (&optional function)
  "Display or return message saying how to restore windows after help command.
Computes a message and applies the optional argument FUNCTION to it.
If FUNCTION is nil, applies `message' to it, thus printing it."
  (and (not (get-buffer-window standard-output))
267
       (let ((first-message
268
	      (cond ((special-display-p (buffer-name standard-output))
269
		     (setq help-return-method (cons (selected-window) t))
270 271 272 273 274 275 276 277
		     ;; If the help output buffer is a special display buffer,
		     ;; don't say anything about how to get rid of it.
		     ;; First of all, the user will do that with the window
		     ;; manager, not with Emacs.
		     ;; Secondly, the buffer has not been displayed yet,
		     ;; so we don't know whether its frame will be selected.
		     nil)
		    ((not (one-window-p t))
278 279
		     (setq help-return-method
			   (cons (selected-window) 'quit-window))
280 281
		     "Type \\[switch-to-buffer-other-window] RET to restore the other window.")
		    (pop-up-windows
282
		     (setq help-return-method (cons (selected-window) t))
283 284
		     "Type \\[delete-other-windows] to remove help window.")
		    (t
285 286 287
		     (setq help-return-method
			   (list (selected-window) (window-buffer)
				 (window-start) (window-point)))
288 289 290 291
		     "Type \\[switch-to-buffer] RET to remove help window."))))
	 (funcall (or function 'message)
		  (concat
		   (if first-message
Dave Love's avatar
Dave Love committed
292 293
		       (substitute-command-keys first-message))
		   (if first-message "  ")
294 295
		   ;; If the help buffer will go in a separate frame,
		   ;; it's no use mentioning a command to scroll, so don't.
296
		   (if (special-display-p (buffer-name standard-output))
297
		       nil
298
		     (if (same-window-p (buffer-name standard-output))
299 300 301 302
			 ;; Say how to scroll this window.
			 (substitute-command-keys
			  "\\[scroll-up] to scroll the help.")
		       ;; Say how to scroll some other window.
303
		       (substitute-command-keys
304
			"\\[scroll-other-window] to scroll the help."))))))))
Richard M. Stallman's avatar
Richard M. Stallman committed
305 306 307 308

(defun describe-key (key)
  "Display documentation of the function invoked by KEY.  KEY is a string."
  (interactive "kDescribe key: ")
309 310 311 312 313 314 315 316 317 318 319 320 321 322
  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
323
      (let ((defn (or (mode-line-key-binding key) (key-binding key))))
324 325 326 327 328 329 330 331
	(if (or (null defn) (integerp defn))
	    (message "%s is undefined" (key-description key))
	  (with-output-to-temp-buffer "*Help*"
	    (princ (key-description key))
	    (if (windowp window)
		(princ " at that spot"))
	    (princ " runs the command ")
	    (prin1 defn)
332
	    (princ "\n   which is ")
333
	    (describe-function-1 defn nil (interactive-p))
334
	    (print-help-return-message)))))))
Richard M. Stallman's avatar
Richard M. Stallman committed
335

336 337
(defun describe-mode ()
  "Display documentation of current major mode and minor modes.
338 339 340
The major mode description comes first, followed by the minor modes,
each on a separate page.

Richard M. Stallman's avatar
Richard M. Stallman committed
341
For this to work correctly for a minor mode, the mode's indicator variable
342
\(listed in `minor-mode-alist') must also be a function whose documentation
Richard M. Stallman's avatar
Richard M. Stallman committed
343
describes the minor mode."
344
  (interactive)
Richard M. Stallman's avatar
Richard M. Stallman committed
345
  (with-output-to-temp-buffer "*Help*"
346 347 348 349 350 351 352 353
    (when minor-mode-alist
      (princ "The major mode is described first.
For minor modes, see following pages.\n\n"))
    (princ mode-name)
    (princ " mode:\n")
    (princ (documentation major-mode))
    (help-setup-xref (list #'help-xref-mode (current-buffer)) (interactive-p))
    (let ((minor-modes minor-mode-alist))
354 355
      (while minor-modes
	(let* ((minor-mode (car (car minor-modes)))
356
	       (indicator (car (cdr (car minor-modes)))))
357 358 359
	  ;; Document a minor mode if it is listed in minor-mode-alist,
	  ;; bound locally in this buffer, non-nil, and has a function
	  ;; definition.
360
	  (if (and (symbol-value minor-mode)
361 362 363 364 365 366 367
		   (fboundp minor-mode))
	      (let ((pretty-minor-mode minor-mode))
		(if (string-match "-mode$" (symbol-name minor-mode))
		    (setq pretty-minor-mode
			  (capitalize
			   (substring (symbol-name minor-mode)
				      0 (match-beginning 0)))))
368 369 370
		(while (and indicator (symbolp indicator)
			    (boundp indicator)
			    (not (eq indicator (symbol-value indicator))))
371
		  (setq indicator (symbol-value indicator)))
372
		(princ "\n\f\n")
373 374 375 376 377
		(princ (format "%s minor mode (%s):\n"
			       pretty-minor-mode
			       (if indicator
				   (format "indicator%s" indicator)
				 "no indicator")))
378
		(princ (documentation minor-mode)))))
379
	(setq minor-modes (cdr minor-modes))))
Richard M. Stallman's avatar
Richard M. Stallman committed
380 381 382 383 384 385 386 387 388
    (print-help-return-message)))

