lisp/help-mode.el

   1 ;;; help-mode.el --- `help-mode' used by *Help* buffers
   2
   3 ;; Copyright (C) 1985, 1986, 1993, 1994, 1998, 1999, 2000, 2001
   4 ;;   Free Software Foundation, Inc.
   5
   6 ;; Maintainer: FSF
   7 ;; Keywords: help, internal
   8
   9 ;; This file is part of GNU Emacs.
  10
  11 ;; GNU Emacs is free software; you can redistribute it and/or modify
  12 ;; it under the terms of the GNU General Public License as published by
  13 ;; the Free Software Foundation; either version 2, or (at your option)
  14 ;; any later version.
  15
  16 ;; GNU Emacs is distributed in the hope that it will be useful,
  17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 ;; GNU General Public License for more details.
  20
  21 ;; You should have received a copy of the GNU General Public License
  22 ;; along with GNU Emacs; see the file COPYING.  If not, write to the
  23 ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  24 ;; Boston, MA 02111-1307, USA.
  25
  26 ;;; Commentary:
  27
  28 ;; Defines `help-mode', which is the mode used by *Help* buffers, and
  29 ;; associated support machinery, such as adding hyperlinks, etc.,
  30
  31 ;;; Code:
  32
  33 (require 'button)
  34 (eval-when-compile (require 'view))
  35
  36 (defvar help-mode-map (make-sparse-keymap)
  37   "Keymap for help mode.")
  38
  39 (set-keymap-parent help-mode-map button-buffer-map)
  40
  41 (define-key help-mode-map [mouse-2] 'help-follow-mouse)
  42 (define-key help-mode-map "\C-c\C-b" 'help-go-back)
  43 (define-key help-mode-map "\C-c\C-c" 'help-follow)
  44 ;; Documentation only, since we use minor-mode-overriding-map-alist.
  45 (define-key help-mode-map "\r" 'help-follow)
  46
  47 (defvar help-xref-stack nil
  48   "A stack of ways by which to return to help buffers after following xrefs.
  49 Used by `help-follow' and `help-xref-go-back'.
  50 An element looks like (POSITION FUNCTION ARGS...).
  51 To use the element, do (apply FUNCTION ARGS) then goto the point.")
  52 (put 'help-xref-stack 'permanent-local t)
  53 (make-variable-buffer-local 'help-xref-stack)
  54
  55 (defvar help-xref-stack-item nil
  56   "An item for `help-follow' in this buffer to push onto `help-xref-stack'.
  57 The format is (FUNCTION ARGS...).")
  58 (put 'help-xref-stack-item 'permanent-local t)
  59 (make-variable-buffer-local 'help-xref-stack-item)
  60
  61 (setq-default help-xref-stack nil help-xref-stack-item nil)
  62
  63 (defcustom help-mode-hook nil
  64   "Hook run by `help-mode'."
  65   :type 'hook
  66   :group 'help)
  67 \f
  68 ;; Button types used by help
  69
  70 (define-button-type 'help-xref
  71   'action #'help-button-action)
  72
  73 (defun help-button-action (button)
  74   "Call BUTTON's help function."
  75   (help-do-xref (button-start button)
  76                 (button-get button 'help-function)
  77                 (button-get button 'help-args)))
  78
  79 ;; These 6 calls to define-button-type were generated in a dolist
  80 ;; loop, but that is bad because it means these button types don't
  81 ;; have an easily found definition.
