lisp/help-mode.el

   1 ;;; help-mode.el --- `help-mode' used by *Help* buffers
   2
   3 ;; Copyright (C) 1985-1986, 1993-1994, 1998-2011
   4 ;;   Free Software Foundation, Inc.
   5
   6 ;; Maintainer: FSF
   7 ;; Keywords: help, internal
   8 ;; Package: emacs
   9
  10 ;; This file is part of GNU Emacs.
  11
  12 ;; GNU Emacs is free software: you can redistribute it and/or modify
  13 ;; it under the terms of the GNU General Public License as published by
  14 ;; the Free Software Foundation, either version 3 of the License, or
  15 ;; (at your option) any later version.
  16
  17 ;; GNU Emacs is distributed in the hope that it will be useful,
  18 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  19 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20 ;; GNU General Public License for more details.
  21
  22 ;; You should have received a copy of the GNU General Public License
  23 ;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
  24
  25 ;;; Commentary:
  26
  27 ;; Defines `help-mode', which is the mode used by *Help* buffers, and
  28 ;; associated support machinery, such as adding hyperlinks, etc.,
  29
  30 ;;; Code:
  31
  32 (require 'button)
  33 (require 'view)
  34 (eval-when-compile (require 'easymenu))
  35
  36 (defvar help-mode-map
  37   (let ((map (make-sparse-keymap)))
  38     (set-keymap-parent map button-buffer-map)
  39
  40     (define-key map [mouse-2] 'help-follow-mouse)
  41     (define-key map "\C-c\C-b" 'help-go-back)
  42     (define-key map "\C-c\C-f" 'help-go-forward)
  43     (define-key map "\C-c\C-c" 'help-follow-symbol)
  44     ;; Documentation only, since we use minor-mode-overriding-map-alist.
  45     (define-key map "\r" 'help-follow)
  46     map)
  47   "Keymap for help mode.")
  48
  49 (easy-menu-define help-mode-menu help-mode-map
  50   "Menu for Help Mode."
  51   '("Help-Mode"
  52     ["Show Help for Symbol" help-follow-symbol
  53      :help "Show the docs for the symbol at point"]
  54     ["Previous Topic" help-go-back
  55      :help "Go back to previous topic in this help buffer"]
  56     ["Next Topic" help-go-forward
  57      :help "Go back to next topic in this help buffer"]
  58     ["Move to Previous Button" backward-button
  59      :help "Move to the Next Button in the help buffer"]
  60     ["Move to Next Button" forward-button
  61       :help "Move to the Next Button in the help buffer"]))
  62
  63 (defvar help-xref-stack nil
  64   "A stack of ways by which to return to help buffers after following xrefs.
  65 Used by `help-follow' and `help-xref-go-back'.
  66 An element looks like (POSITION FUNCTION ARGS...).
  67 To use the element, do (apply FUNCTION ARGS) then goto the point.")
  68 (put 'help-xref-stack 'permanent-local t)
  69 (make-variable-buffer-local 'help-xref-stack)
  70
  71 (defvar help-xref-forward-stack nil
  72   "A stack used to navigate help forwards after using the back button.
  73 Used by `help-follow' and `help-xref-go-forward'.
  74 An element looks like (POSITION FUNCTION ARGS...).
  75 To use the element, do (apply FUNCTION ARGS) then goto the point.")
  76 (put 'help-xref-forward-stack 'permanent-local t)
  77 (make-variable-buffer-local 'help-xref-forward-stack)
  78
  79 (defvar help-xref-stack-item nil
  80   "An item for `help-follow' in this buffer to push onto `help-xref-stack'.
  81 The format is (FUNCTION ARGS...).")
  82 (put 'help-xref-stack-item 'permanent-local t)
  83 (make-variable-buffer-local 'help-xref-stack-item)
  84
  85 (defvar help-xref-stack-forward-item nil
  86   "An item for `help-go-back' to push onto `help-xref-forward-stack'.