;; So keyboard macro definitions are documented correctly
(fset 'defining-kbd-macro (symbol-function 'start-kbd-macro))

(defun describe-distribution ()
  "Display info on how to obtain the latest version of GNU Emacs."
  (interactive)
  (find-file-read-only
Jim Blandy's avatar
Jim Blandy committed
389
   (expand-file-name "DISTRIB" data-directory)))
Richard M. Stallman's avatar
Richard M. Stallman committed
390 391 392 393 394

(defun describe-copying ()
  "Display info on how you may redistribute copies of GNU Emacs."
  (interactive)
  (find-file-read-only
Jim Blandy's avatar
Jim Blandy committed
395
   (expand-file-name "COPYING" data-directory))
Richard M. Stallman's avatar
Richard M. Stallman committed
396 397
  (goto-char (point-min)))

398 399 400 401 402 403 404
(defun describe-project ()
  "Display info on the GNU project."
  (interactive)
  (find-file-read-only
   (expand-file-name "GNU" data-directory))
  (goto-char (point-min)))

Richard M. Stallman's avatar
Richard M. Stallman committed
405 406 407 408 409 410 411 412
(defun describe-no-warranty ()
  "Display info on all the kinds of warranty Emacs does NOT have."
  (interactive)
  (describe-copying)
  (let (case-fold-search)
    (search-forward "NO WARRANTY")
    (recenter 0)))

413
(defun describe-prefix-bindings ()
414 415 416
  "Describe the bindings of the prefix used to reach this command.
The prefix described consists of all but the last event
of the key sequence that ran this command."
417
  (interactive)
418 419 420 421 422 423 424 425 426 427
  (let* ((key (this-command-keys)))
    (describe-bindings
     (if (stringp key)
	 (substring key 0 (1- (length key)))
       (let ((prefix (make-vector (1- (length key)) nil))
	     (i 0))
	 (while (< i (length prefix))
	   (aset prefix i (aref key i))
	   (setq i (1+ i)))
	 prefix)))))
428 429
;; Make C-h after a prefix, when not specifically bound, 
;; run describe-prefix-bindings.
430 431
(setq prefix-help-command 'describe-prefix-bindings)

432 433 434 435 436 437 438 439
(defun view-emacs-news (&optional arg)
  "Display info on recent changes to Emacs.
With numeric argument display information on correspondingly older changes."
  (interactive "P")
  (let* ((arg (if arg (prefix-numeric-value arg) 0)))
    (find-file-read-only
     (expand-file-name (concat (make-string arg ?O) "NEWS")
		       data-directory))))
Richard M. Stallman's avatar
Richard M. Stallman committed
440

441 442 443
(defun view-emacs-FAQ ()
  "Display the Emacs Frequently Asked Questions (FAQ) file."
  (interactive)
Dave Love's avatar
Dave Love committed
444
;;;  (find-file-read-only (expand-file-name "FAQ" data-directory))
445
  (info "(efaq)"))
446

447 448 449 450 451
(defun view-emacs-problems ()
  "Display info on known problems with Emacs and possible workarounds."
  (interactive)
  (view-file (expand-file-name "PROBLEMS" data-directory)))

Richard M. Stallman's avatar
Richard M. Stallman committed
452 453 454 455
(defun view-lossage ()
  "Display last 100 input keystrokes."
  (interactive)
  (with-output-to-temp-buffer "*Help*"
456 457 458 459 460 461 462 463
    (princ (mapconcat (function (lambda (key)
				  (if (or (integerp key)
					  (symbolp key)
					  (listp key))
				      (single-key-description key)
				    (prin1-to-string key nil))))
		      (recent-keys)
		      " "))
Richard M. Stallman's avatar
Richard M. Stallman committed
464 465 466 467 468
    (save-excursion
      (set-buffer standard-output)
      (goto-char (point-min))
      (while (progn (move-to-column 50) (not (eobp)))
	(search-forward " " nil t)
469 470 471
	(insert "\n"))
      (setq help-xref-stack nil
	    help-xref-stack-item nil))
Richard M. Stallman's avatar
Richard M. Stallman committed
472 473
    (print-help-return-message)))