  82
  83 (define-button-type 'help-function
  84   :supertype 'help-xref
  85   'help-function 'describe-function
  86   'help-echo (purecopy "mouse-2, RET: describe this function"))
  87
  88 (define-button-type 'help-variable
  89   :supertype 'help-xref
  90   'help-function 'describe-variable
  91   'help-echo (purecopy "mouse-2, RET: describe this variable"))
  92
  93 (define-button-type 'help-face
  94   :supertype 'help-xref
  95   'help-function 'describe-face
  96   'help-echo (purecopy "mouse-2, RET: describe this face"))
  97
  98 (define-button-type 'help-coding-system
  99   :supertype 'help-xref
 100   'help-function 'describe-coding-system
 101   'help-echo (purecopy "mouse-2, RET: describe this coding system"))
 102
 103 (define-button-type 'help-input-method
 104   :supertype 'help-xref
 105   'help-function 'describe-input-method
 106   'help-echo (purecopy "mouse-2, RET: describe this input method"))
 107
 108 (define-button-type 'help-character-set
 109   :supertype 'help-xref
 110   'help-function 'describe-character-set
 111   'help-echo (purecopy "mouse-2, RET: describe this character set"))
 112
 113 ;; make some more ideosyncratic button types
 114
 115 (define-button-type 'help-symbol
 116   :supertype 'help-xref
 117   'help-function #'help-xref-interned
 118   'help-echo (purecopy "mouse-2, RET: describe this symbol"))
 119
 120 (define-button-type 'help-back
 121   :supertype 'help-xref
 122   'help-function #'help-xref-go-back
 123   'help-echo (purecopy "mouse-2, RET: go back to previous help buffer"))
 124
 125 (define-button-type 'help-info
 126   :supertype 'help-xref
 127   'help-function #'info
 128   'help-echo (purecopy"mouse-2, RET: read this Info node"))
 129
 130 (define-button-type 'help-customize-variable
 131   :supertype 'help-xref
 132   'help-function (lambda (v)
 133                    (if help-xref-stack
 134                        (pop help-xref-stack))
 135                    (customize-variable v))
 136   'help-echo (purecopy "mouse-2, RET: customize variable"))
 137
 138 (define-button-type 'help-customize-face
 139   :supertype 'help-xref
 140   'help-function (lambda (v)
 141                    (if help-xref-stack
 142                        (pop help-xref-stack))
 143                    (customize-face v))
 144   'help-echo (purecopy "mouse-2, RET: customize face"))
 145
 146 (define-button-type 'help-function-def
 147   :supertype 'help-xref
 148   'help-function (lambda (fun file)
 149                    (require 'find-func)
 150                   ;; Don't use find-function-noselect because it follows
 151                    ;; aliases (which fails for built-in functions).
 152                    (let* ((location (find-function-search-for-symbol
 153                                      fun nil file)))
 154                      (pop-to-buffer (car location))
 155                      (goto-char (cdr location))))
 156   'help-echo (purecopy "mouse-2, RET: find function's definition"))
 157
 158 (define-button-type 'help-variable-def
 159   :supertype 'help-xref
 160   'help-function (lambda (var &optional file)
 161                    (let ((location
 162                           (find-variable-noselect var file)))
 163                      (pop-to-buffer (car location))
 164                      (goto-char (cdr location))))
 165   'help-echo (purecopy"mouse-2, RET: find variable's definition"))
 166
 167 \f
 168 ;;;###autoload
 169 (defun help-mode ()
 170   "Major mode for viewing help text and navigating references in it.
 171 Entry to this mode runs the normal hook `help-mode-hook'.
 172 Commands:
 173 \\{help-mode-map}"
 174   (interactive)
 175   (kill-all-local-variables)
 176   (use-local-map help-mode-map)
 177   (setq mode-name "Help")
 178   (setq major-mode 'help-mode)
 179   (make-local-variable 'font-lock-defaults)
 180   (setq font-lock-defaults nil)         ; font-lock would defeat xref
 181   (view-mode)
 182   (make-local-variable 'view-no-disable-on-exit)
 183   (setq view-no-disable-on-exit t)
 184   (run-hooks 'help-mode-hook))
 185
 186 ;;;###autoload
 187 (defun help-mode-setup ()
 188   (help-mode)
 189   (setq buffer-read-only nil))
 190
 191 ;;;###autoload
 192 (defun help-mode-finish ()
 193   (when (eq major-mode 'help-mode)
 194     ;; View mode's read-only status of existing *Help* buffer is lost
 195     ;; by with-output-to-temp-buffer.
 196     (toggle-read-only 1)
 197     (help-make-xrefs (current-buffer)))
 198   (setq view-return-to-alist
 199         (list (cons (selected-window) help-return-method))))
 200
 201 \f
 202 ;;; Grokking cross-reference information in doc strings and
 203 ;;; hyperlinking it.
 204
 205 ;; This may have some scope for extension and the same or something
 206 ;; similar should be done for widget doc strings, which currently use
 207 ;; another mechanism.
 208
 209 (defcustom help-highlight-p t
 210   "*If non-nil, `help-make-xrefs' highlight cross-references.
 211 Under a window system it highlights them with face defined by
 212 `help-highlight-face'."
 213  :group 'help
 214  :version "20.3"
 215  :type 'boolean)
 216
 217 (defcustom help-highlight-face 'underline
 218   "Face used by `help-make-xrefs' to highlight cross-references.