  87 The format is (FUNCTION ARGS...).")
  88 (put 'help-xref-stack-forward-item 'permanent-local t)
  89 (make-variable-buffer-local 'help-xref-stack-forward-item)
  90
  91 (setq-default help-xref-stack nil help-xref-stack-item nil)
  92 (setq-default help-xref-forward-stack nil help-xref-forward-stack-item nil)
  93
  94 (defcustom help-mode-hook nil
  95   "Hook run by `help-mode'."
  96   :type 'hook
  97   :group 'help)
  98 \f
  99 ;; Button types used by help
 100
 101 (define-button-type 'help-xref
 102   'follow-link t
 103   'action #'help-button-action)
 104
 105 (defun help-button-action (button)
 106   "Call BUTTON's help function."
 107   (help-do-xref (button-start button)
 108                 (button-get button 'help-function)
 109                 (button-get button 'help-args)))
 110
 111 ;; These 6 calls to define-button-type were generated in a dolist
 112 ;; loop, but that is bad because it means these button types don't
 113 ;; have an easily found definition.
 114
 115 (define-button-type 'help-function
 116   :supertype 'help-xref
 117   'help-function 'describe-function
 118   'help-echo (purecopy "mouse-2, RET: describe this function"))
 119
 120 (define-button-type 'help-variable
 121   :supertype 'help-xref
 122   'help-function 'describe-variable
 123   'help-echo (purecopy "mouse-2, RET: describe this variable"))
 124
 125 (define-button-type 'help-face
 126   :supertype 'help-xref
 127   'help-function 'describe-face
 128   'help-echo (purecopy "mouse-2, RET: describe this face"))
 129
 130 (define-button-type 'help-coding-system
 131   :supertype 'help-xref
 132   'help-function 'describe-coding-system
 133   'help-echo (purecopy "mouse-2, RET: describe this coding system"))
 134
 135 (define-button-type 'help-input-method
 136   :supertype 'help-xref
 137   'help-function 'describe-input-method
 138   'help-echo (purecopy "mouse-2, RET: describe this input method"))
 139
 140 (define-button-type 'help-character-set
 141   :supertype 'help-xref
 142   'help-function 'describe-character-set
 143   'help-echo (purecopy "mouse-2, RET: describe this character set"))
 144
 145 ;; make some more ideosyncratic button types
 146
 147 (define-button-type 'help-symbol
 148   :supertype 'help-xref
 149   'help-function #'help-xref-interned
 150   'help-echo (purecopy "mouse-2, RET: describe this symbol"))
 151
 152 (define-button-type 'help-back
 153   :supertype 'help-xref
 154   'help-function #'help-xref-go-back
 155   'help-echo (purecopy "mouse-2, RET: go back to previous help buffer"))
 156
 157 (define-button-type 'help-forward
 158   :supertype 'help-xref
 159   'help-function #'help-xref-go-forward
 160   'help-echo (purecopy "mouse-2, RET: move forward to next help buffer"))
 161
 162 (define-button-type 'help-info-variable
 163   :supertype 'help-xref
 164   ;; the name of the variable is put before the argument to Info
 165   'help-function (lambda (_a v) (info v))
 166   'help-echo (purecopy "mouse-2, RET: read this Info node"))
 167
 168 (define-button-type 'help-info
 169   :supertype 'help-xref
 170   'help-function #'info
 171   'help-echo (purecopy "mouse-2, RET: read this Info node"))
 172
 173 (define-button-type 'help-url
 174   :supertype 'help-xref
 175   'help-function #'browse-url
 176   'help-echo (purecopy "mouse-2, RET: view this URL in a browser"))
 177
 178 (define-button-type 'help-customize-variable
 179   :supertype 'help-xref
 180   'help-function (lambda (v)
 181                    (customize-variable v))
 182   'help-echo (purecopy "mouse-2, RET: customize variable"))
 183
 184 (define-button-type 'help-customize-face
 185   :supertype 'help-xref
 186   'help-function (lambda (v)
 187                    (customize-face v))
 188   'help-echo (purecopy "mouse-2, RET: customize face"))
 189
 190 (define-button-type 'help-function-def
 191   :supertype 'help-xref
 192   'help-function (lambda (fun file)
 193                    (require 'find-func)
 194                    (when (eq file 'C-source)
 195                      (setq file
 196                            (help-C-file-name (indirect-function fun) 'fun)))
 197                    ;; Don't use find-function-noselect because it follows
 198                    ;; aliases (which fails for built-in functions).