Richard M. Stallman's avatar
Richard M. Stallman committed
474
(defalias 'help 'help-for-help)
475
(make-help-screen help-for-help
476
  "a b c C f F C-f i I k C-k l L m n p s t v w C-c C-d C-n C-p C-w; ? for help:"
477
  "You have typed %THIS-KEY%, the help character.  Type a Help option:
478
\(Use SPC or DEL to scroll through this text.  Type \\<help-map>\\[help-quit] to exit the Help command.)
Richard M. Stallman's avatar
Richard M. Stallman committed
479

480 481 482
a  command-apropos.  Give a substring, and see a list of commands
	(functions interactively callable) that contain
	that substring.  See also the  apropos  command.
483 484
b  describe-bindings.  Display table of all key bindings.
c  describe-key-briefly.  Type a command key sequence;
485
	it prints the function name that sequence runs.
486 487 488
C  describe-coding-system.  This describes either a specific coding system
        (if you type its name) or the coding systems currently in use
	(if you type just RET).
489
f  describe-function.  Type a function name and get documentation of it.
490 491
C-f Info-goto-emacs-command-node.  Type a function name;
	it takes you to the Info node for that command.
492
i  info. The  info  documentation reader.
493 494
I  describe-input-method.  Describe a specific input method (if you type
	its name) or the current input method (if you type just RET).
495 496
C-i  info-lookup-symbol.  Display the definition of a specific symbol
        as found in the manual for the language this buffer is written in.
497
k  describe-key.  Type a command key sequence;
498 499 500
	it displays the full documentation.
C-k Info-goto-emacs-key-command-node.  Type a command key sequence;
	it takes you to the Info node for the command bound to that key.
501
l  view-lossage.  Show last 100 characters you typed.
502 503 504
L  describe-language-environment.  This describes either the a
	specific language environment (if you type its name)
	or the current language environment (if you type just RET).
505 506
m  describe-mode.  Print documentation of current minor modes,
	and the current major mode, including their special commands.
507
n  view-emacs-news.  Display news of recent Emacs changes.
508 509 510 511
p  finder-by-keyword. Find packages matching a given topic keyword.
s  describe-syntax.  Display contents of syntax table, plus explanations
t  help-with-tutorial.  Select the Emacs learn-by-doing tutorial.
v  describe-variable.  Type name of a variable;
512
	it displays the variable's documentation and value.
513
w  where-is.  Type command name; it prints which keystrokes
514
	invoke that command.
515 516 517 518 519 520 521 522

F  Display the frequently asked questions file.
h  Display the HELLO file which illustrates various scripts.
C-c Display Emacs copying permission (General Public License).
C-d Display Emacs ordering information.
C-n Display news of recent Emacs changes.
C-p Display information about the GNU project.
C-w Display information on absence of warranty for GNU Emacs."
523
  help-map)
Richard M. Stallman's avatar
Richard M. Stallman committed
524 525

(defun function-called-at-point ()
526 527
  "Return a function around point or else called by the list containing point.
If that doesn't give a function, return nil."
528 529 530 531
  (let ((stab (syntax-table)))
    (set-syntax-table emacs-lisp-mode-syntax-table)
    (unwind-protect
	(or (condition-case ()
532 533 534 535 536 537 538 539 540 541
		(save-excursion
		  (or (not (zerop (skip-syntax-backward "_w")))
		      (eq (char-syntax (following-char)) ?w)
		      (eq (char-syntax (following-char)) ?_)
		      (forward-sexp -1))
		  (skip-chars-forward "'")
		  (let ((obj (read (current-buffer))))
		    (and (symbolp obj) (fboundp obj) obj)))
	      (error nil))
	    (condition-case ()
542 543 544 545 546 547 548 549 550 551 552 553 554 555 556
		(save-excursion
		  (save-restriction
		    (narrow-to-region (max (point-min) (- (point) 1000)) (point-max))
		    ;; Move up to surrounding paren, then after the open.
		    (backward-up-list 1)
		    (forward-char 1)
		    ;; If there is space here, this is probably something
		    ;; other than a real Lisp function call, so ignore it.
		    (if (looking-at "[ \t]")
			(error "Probably not a Lisp function call"))
		    (let (obj)
		      (setq obj (read (current-buffer)))
		      (and (symbolp obj) (fboundp obj) obj))))
	      (error nil)))
      (set-syntax-table stab))))
Richard M. Stallman's avatar
Richard M. Stallman committed
557

558 559 560 561 562 563
(defvar symbol-file-load-history-loaded nil
  "Non-nil means we have loaded the file `fns-VERSION.el' in `exec-directory'.
That file records the part of `load-history' for preloaded files,
which is cleared out before dumping to make Emacs smaller.")