 219 Must be previously-defined."
 220   :group 'help
 221   :version "20.3"
 222   :type 'face)
 223
 224 (defvar help-back-label (purecopy "[back]")
 225   "Label to use by `help-make-xrefs' for the go-back reference.")
 226
 227 (defconst help-xref-symbol-regexp
 228   (purecopy (concat "\\(\\<\\(\\(variable\\|option\\)\\|"
 229                     "\\(function\\|command\\)\\|"
 230                     "\\(face\\)\\|"
 231                     "\\(symbol\\)\\|"
 232                     "\\(source \\(?:code \\)?\\(?:of\\|for\\)\\)\\)\\s-+\\)?"
 233                     ;; Note starting with word-syntax character:
 234                     "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'"))
 235   "Regexp matching doc string references to symbols.
 236
 237 The words preceding the quoted symbol can be used in doc strings to
 238 distinguish references to variables, functions and symbols.")
 239
 240 (defconst help-xref-mule-regexp nil
 241   "Regexp matching doc string references to MULE-related keywords.
 242
 243 It is usually nil, and is temporarily bound to an appropriate regexp
 244 when help commands related to multilingual environment (e.g.,
 245 `describe-coding-system') are invoked.")
 246
 247
 248 (defconst help-xref-info-regexp
 249   (purecopy "\\<[Ii]nfo[ \t\n]+node[ \t\n]+`\\([^']+\\)'")
 250   "Regexp matching doc string references to an Info node.")
 251
 252 ;;;###autoload
 253 (defun help-setup-xref (item interactive-p)
 254   "Invoked from commands using the \"*Help*\" buffer to install some xref info.
 255
 256 ITEM is a (FUNCTION . ARGS) pair appropriate for recreating the help
 257 buffer after following a reference.  INTERACTIVE-P is non-nil if the
 258 calling command was invoked interactively.  In this case the stack of
 259 items for help buffer \"back\" buttons is cleared.
 260
 261 This should be called very early, before the output buffer is cleared,
 262 because we want to record the \"previous\" position of point so we can
 263 restore it properly when going back."
 264   (with-current-buffer (help-buffer)
 265     (if interactive-p
 266         ;; Why do we want to prevent the user from going back ??  -stef
 267         (setq help-xref-stack nil)
 268       (when help-xref-stack-item
 269         (push (cons (point) help-xref-stack-item) help-xref-stack)))
 270     (setq help-xref-stack-item item)))
 271
 272 (defvar help-xref-following nil
 273   "Non-nil when following a help cross-reference.")
 274
 275 (defun help-buffer ()
 276   (buffer-name                          ;for with-output-to-temp-buffer
 277    (if help-xref-following
 278        (current-buffer)
 279      (get-buffer-create "*Help*"))))
 280
 281 ;;;###autoload
 282 (defun help-make-xrefs (&optional buffer)
 283   "Parse and hyperlink documentation cross-references in the given BUFFER.
 284
 285 Find cross-reference information in a buffer and, if
 286 `help-highlight-p' is non-nil, highlight it with face defined by
 287 `help-highlight-face'; activate such cross references for selection
 288 with `help-follow'.  Cross-references have the canonical form `...'
 289 and the type of reference may be disambiguated by the preceding
 290 word(s) used in `help-xref-symbol-regexp'.
 291
 292 If the variable `help-xref-mule-regexp' is non-nil, find also
 293 cross-reference information related to multilingual environment
 294 \(e.g., coding-systems).  This variable is also used to disambiguate
 295 the type of reference as the same way as `help-xref-symbol-regexp'.
 296
 297 A special reference `back' is made to return back through a stack of
 298 help buffers.  Variable `help-back-label' specifies the text for
 299 that."
 300   (interactive "b")
 301   (save-excursion
 302     (set-buffer (or buffer (current-buffer)))
 303     (goto-char (point-min))
 304     ;; Skip the header-type info, though it might be useful to parse
 305     ;; it at some stage (e.g. "function in `library'").
 306     (forward-paragraph)
 307     (let ((old-modified (buffer-modified-p)))
 308       (let ((stab (syntax-table))
 309             (case-fold-search t)
 310             (inhibit-read-only t))
 311         (set-syntax-table emacs-lisp-mode-syntax-table)
 312         ;; The following should probably be abstracted out.