 199                    (let ((location
 200                           (find-function-search-for-symbol fun nil file)))
 201                      (pop-to-buffer (car location))
 202                      (if (cdr location)
 203                          (goto-char (cdr location))
 204                        (message "Unable to find location in file"))))
 205   'help-echo (purecopy "mouse-2, RET: find function's definition"))
 206
 207 (define-button-type 'help-function-cmacro
 208   :supertype 'help-xref
 209   'help-function (lambda (fun file)
 210                    (setq file (locate-library file t))
 211                    (if (and file (file-readable-p file))
 212                        (progn
 213                          (pop-to-buffer (find-file-noselect file))
 214                          (goto-char (point-min))
 215                          (if (re-search-forward
 216                               (format "^[ \t]*(define-compiler-macro[ \t]+%s"
 217                                       (regexp-quote (symbol-name fun))) nil t)
 218                              (forward-line 0)
 219                            (message "Unable to find location in file")))
 220                      (message "Unable to find file")))
 221   'help-echo (purecopy "mouse-2, RET: find function's compiler macro"))
 222
 223 (define-button-type 'help-variable-def
 224   :supertype 'help-xref
 225   'help-function (lambda (var &optional file)
 226                    (when (eq file 'C-source)
 227                      (setq file (help-C-file-name var 'var)))
 228                    (let ((location (find-variable-noselect var file)))
 229                      (pop-to-buffer (car location))
 230                      (if (cdr location)
 231                        (goto-char (cdr location))
 232                        (message "Unable to find location in file"))))
 233   'help-echo (purecopy "mouse-2, RET: find variable's definition"))
 234
 235 (define-button-type 'help-face-def
 236   :supertype 'help-xref
 237   'help-function (lambda (fun file)
 238                    (require 'find-func)
 239                    ;; Don't use find-function-noselect because it follows
 240                    ;; aliases (which fails for built-in functions).
 241                    (let ((location
 242                           (find-function-search-for-symbol fun 'defface file)))
 243                      (pop-to-buffer (car location))
 244                      (if (cdr location)
 245                          (goto-char (cdr location))
 246                        (message "Unable to find location in file"))))
 247   'help-echo (purecopy "mouse-2, RET: find face's definition"))
 248
 249 (define-button-type 'help-package
 250   :supertype 'help-xref
 251   'help-function 'describe-package
 252   'help-echo (purecopy "mouse-2, RET: Describe package"))
 253
 254 (define-button-type 'help-package-def
 255   :supertype 'help-xref
 256   'help-function (lambda (file) (dired file))
 257   'help-echo (purecopy "mouse-2, RET: visit package directory"))
 258
 259 (define-button-type 'help-theme-def
 260   :supertype 'help-xref
 261   'help-function 'find-file
 262   'help-echo (purecopy "mouse-2, RET: visit theme file"))
 263
 264 (define-button-type 'help-theme-edit
 265   :supertype 'help-xref
 266   'help-function 'customize-create-theme
 267   'help-echo (purecopy "mouse-2, RET: edit this theme file"))
 268 \f
 269 ;;;###autoload
 270 (defun help-mode ()
 271   "Major mode for viewing help text and navigating references in it.
 272 Entry to this mode runs the normal hook `help-mode-hook'.
 273 Commands:
 274 \\{help-mode-map}"
 275   (interactive)
 276   (kill-all-local-variables)
 277   (use-local-map help-mode-map)
 278   (setq mode-name "Help")
 279   (setq major-mode 'help-mode)
 280
 281   (view-mode)
 282   (set (make-local-variable 'view-no-disable-on-exit) t)
 283   ;; With Emacs 22 `view-exit-action' could delete the selected window
 284   ;; disregarding whether the help buffer was shown in that window at
 285   ;; all.  Since `view-exit-action' is called with the help buffer as
 286   ;; argument it seems more appropriate to have it work on the buffer
 287   ;; only and leave it to `view-mode-exit' to delete any associated
 288   ;; window(s).