(defun symbol-file (function)
564
  "Return the input source from which FUNCTION was loaded.
565 566 567 568 569 570 571 572 573 574 575 576 577 578
The value is normally a string that was passed to `load':
either an absolute file name, or a library name
\(with no directory name and no `.el' or `.elc' at the end).
It can also be nil, if the definition is not associated with any file."
  (unless symbol-file-load-history-loaded
    (load (expand-file-name
	   ;; fns-XX.YY.ZZ.el does not work on DOS filesystem.
	   (if (eq system-type 'ms-dos)
	       "fns.el"
	     (format "fns-%s.el" emacs-version))
	   exec-directory)
	  ;; The file name fns-%s.el already has a .el extension.
	  nil nil t)
    (setq symbol-file-load-history-loaded t))
579 580 581 582 583 584 585 586
  (let ((files load-history)
	file functions)
    (while files
      (if (memq function (cdr (car files)))
	  (setq file (car (car files)) files nil))
      (setq files (cdr files)))
    file))

Richard M. Stallman's avatar
Richard M. Stallman committed
587 588 589 590 591 592 593 594 595
(defun describe-function (function)
  "Display the full documentation of FUNCTION (a symbol)."
  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Describe function (default %s): " fn)
				  "Describe function: ")
596
				obarray 'fboundp t nil nil (symbol-name fn)))
Richard M. Stallman's avatar
Richard M. Stallman committed
597 598
     (list (if (equal val "")
	       fn (intern val)))))
599 600 601
  (if function
      (with-output-to-temp-buffer "*Help*"
	(prin1 function)
602 603 604
	;; Use " is " instead of a colon so that
	;; it is easier to get out the function name using forward-sexp.
	(princ " is ")
605
	(describe-function-1 function nil (interactive-p))
606 607 608 609 610 611 612
	(print-help-return-message)
	(save-excursion
	  (set-buffer standard-output)
	  ;; Return the text we displayed.
	  (buffer-string)))
    (message "You didn't specify a function")))