 313         (unwind-protect
 314             (progn
 315               ;; Info references
 316               (save-excursion
 317                 (while (re-search-forward help-xref-info-regexp nil t)
 318                   (let ((data (match-string 1)))
 319                     (save-match-data
 320                       (unless (string-match "^([^)]+)" data)
 321                         (setq data (concat "(emacs)" data))))
 322                     (help-xref-button 1 'help-info data))))
 323               ;; Mule related keywords.  Do this before trying
 324               ;; `help-xref-symbol-regexp' because some of Mule
 325               ;; keywords have variable or function definitions.
 326               (if help-xref-mule-regexp
 327                   (save-excursion
 328                     (while (re-search-forward help-xref-mule-regexp nil t)
 329                       (let* ((data (match-string 7))
 330                              (sym (intern-soft data)))
 331                         (cond
 332                          ((match-string 3) ; coding system
 333                           (and sym (coding-system-p sym)
 334                                (help-xref-button 6 'help-coding-system sym)))
 335                          ((match-string 4) ; input method
 336                           (and (assoc data input-method-alist)
 337                                (help-xref-button 7 'help-input-method data)))
 338                          ((or (match-string 5) (match-string 6)) ; charset
 339                           (and sym (charsetp sym)
 340                                (help-xref-button 7 'help-character-set sym)))
 341                          ((assoc data input-method-alist)
 342                           (help-xref-button 7 'help-character-set data))
 343                          ((and sym (coding-system-p sym))
 344                           (help-xref-button 7 'help-coding-system sym))
 345                          ((and sym (charsetp sym))
 346                           (help-xref-button 7 'help-character-set sym)))))))
 347               ;; Quoted symbols
 348               (save-excursion
 349                 (while (re-search-forward help-xref-symbol-regexp nil t)
 350                   (let* ((data (match-string 8))
 351                          (sym (intern-soft data)))
 352                     (if sym
 353                         (cond
 354                          ((match-string 3) ; `variable' &c
 355                           (and (boundp sym) ; `variable' doesn't ensure
 356                                         ; it's actually bound
 357                                (help-xref-button 8 'help-variable sym)))
 358                          ((match-string 4) ; `function' &c
 359                           (and (fboundp sym) ; similarly
 360                                (help-xref-button 8 'help-function sym)))
 361                          ((match-string 5) ; `face'
 362                           (and (facep sym)
 363                                (help-xref-button 8 'help-face sym)))
 364                          ((match-string 6)) ; nothing for `symbol'
 365                          ((match-string 7)
 366 ;; this used:
 367 ;;                         #'(lambda (arg)
 368 ;;                             (let ((location
 369 ;;                                    (find-function-noselect arg)))
 370 ;;                               (pop-to-buffer (car location))
 371 ;;                               (goto-char (cdr location))))
 372                           (help-xref-button 8 'help-function-def sym))
 373                          ((and (boundp sym) (fboundp sym))
 374                           ;; We can't intuit whether to use the
 375                           ;; variable or function doc -- supply both.
 376                           (help-xref-button 8 'help-symbol sym))
 377                          ((boundp sym)
 378                           (help-xref-button 8 'help-variable sym))
 379                          ((fboundp sym)
 380                           (help-xref-button 8 'help-function sym))
 381                          ((facep sym)
 382                           (help-xref-button 8 'help-face sym)))))))
 383               ;; An obvious case of a key substitution:
 384               (save-excursion
 385                 (while (re-search-forward
 386                         ;; Assume command name is only word characters
 387                         ;; and dashes to get things like `use M-x foo.'.
 388                         "\\<M-x\\s-+\\(\\sw\\(\\sw\\|-\\)+\\)" nil t)
 389                   (let ((sym (intern-soft (match-string 1))))
 390                     (if (fboundp sym)
 391                         (help-xref-button 1 'help-function sym)))))
 392               ;; Look for commands in whole keymap substitutions:
 393               (save-excursion
 394                 ;; Make sure to find the first keymap.
 395                 (goto-char (point-min))
 396                 ;; Find a header and the column at which the command
 397                 ;; name will be found.
 398                 (while (re-search-forward "^key +binding\n\\(-+ +\\)-+\n\n"
 399                                           nil t)
 400                   (let ((col (- (match-end 1) (match-beginning 1))))
 401                     (while
 402                         ;; Ignore single blank lines in table, but not
 403                         ;; double ones, which should terminate it.