 289   (setq view-exit-action
 290         (lambda (buffer)
 291           ;; Use `with-current-buffer' to make sure that `bury-buffer'
 292           ;; also removes BUFFER from the selected window.
 293           (with-current-buffer buffer
 294             (bury-buffer))))
 295
 296   (set (make-local-variable 'revert-buffer-function)
 297        'help-mode-revert-buffer)
 298
 299   (run-mode-hooks 'help-mode-hook))
 300
 301 ;;;###autoload
 302 (defun help-mode-setup ()
 303   (help-mode)
 304   (setq buffer-read-only nil))
 305
 306 ;;;###autoload
 307 (defun help-mode-finish ()
 308   (if (eq help-window t)
 309       ;; If `help-window' is t, `view-return-to-alist' is handled by
 310       ;; `with-help-window'.  In this case set `help-window' to the
 311       ;; selected window since now is the only moment where we can
 312       ;; unambiguously identify it.
 313       (setq help-window (selected-window))
 314     (let ((entry (assq (selected-window) view-return-to-alist)))
 315       (if entry
 316           ;; When entering Help mode from the Help window,
 317           ;; such as by following a link, preserve the same
 318           ;; meaning for the q command.
 319           ;; (setcdr entry (cons (selected-window) help-return-method))
 320           nil
 321         (setq view-return-to-alist
 322               (cons (cons (selected-window) help-return-method)
 323                     view-return-to-alist)))))
 324
 325   (when (eq major-mode 'help-mode)
 326     ;; View mode's read-only status of existing *Help* buffer is lost
 327     ;; by with-output-to-temp-buffer.
 328     (toggle-read-only 1)
 329
 330     (save-excursion
 331       (goto-char (point-min))
 332       (let ((inhibit-read-only t))
 333         (when (re-search-forward "^This [^[:space:]]+ is advised.$" nil t)
 334           (put-text-property (match-beginning 0)
 335                              (match-end 0)
 336                              'face 'font-lock-warning-face))))
 337
 338     (help-make-xrefs (current-buffer))))
 339 \f
 340 ;; Grokking cross-reference information in doc strings and
 341 ;; hyperlinking it.
 342
 343 ;; This may have some scope for extension and the same or something
 344 ;; similar should be done for widget doc strings, which currently use
 345 ;; another mechanism.
 346
 347 (defvar help-back-label (purecopy "[back]")
 348   "Label to use by `help-make-xrefs' for the go-back reference.")
 349
 350 (defvar help-forward-label (purecopy "[forward]")
 351   "Label to use by `help-make-xrefs' for the go-forward reference.")
 352
 353 (defconst help-xref-symbol-regexp
 354   (purecopy (concat "\\(\\<\\(\\(variable\\|option\\)\\|"  ; Link to var
 355                     "\\(function\\|command\\)\\|"          ; Link to function
 356                     "\\(face\\)\\|"                        ; Link to face
 357                     "\\(symbol\\|program\\|property\\)\\|" ; Don't link
 358                     "\\(source \\(?:code \\)?\\(?:of\\|for\\)\\)\\)"
 359                     "[ \t\n]+\\)?"
 360                     ;; Note starting with word-syntax character:
 361                     "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'"))
 362   "Regexp matching doc string references to symbols.
 363
 364 The words preceding the quoted symbol can be used in doc strings to
 365 distinguish references to variables, functions and symbols.")
 366
 367 (defvar help-xref-mule-regexp nil
 368   "Regexp matching doc string references to MULE-related keywords.
 369
 370 It is usually nil, and is temporarily bound to an appropriate regexp
 371 when help commands related to multilingual environment (e.g.,
 372 `describe-coding-system') are invoked.")
 373
 374
 375 (defconst help-xref-info-regexp
 376   (purecopy "\\<[Ii]nfo[ \t\n]+\\(node\\|anchor\\)[ \t\n]+`\\([^']+\\)'")
 377   "Regexp matching doc string references to an Info node.")
 378
 379 (defconst help-xref-url-regexp
 380   (purecopy "\\<[Uu][Rr][Ll][ \t\n]+`\\([^']+\\)'")
 381   "Regexp matching doc string references to a URL.")