613
(defun describe-function-1 (function parens interactive-p)
614 615 616
  (let* ((def (if (symbolp function)
		  (symbol-function function)
		function))
617 618 619 620 621 622 623
	 file-name string need-close
	 (beg (if (commandp def) "an interactive " "a ")))
    (setq string
	  (cond ((or (stringp def)
		     (vectorp def))
		 "a keyboard macro")
		((subrp def)
624 625 626
		 (if (eq 'unevalled (cdr (subr-arity def)))
		     (concat beg "special form")
		   (concat beg "built-in function")))
627 628 629
		((byte-code-function-p def)
		 (concat beg "compiled Lisp function"))
		((symbolp def)
630 631
		 (while (symbolp (symbol-function def))
		   (setq def (symbol-function def)))
632
		 (format "an alias for `%s'" def))
633 634 635 636 637 638 639 640
		((eq (car-safe def) 'lambda)
		 (concat beg "Lisp function"))
		((eq (car-safe def) 'macro)
		 "a Lisp macro")
		((eq (car-safe def) 'mocklisp)
		 "a mocklisp function")
		((eq (car-safe def) 'autoload)
		 (setq file-name (nth 1 def))
641
		 (format "%s autoloaded %s"
642
			 (if (commandp def) "an interactive" "an")
643 644
			 (if (eq (nth 4 def) 'keymap) "keymap"
			   (if (nth 4 def) "Lisp macro" "Lisp function"))
645
			 ))
646 647 648 649 650 651 652 653 654 655 656 657
                ;; perhaps use keymapp here instead
                ((eq (car-safe def) 'keymap)
                 (let ((is-full nil)
                       (elts (cdr-safe def)))
                   (while elts
                     (if (char-table-p (car-safe elts))
                         (setq is-full t
                               elts nil))
                     (setq elts (cdr-safe elts)))
                   (if is-full
                       "a full keymap"
                     "a sparse keymap")))
658 659 660 661 662
		(t "")))
    (when (and parens (not (equal string "")))
      (setq need-close t)
      (princ "("))
    (princ string)
663 664 665 666
    (with-current-buffer "*Help*"
      (save-excursion
	(save-match-data
	  (if (re-search-backward "alias for `\\([^`']+\\)'" nil t)
667 668
	      (help-xref-button 1 #'describe-function def
				"mouse-2, RET: describe this function")))))
669
    (or file-name
670
	(setq file-name (symbol-file function)))
671 672 673 674 675 676
    (if file-name
	(progn
	  (princ " in `")
	  ;; We used to add .el to the file name,
	  ;; but that's completely wrong when the user used load-file.
	  (princ file-name)
677 678 679 680 681
	  (princ "'")
	  ;; Make a hyperlink to the library.
	  (with-current-buffer "*Help*"
	    (save-excursion
	      (re-search-backward "`\\([^`']+\\)'" nil t)
682 683 684
	      (help-xref-button 1 #'(lambda (arg)
				      (let ((location
					     (find-function-noselect arg)))
685 686
					(pop-to-buffer (car location))
					(goto-char (cdr location))))
687 688
				function
				"mouse-2, RET: find function's definition")))))
689 690 691
    (if need-close (princ ")"))
    (princ ".")
    (terpri)
692 693 694 695 696 697 698 699 700
    ;; Handle symbols aliased to other symbols.
    (setq def (indirect-function def))
    ;; If definition is a macro, find the function inside it.
    (if (eq (car-safe def) 'macro)
	(setq def (cdr def)))
    (let ((arglist (cond ((byte-code-function-p def)
			  (car (append def nil)))
			 ((eq (car-safe def) 'lambda)
			  (nth 1 def))
701 702 703 704
			 ((and (eq (car-safe def) 'autoload)
			       (not (eq (nth 4 def) 'keymap)))
			  (concat "[Arg list not available until "
				  "function definition is loaded.]"))
705
			 (t t))))
706 707 708 709 710 711 712 713 714 715 716
      (cond ((listp arglist)
	     (princ (cons (if (symbolp function) function "anonymous")
			  (mapcar (lambda (arg)
				    (if (memq arg '(&optional &rest))
					arg
				      (intern (upcase (symbol-name arg)))))
				  arglist)))
	     (terpri))
	    ((stringp arglist)
	     (princ arglist)
	     (terpri))))
717 718 719 720
    (let ((doc (documentation function)))
      (if doc
	  (progn (terpri)
		 (princ doc)
721 722 723 724 725 726
		 (if (subrp (symbol-function function))
		     (with-current-buffer standard-output
		       (beginning-of-line)
		       ;; Builtins get the calling sequence at the end of
		       ;; the doc string.  Move it to the same place as
		       ;; for other functions.
727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748

		       ;; In cases where `function' has been fset to a
		       ;; subr we can't search for function's name in
		       ;; the doc string.  Kluge round that using the
		       ;; printed representation.  The arg list then
		       ;; shows the wrong function name, but that
		       ;; might be a useful hint.
		       (let* ((rep (prin1-to-string def))
			      (name (progn
				      (string-match " \\([^ ]+\\)>$" rep)
				      (match-string 1 rep))))
			 (if (looking-at (format "(%s[ )]" name))
			     (let ((start (point-marker)))
			       (goto-char (point-min))
			       (forward-paragraph)
			       (insert-buffer-substring (current-buffer) start)
			       (insert ?\n)
			       (delete-region (1- start) (point-max)))
			   (goto-char (point-min))
			   (forward-paragraph)
			   (insert
			    "[Missing arglist.  Please make a bug report.]\n")))
749
		       (goto-char (point-max))))
750 751
		 (help-setup-xref (list #'describe-function function)
				  interactive-p))
752 753
	(princ "not documented")))))

Richard M. Stallman's avatar
Richard M. Stallman committed
754
(defun variable-at-point ()
755 756
  "Return the bound variable symbol found around point.
Return 0 if there is no such symbol."
Richard M. Stallman's avatar
Richard M. Stallman committed
757
  (condition-case ()
758 759 760 761 762 763 764 765 766 767
      (let ((stab (syntax-table)))
	(unwind-protect
	    (save-excursion
	      (set-syntax-table emacs-lisp-mode-syntax-table)
	      (or (not (zerop (skip-syntax-backward "_w")))
		  (eq (char-syntax (following-char)) ?w)
		  (eq (char-syntax (following-char)) ?_)
		  (forward-sexp -1))
	      (skip-chars-forward "'")
	      (let ((obj (read (current-buffer))))
768 769
		(or (and (symbolp obj) (boundp obj) obj)
		    0)))
770
	  (set-syntax-table stab)))
771
    (error 0)))
Richard M. Stallman's avatar
Richard M. Stallman committed
772