 404                         (and (not (looking-at "\n\\s-*\n"))
 405                              (progn
 406                                (and (eolp) (forward-line))
 407                                (end-of-line)
 408                                (skip-chars-backward "^\t\n")
 409                                (if (and (>= (current-column) col)
 410                                         (looking-at "\\(\\sw\\|-\\)+$"))
 411                                    (let ((sym (intern-soft (match-string 0))))
 412                                      (if (fboundp sym)
 413                                          (help-xref-button 0 'help-function sym))))
 414                                (zerop (forward-line)))))))))
 415           (set-syntax-table stab))
 416         ;; Delete extraneous newlines at the end of the docstring
 417         (goto-char (point-max))
 418         (while (and (not (bobp)) (bolp))
 419           (delete-char -1))
 420         ;; Make a back-reference in this buffer if appropriate.
 421         (when help-xref-stack
 422           (insert "\n\n")
 423           (help-insert-xref-button help-back-label 'help-back
 424                                    (current-buffer))))
 425       ;; View mode steals RET from us.
 426       (set (make-local-variable 'minor-mode-overriding-map-alist)
 427            (list (cons 'view-mode
 428                        (let ((map (make-sparse-keymap)))
 429                          (set-keymap-parent map view-mode-map)
 430                          (define-key map "\r" 'help-follow)
 431                          map))))
 432       (set-buffer-modified-p old-modified))))
 433
 434 ;;;###autoload
 435 (defun help-xref-button (match-number type &rest args)
 436   "Make a hyperlink for cross-reference text previously matched.
 437 MATCH-NUMBER is the subexpression of interest in the last matched
 438 regexp.  TYPE is the type of button to use.  Any remaining arguments are
 439 passed to the button's help-function when it is invoked.
 440 See `help-make-xrefs'."
 441   ;; Don't mung properties we've added specially in some instances.
 442   (unless (button-at (match-beginning match-number))
 443     (make-text-button (match-beginning match-number)
 444                       (match-end match-number)
 445                       'type type 'help-args args)))
 446
 447 ;;;###autoload
 448 (defun help-insert-xref-button (string type &rest args)
 449   "Insert STRING and make a hyperlink from cross-reference text on it.
 450 TYPE is the type of button to use.  Any remaining arguments are passed
 451 to the button's help-function when it is invoked.
 452 See `help-make-xrefs'."
 453   (unless (button-at (point))
 454     (insert-text-button string 'type type 'help-args args)))
 455
 456 ;;;###autoload
 457 (defun help-xref-on-pp (from to)
 458   "Add xrefs for symbols in `pp's output between FROM and TO."
 459   (let ((ost (syntax-table)))
 460     (unwind-protect
 461         (save-excursion
 462           (save-restriction
 463             (set-syntax-table emacs-lisp-mode-syntax-table)
 464             (narrow-to-region from to)
 465             (goto-char (point-min))
 466             (condition-case nil
 467                 (while (not (eobp))
 468                   (cond
 469                    ((looking-at "\"") (forward-sexp 1))
 470                    ((looking-at "#<") (search-forward ">" nil 'move))
 471                    ((looking-at "\\(\\(\\sw\\|\\s_\\)+\\)")
 472                     (let* ((sym (intern-soft (match-string 1)))
 473                            (type (cond ((fboundp sym) 'help-function)
 474                                        ((or (memq sym '(t nil))
 475                                             (keywordp sym))
 476                                         nil)
 477                                        ((and sym (boundp sym))
 478                                         'help-variable))))
 479                       (when type (help-xref-button 1 type sym)))
 480                     (goto-char (match-end 1)))
 481                    (t (forward-char 1))))
 482               (error nil))))
 483       (set-syntax-table ost))))
 484
 485 \f
 486 ;; Additional functions for (re-)creating types of help buffers.
 487 (defun help-xref-interned (symbol)
 488   "Follow a hyperlink which appeared to be an arbitrary interned SYMBOL.
 489 Both variable, function and face documentation are extracted into a single
 490 help buffer."
 491   (with-current-buffer (help-buffer)
 492     ;; Push the previous item on the stack before clobbering the output buffer.
 493     (help-setup-xref nil nil)
 494     (let ((facedoc (when (facep symbol)
 495                      ;; Don't record the current entry in the stack.