 382
 383 ;;;###autoload
 384 (defun help-setup-xref (item interactive-p)
 385   "Invoked from commands using the \"*Help*\" buffer to install some xref info.
 386
 387 ITEM is a (FUNCTION . ARGS) pair appropriate for recreating the help
 388 buffer after following a reference.  INTERACTIVE-P is non-nil if the
 389 calling command was invoked interactively.  In this case the stack of
 390 items for help buffer \"back\" buttons is cleared.
 391
 392 This should be called very early, before the output buffer is cleared,
 393 because we want to record the \"previous\" position of point so we can
 394 restore it properly when going back."
 395   (with-current-buffer (help-buffer)
 396     (when help-xref-stack-item
 397       (push (cons (point) help-xref-stack-item) help-xref-stack)
 398       (setq help-xref-forward-stack nil))
 399     (when interactive-p
 400       (let ((tail (nthcdr 10 help-xref-stack)))
 401         ;; Truncate the stack.
 402         (if tail (setcdr tail nil))))
 403     (setq help-xref-stack-item item)))
 404
 405 (defvar help-xref-following nil
 406   "Non-nil when following a help cross-reference.")
 407
 408 ;;;###autoload
 409 (defun help-buffer ()
 410   "Return the name of a buffer for inserting help.
 411 If `help-xref-following' is non-nil, this is the name of the
 412 current buffer.  Signal an error if this buffer is not derived
 413 from `help-mode'.
 414 Otherwise, return \"*Help*\", creating a buffer with that name if
 415 it does not already exist."
 416   (buffer-name                          ;for with-output-to-temp-buffer
 417    (if (not help-xref-following)
 418        (get-buffer-create "*Help*")
 419      (unless (derived-mode-p 'help-mode)
 420        (error "Current buffer is not in Help mode"))
 421      (current-buffer))))
 422
 423 (defvar help-xref-override-view-map
 424   (let ((map (make-sparse-keymap)))
 425     (set-keymap-parent map view-mode-map)
 426     (define-key map "\r" nil)
 427     map)
 428   "Replacement keymap for `view-mode' in help buffers.")
 429
 430 ;;;###autoload
 431 (defun help-make-xrefs (&optional buffer)
 432   "Parse and hyperlink documentation cross-references in the given BUFFER.
 433
 434 Find cross-reference information in a buffer and activate such cross
 435 references for selection with `help-follow'.  Cross-references have
 436 the canonical form `...'  and the type of reference may be
 437 disambiguated by the preceding word(s) used in
 438 `help-xref-symbol-regexp'.  Faces only get cross-referenced if
 439 preceded or followed by the word `face'.  Variables without
 440 variable documentation do not get cross-referenced, unless
 441 preceded by the word `variable' or `option'.
 442
 443 If the variable `help-xref-mule-regexp' is non-nil, find also
 444 cross-reference information related to multilingual environment
 445 \(e.g., coding-systems).  This variable is also used to disambiguate
 446 the type of reference as the same way as `help-xref-symbol-regexp'.
 447
 448 A special reference `back' is made to return back through a stack of
 449 help buffers.  Variable `help-back-label' specifies the text for
 450 that."
 451   (interactive "b")
 452   (with-current-buffer (or buffer (current-buffer))
 453     (save-excursion
 454       (goto-char (point-min))
 455       ;; Skip the header-type info, though it might be useful to parse
 456       ;; it at some stage (e.g. "function in `library'").
 457       (forward-paragraph)
 458       (let ((old-modified (buffer-modified-p)))
 459         (let ((stab (syntax-table))
 460               (case-fold-search t)
 461               (inhibit-read-only t))
 462           (set-syntax-table emacs-lisp-mode-syntax-table)
 463           ;; The following should probably be abstracted out.