773 774 775 776 777 778 779 780 781 782 783 784 785 786 787
(defun help-xref-on-pp (from to)
  "Add xrefs for symbols in `pp's output between FROM and TO."
  (let ((ost (syntax-table)))
    (unwind-protect
	(save-excursion
	  (save-restriction
	    (set-syntax-table emacs-lisp-mode-syntax-table)
	    (narrow-to-region from to)
	    (goto-char (point-min))
	    (while (not (eobp))
	      (cond
	       ((looking-at "\"") (forward-sexp 1))
	       ((looking-at "#<") (search-forward ">" nil 'move))
	       ((looking-at "\\(\\(\\sw\\|\\s_\\)+\\)")
		(let* ((sym (intern-soft
788 789
			     (buffer-substring (match-beginning 1)
					       (match-end 1))))
790
		       (fn (cond ((fboundp sym) #'describe-function)
791 792 793 794 795
				 ((or (memq sym '(t nil))
				      (keywordp sym))
				  nil)
				 ((and sym (boundp sym))
				  #'describe-variable))))
796 797 798 799 800
		  (when fn (help-xref-button 1 fn sym)))
		(goto-char (match-end 1)))
	       (t (forward-char 1))))))
      (set-syntax-table ost))))

Richard M. Stallman's avatar
Richard M. Stallman committed
801 802 803 804 805 806 807
(defun describe-variable (variable)
  "Display the full documentation of VARIABLE (a symbol).
Returns the documentation as a string, also."
  (interactive 
   (let ((v (variable-at-point))
	 (enable-recursive-minibuffers t)
	 val)
808
     (setq val (completing-read (if (symbolp v)
809 810
				    (format
				     "Describe variable (default %s): " v)
Richard M. Stallman's avatar
Richard M. Stallman committed
811
				  "Describe variable: ")
812 813
				obarray 'boundp t nil nil
				(if (symbolp v) (symbol-name v))))
Richard M. Stallman's avatar
Richard M. Stallman committed
814 815
     (list (if (equal val "")
	       v (intern val)))))
816
  (if (symbolp variable)
817 818 819 820 821 822 823
      (let (valvoid)
	(with-output-to-temp-buffer "*Help*"
	  (prin1 variable)
	  (if (not (boundp variable))
	      (progn
		(princ " is void")
		(setq valvoid t))
824 825 826 827 828 829 830 831
	    (let ((val (symbol-value variable)))
	      (with-current-buffer standard-output
		(princ "'s value is ")
		(terpri)
		(let ((from (point)))
		  (pp val)
		  (help-xref-on-pp from (point))))))
	  (terpri)
832 833 834 835 836
	  (if (local-variable-p variable)
	      (progn
		(princ (format "Local in buffer %s; " (buffer-name)))
		(if (not (default-boundp variable))
		    (princ "globally void")
837 838 839 840 841 842 843
		  (let ((val (default-value variable)))
		    (with-current-buffer standard-output
		      (princ "global value is ")
		      (terpri)
		      (let ((from (point)))
			(pp val)
			(help-xref-on-pp from (point))))))
844 845 846 847 848 849
		(terpri)))
	  (terpri)
	  (save-current-buffer
	    (set-buffer standard-output)
	    (if (> (count-lines (point-min) (point-max)) 10)
		(progn
850
		  (set-syntax-table emacs-lisp-mode-syntax-table)
851 852 853 854 855 856 857 858 859 860 861 862
		  (goto-char (point-min))
		  (if valvoid
		      (forward-line 1)
		    (forward-sexp 1)
		    (delete-region (point) (progn (end-of-line) (point)))
		    (insert "'s value is shown below.\n\n")
		    (save-excursion
		      (insert "\n\nValue:"))))))
	  (princ "Documentation:")
	  (terpri)
	  (let ((doc (documentation-property variable 'variable-documentation)))
	    (princ (or doc "not documented as a variable.")))
863
          (help-setup-xref (list #'describe-variable variable) (interactive-p))
864 865