 496                      (setq help-xref-stack-item nil)
 497                      (describe-face symbol)))
 498           (fdoc (when (fboundp symbol)
 499                   ;; Don't record the current entry in the stack.
 500                   (setq help-xref-stack-item nil)
 501                   (describe-function symbol)))
 502           (sdoc (when (boundp symbol)
 503                   ;; Don't record the current entry in the stack.
 504                   (setq help-xref-stack-item nil)
 505                   (describe-variable symbol))))
 506       (cond
 507        (sdoc
 508         ;; We now have a help buffer on the variable.
 509         ;; Insert the function and face text before it.
 510         (when (or fdoc facedoc)
 511           (goto-char (point-min))
 512           (let ((inhibit-read-only t))
 513             (when fdoc
 514               (insert fdoc "\n\n")
 515               (when facedoc
 516                 (insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
 517                         " is also a " "face." "\n\n")))
 518             (when facedoc
 519               (insert facedoc "\n\n"))
 520             (insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
 521                     " is also a " "variable." "\n\n"))
 522           ;; Don't record the `describe-variable' item in the stack.
 523           (setq help-xref-stack-item nil)
 524           (help-setup-xref (list #'help-xref-interned symbol) nil)))
 525        (fdoc
 526         ;; We now have a help buffer on the function.
 527         ;; Insert face text before it.
 528         (when facedoc
 529           (goto-char (point-max))
 530           (let ((inhibit-read-only t))
 531             (insert "\n\n" (make-string 30 ?-) "\n\n" (symbol-name symbol)
 532                     " is also a " "face." "\n\n" facedoc))
 533           ;; Don't record the `describe-function' item in the stack.
 534           (setq help-xref-stack-item nil)
 535           (help-setup-xref (list #'help-xref-interned symbol) nil)))))))
 536
 537 \f
 538 ;;; Navigation/hyperlinking with xrefs
 539
 540 (defun help-follow-mouse (click)
 541   "Follow the cross-reference that you CLICK on."
 542   (interactive "e")
 543   (let* ((start (event-start click))
 544          (window (car start))
 545          (pos (car (cdr start))))
 546     (with-current-buffer (window-buffer window)
 547       (help-follow pos))))
 548
 549 (defun help-xref-go-back (buffer)
 550   "From BUFFER, go back to previous help buffer text using `help-xref-stack'."
 551   (let (item position method args)
 552     (with-current-buffer buffer
 553       (when help-xref-stack
 554         (setq item (pop help-xref-stack)
 555               ;; Clear the current item so that it won't get pushed
 556               ;; by the function we're about to call.  TODO: We could also
 557               ;; push it onto a "forward" stack and add a `forw' button.
 558               help-xref-stack-item nil
 559               position (car item)
 560               method (cadr item)
 561               args (cddr item))))
 562     (apply method args)
 563     (with-current-buffer buffer
 564       (if (get-buffer-window buffer)
 565           (set-window-point (get-buffer-window buffer) position)
 566         (goto-char position)))))
 567
 568 (defun help-go-back ()
 569   "Invoke the [back] button (if any) in the Help mode buffer."
 570   (interactive)
 571   (let ((back-button (button-at (1- (point-max)))))
 572     (if back-button
 573         (button-activate back-button)
 574       (error "No [back] button"))))
 575
 576 (defun help-do-xref (pos function args)
 577   "Call the help cross-reference function FUNCTION with args ARGS.
 578 Things are set up properly so that the resulting help-buffer has
 579 a proper [back] button."
 580   ;; There is a reference at point.  Follow it.
 581   (let ((help-xref-following t))
 582     (apply function args)))
 583
 584 (defun help-follow (&optional pos)
 585   "Follow cross-reference at POS, defaulting to point.
 586
 587 For the cross-reference format, see `help-make-xrefs'."
 588   (interactive "d")
 589   (unless pos
 590     (setq pos (point)))
 591   (unless (push-button pos)
 592     ;; check if the symbol under point is a function or variable
 593     (let ((sym
 594            (intern
 595             (save-excursion
 596               (goto-char pos) (skip-syntax-backward "w_")
 597               (buffer-substring (point)
 598                                 (progn (skip-syntax-forward "w_")
 599                                        (point)))))))
 600       (when (or (boundp sym) (fboundp sym) (facep sym))
 601         (help-do-xref pos #'help-xref-interned (list sym))))))
 602
 603
 604 (provide 'help-mode)
 605
 606 ;;; help-mode.el ends here