 464           (unwind-protect
 465               (progn
 466                 ;; Info references
 467                 (save-excursion
 468                   (while (re-search-forward help-xref-info-regexp nil t)
 469                     (let ((data (match-string 2)))
 470                       (save-match-data
 471                         (unless (string-match "^([^)]+)" data)
 472                           (setq data (concat "(emacs)" data)))
 473                         (setq data ;; possible newlines if para filled
 474                               (replace-regexp-in-string "[ \t\n]+" " " data t t)))
 475                       (help-xref-button 2 'help-info data))))
 476                 ;; URLs
 477                 (save-excursion
 478                   (while (re-search-forward help-xref-url-regexp nil t)
 479                     (let ((data (match-string 1)))
 480                       (help-xref-button 1 'help-url data))))
 481                 ;; Mule related keywords.  Do this before trying
 482                 ;; `help-xref-symbol-regexp' because some of Mule
 483                 ;; keywords have variable or function definitions.
 484                 (if help-xref-mule-regexp
 485                     (save-excursion
 486                       (while (re-search-forward help-xref-mule-regexp nil t)
 487                         (let* ((data (match-string 7))
 488                                (sym (intern-soft data)))
 489                           (cond
 490                            ((match-string 3) ; coding system
 491                             (and sym (coding-system-p sym)
 492                                  (help-xref-button 6 'help-coding-system sym)))
 493                            ((match-string 4) ; input method
 494                             (and (assoc data input-method-alist)
 495                                  (help-xref-button 7 'help-input-method data)))
 496                            ((or (match-string 5) (match-string 6)) ; charset
 497                             (and sym (charsetp sym)
 498                                  (help-xref-button 7 'help-character-set sym)))
 499                            ((assoc data input-method-alist)
 500                             (help-xref-button 7 'help-character-set data))
 501                            ((and sym (coding-system-p sym))
 502                             (help-xref-button 7 'help-coding-system sym))
 503                            ((and sym (charsetp sym))
 504                             (help-xref-button 7 'help-character-set sym)))))))
 505                 ;; Quoted symbols
 506                 (save-excursion
 507                   (while (re-search-forward help-xref-symbol-regexp nil t)
 508                     (let* ((data (match-string 8))
 509                            (sym (intern-soft data)))
 510                       (if sym
 511                           (cond
 512                            ((match-string 3) ; `variable' &c
 513                             (and (or (boundp sym) ; `variable' doesn't ensure
 514                                         ; it's actually bound
 515                                      (get sym 'variable-documentation))
 516                                  (help-xref-button 8 'help-variable sym)))
 517                            ((match-string 4) ; `function' &c
 518                             (and (fboundp sym) ; similarly
 519                                  (help-xref-button 8 'help-function sym)))
 520                            ((match-string 5) ; `face'
 521                             (and (facep sym)
 522                                  (help-xref-button 8 'help-face sym)))
 523                            ((match-string 6)) ; nothing for `symbol'
 524                            ((match-string 7)
 525                             ;; this used:
 526                             ;; #'(lambda (arg)
 527                             ;;     (let ((location
 528                             ;;            (find-function-noselect arg)))
 529                             ;;       (pop-to-buffer (car location))
 530                             ;;  (goto-char (cdr location))))
 531                             (help-xref-button 8 'help-function-def sym))
 532                            ((and
 533                              (facep sym)
 534                              (save-match-data (looking-at "[ \t\n]+face\\W")))
 535                             (help-xref-button 8 'help-face sym))
 536                            ((and (or (boundp sym)
 537                                      (get sym 'variable-documentation))
 538                                  (fboundp sym))
 539                             ;; We can't intuit whether to use the
 540                             ;; variable or function doc -- supply both.
 541                             (help-xref-button 8 'help-symbol sym))
 542                            ((and
 543                              (or (boundp sym)
 544                                  (get sym 'variable-documentation))
 545                              (or
 546                               (documentation-property
 547                                sym 'variable-documentation)
 548                               (condition-case nil
 549                                   (documentation-property
 550                                    (indirect-variable sym)
 551                                    'variable-documentation)
 552                                 (cyclic-variable-indirection nil))))
 553                             (help-xref-button 8 'help-variable sym))
 554                            ((fboundp sym)
 555                             (help-xref-button 8 'help-function sym)))))))
 556                 ;; An obvious case of a key substitution:
 557                 (save-excursion
 558                   (while (re-search-forward
 559                           ;; Assume command name is only word and symbol
 560                           ;; characters to get things like `use M-x foo->bar'.
 561                           ;; Command required to end with word constituent
 562                           ;; to avoid `.' at end of a sentence.