	  ;; Make a link to customize if this variable can be customized.
866
	  ;; Note, it is not reliable to test only for a custom-type property
Richard M. Stallman's avatar
Richard M. Stallman committed
867 868
	  ;; because those are only present after the var's definition
	  ;; has been loaded.
869 870 871
	  (if (or (get variable 'custom-type) ; after defcustom
		  (get variable 'custom-loads) ; from loaddefs.el
		  (get variable 'standard-value)) ; from cus-start.el
872 873 874 875 876 877 878 879
	      (let ((customize-label "customize"))
		(terpri)
		(terpri)
		(princ (concat "You can " customize-label " this variable."))
		(with-current-buffer "*Help*"
		  (save-excursion
		    (re-search-backward 
		     (concat "\\(" customize-label "\\)") nil t)
880 881 882 883
		    (help-xref-button 1 (lambda (v)
					  (if help-xref-stack
					      (pop help-xref-stack))
					  (customize-variable v))
884
				      variable
885
				      "mouse-2, RET: customize variable")))))
886 887 888
	  ;; Make a hyperlink to the library if appropriate.  (Don't
	  ;; change the format of the buffer's initial line in case
	  ;; anything expects the current format.)
889
	  (let ((file-name (symbol-file variable)))
890
	    (when file-name
891
	      (princ "\n\nDefined in `")
892
	      (princ file-name)
893
	      (princ "'.")
894 895
	      (with-current-buffer "*Help*"
		(save-excursion
896
		  (re-search-backward "`\\([^`']+\\)'" nil t)
897 898 899 900 901 902 903
		  (help-xref-button
		   1 (lambda (arg)
		       (let ((location
			      (find-variable-noselect arg)))
			 (pop-to-buffer (car location))
			 (goto-char (cdr location))))
		   variable "mouse-2, RET: find variable's definition")))))
904

905 906 907 908 909
	  (print-help-return-message)
	  (save-excursion
	    (set-buffer standard-output)
	    ;; Return the text we displayed.
	    (buffer-string))))
910
    (message "You did not specify a variable")))
Richard M. Stallman's avatar
Richard M. Stallman committed
911

912
(defun describe-bindings (&optional prefix buffer)
913 914 915 916
  "Show a list of all defined keys, and their definitions.
We put that list in a buffer, and display the buffer.

The optional argument PREFIX, if non-nil, should be a key sequence;
917 918 919
then we display only bindings that start with that prefix.
The optional argument BUFFER specifies which buffer's bindings
to display (default, the current buffer)."
920
  (interactive "P")
921 922 923
  (or buffer (setq buffer (current-buffer)))
  (with-current-buffer buffer
    (describe-bindings-internal nil prefix))
924
  (with-current-buffer "*Help*"
925 926
    (help-setup-xref (list #'describe-bindings prefix buffer)
		     (interactive-p))))
927

928
(defun where-is (definition &optional insert)
929
  "Print message listing key sequences that invoke the command DEFINITION.
930 931
Argument is a command definition, usually a symbol with a function definition.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
932 933 934 935 936 937 938
  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Where is command (default %s): " fn)
				  "Where is command: ")
939
				obarray 'commandp t))
940
     (list (if (equal val "")
941 942
	       fn (intern val))
	   current-prefix-arg)))
943
  (let* ((keys (where-is-internal definition overriding-local-map nil nil))
944 945 946 947 948 949 950 951 952
	 (keys1 (mapconcat 'key-description keys ", "))
	 (standard-output (if insert (current-buffer) t)))
    (if insert
	(if (> (length keys1) 0)
	    (princ (format "%s (%s)" keys1 definition))
	  (princ (format "M-x %s RET" definition)))
      (if (> (length keys1) 0)
	  (princ (format "%s is on %s" definition keys1))
	(princ (format "%s is not on any key" definition)))))
953 954
  nil)

955
(defun locate-library (library &optional nosuffix path interactive-call)
956
  "Show the precise file name of Emacs library LIBRARY.
Richard M. Stallman's avatar
Richard M. Stallman committed
957 958 959
This command searches the directories in `load-path' like `M-x load-library'
to find the file that `M-x load-library RET LIBRARY RET' would load.
Optional second arg NOSUFFIX non-nil means don't add suffixes `.elc' or `.el'
960 961 962
to the specified name LIBRARY.

If the optional third arg PATH is specified, that list of directories
963 964 965 966 967
is used instead of `load-path'.

When called from a program, the file name is normaly returned as a
string.  When run interactively, the argument INTERACTIVE-CALL is t,
and the file name is displayed in the echo area."
968 969 970
  (interactive (list (read-string "Locate library: ")
		     nil nil
		     t))
971
  (let (result)
972
    (catch 'answer
Dave Love's avatar
Dave Love committed
973
      (mapc
974
       (lambda (dir)
Dave Love's avatar
Dave Love committed
975
	 (mapc
976 977 978 979 980 981 982 983 984
	  (lambda (suf)
	    (let ((try (expand-file-name (concat library suf) dir)))
	      (and (file-readable-p try)
		   (null (file-directory-p try))
		   (progn
		     (setq result try)
		     (throw 'answer try)))))
	  (if nosuffix
	      '("")
985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000
	    '(".elc" ".el" "")
;;; load doesn't handle this yet.
;;;	    (let ((basic '(".elc" ".el" ""))
;;;		  (compressed '(".Z" ".gz" "")))
;;;	      ;; If autocompression mode is on,
;;;	      ;; consider all combinations of library suffixes
;;;	      ;; and compression suffixes.
;;;	      (if (rassq 'jka-compr-handler file-name-handler-alist)
;;;		  (apply 'nconc
;;;			 (mapcar (lambda (compelt)
;;;				   (mapcar (lambda (baselt)
;;;					     (concat baselt compelt))
;;;					   basic))
;;;				 compressed))
;;;		basic))
	    )))
1001 1002 1003 1004 1005 1006
       (or path load-path)))
    (and interactive-call
	 (if result
	     (message "Library is file %s" result)
	   (message "No library %s in search path" library)))
    result))
Eric S. Raymond's avatar
Eric S. Raymond committed
1007

1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018

;;; Grokking cross-reference information in doc strings and
;;; hyperlinking it.

;; This may have some scope for extension and the same or something
;; similar should be done for widget doc strings, which currently use
;; another mechanism.

(defcustom help-highlight-p t
  "*If non-nil, `help-make-xrefs' highlight cross-references.
Under a window system it highlights them with face defined by
Dave Love's avatar
Dave Love committed
1019
`help-highlight-face'."
1020 1021 1022 1023 1024 1025 1026 1027 1028
 :group 'help
 :version "20.3"
 :type 'boolean)

(defcustom help-highlight-face 'underline
  "Face used by `help-make-xrefs' to highlight cross-references.
Must be previously-defined."
  :group 'help
  :version "20.3"
1029
  :type 'face)
1030

1031
(defvar help-back-label (purecopy "[back]")
1032 1033
  "Label to use by `help-make-xrefs' for the go-back reference.")

1034 1035 1036
(defconst help-xref-symbol-regexp
  (purecopy (concat "\\(\\<\\(\\(variable\\|option\\)\\|"
		    "\\(function\\|command\\)\\|"
1037
		    "\\(face\\)\\|"
1038 1039 1040
		    "\\(symbol\\)\\)\\s-+\\)?"
		    ;; Note starting with word-syntax character:
		    "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'"))
1041 1042 1043 1044 1045
  "Regexp matching doc string references to symbols.

The words preceding the quoted symbol can be used in doc strings to
distinguish references to variables, functions and symbols.")

1046 1047
(defconst help-xref-info-regexp
  (purecopy "\\<[Ii]nfo[ \t\n]+node[ \t\n]+`\\([^']+\\)'")
1048 1049 1050 1051 1052
  "Regexp matching doc string references to an Info node.")

(defun help-setup-xref (item interactive-p)
  "Invoked from commands using the \"*Help*\" buffer to install some xref info.

1053
ITEM is a (FUNCTION . ARGS) pair appropriate for recreating the help
1054 1055 1056 1057 1058 1059 1060
buffer after following a reference.  INTERACTIVE-P is non-nil if the
calling command was invoked interactively.  In this case the stack of
items for help buffer \"back\" buttons is cleared."
  (if interactive-p
      (setq help-xref-stack nil))
  (setq help-xref-stack-item item))

Dave Love's avatar
Dave Love committed
1061 1062 1063
(defvar help-xref-following nil
  "Non-nil when following a help cross-reference.")

1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091
(defun help-make-xrefs (&optional buffer)
  "Parse and hyperlink documentation cross-references in the given BUFFER.

Find cross-reference information in a buffer and, if
`help-highlight-p' is non-nil, highlight it with face defined by
`help-highlight-face'; activate such cross references for selection
with `help-follow'.  Cross-references have the canonical form `...'
and the type of reference may be disambiguated by the preceding
word(s) used in `help-xref-symbol-regexp'.

A special reference `back' is made to return back through a stack of
help buffers.  Variable `help-back-label' specifies the text for
that."
  (interactive "b")
  (save-excursion
    (set-buffer (or buffer (current-buffer)))
    (goto-char (point-min))
    ;; Skip the header-type info, though it might be useful to parse
    ;; it at some stage (e.g. "function in `library'").
    (forward-paragraph)
    (let ((old-modified (buffer-modified-p)))
      (let ((stab (syntax-table))
            (case-fold-search t)
            (inhibit-read-only t))
        (set-syntax-table emacs-lisp-mode-syntax-table)
        ;; The following should probably be abstracted out.
        (unwind-protect
            (progn