 563                           "\\<M-x\\s-+\\(\\sw\\(\\sw\\|\\s_\\)*\\sw\\)" nil t)
 564                     (let ((sym (intern-soft (match-string 1))))
 565                       (if (fboundp sym)
 566                           (help-xref-button 1 'help-function sym)))))
 567                 ;; Look for commands in whole keymap substitutions:
 568                 (save-excursion
 569                   ;; Make sure to find the first keymap.
 570                   (goto-char (point-min))
 571                   ;; Find a header and the column at which the command
 572                   ;; name will be found.
 573
 574                   ;; If the keymap substitution isn't the last thing in
 575                   ;; the doc string, and if there is anything on the same
 576                   ;; line after it, this code won't recognize the end of it.
 577                   (while (re-search-forward "^key +binding\n\\(-+ +\\)-+\n\n"
 578                                             nil t)
 579                     (let ((col (- (match-end 1) (match-beginning 1))))
 580                       (while
 581                           (and (not (eobp))
 582                                ;; Stop at a pair of blank lines.
 583                                (not (looking-at "\n\\s-*\n")))
 584                         ;; Skip a single blank line.
 585                         (and (eolp) (forward-line))
 586                         (end-of-line)
 587                         (skip-chars-backward "^ \t\n")
 588                         (if (and (>= (current-column) col)
 589                                  (looking-at "\\(\\sw\\|\\s_\\)+$"))
 590                             (let ((sym (intern-soft (match-string 0))))
 591                               (if (fboundp sym)
 592                                   (help-xref-button 0 'help-function sym))))
 593                         (forward-line))))))
 594             (set-syntax-table stab))
 595           ;; Delete extraneous newlines at the end of the docstring
 596           (goto-char (point-max))
 597           (while (and (not (bobp)) (bolp))
 598             (delete-char -1))
 599           (insert "\n")
 600           (when (or help-xref-stack help-xref-forward-stack)
 601             (insert "\n"))
 602           ;; Make a back-reference in this buffer if appropriate.
 603           (when help-xref-stack
 604             (help-insert-xref-button help-back-label 'help-back
 605                                      (current-buffer)))
 606           ;; Make a forward-reference in this buffer if appropriate.
 607           (when help-xref-forward-stack
 608             (when help-xref-stack
 609               (insert "\t"))
 610             (help-insert-xref-button help-forward-label 'help-forward
 611                                      (current-buffer)))
 612           (when (or help-xref-stack help-xref-forward-stack)
 613             (insert "\n")))
 614         ;; View mode steals RET from us.
 615         (set (make-local-variable 'minor-mode-overriding-map-alist)
 616              (list (cons 'view-mode help-xref-override-view-map)))
 617         (set-buffer-modified-p old-modified)))))
 618
 619 ;;;###autoload
 620 (defun help-xref-button (match-number type &rest args)
 621   "Make a hyperlink for cross-reference text previously matched.
 622 MATCH-NUMBER is the subexpression of interest in the last matched
 623 regexp.  TYPE is the type of button to use.  Any remaining arguments are
 624 passed to the button's help-function when it is invoked.
 625 See `help-make-xrefs'."
 626   ;; Don't mung properties we've added specially in some instances.
 627   (unless (button-at (match-beginning match-number))
 628     (make-text-button (match-beginning match-number)
 629                       (match-end match-number)
 630                       'type type 'help-args args)))
 631
 632 ;;;###autoload
 633 (defun help-insert-xref-button (string type &rest args)
 634   "Insert STRING and make a hyperlink from cross-reference text on it.
 635 TYPE is the type of button to use.  Any remaining arguments are passed
 636 to the button's help-function when it is invoked.
 637 See `help-make-xrefs'."
 638   (unless (button-at (point))
 639     (insert-text-button string 'type type 'help-args args)))
 640
 641 ;;;###autoload
 642 (defun help-xref-on-pp (from to)
 643   "Add xrefs for symbols in `pp's output between FROM and TO."
 644   (if (> (- to from) 5000) nil
 645     (with-syntax-table emacs-lisp-mode-syntax-table
 646       (save-excursion
 647         (save-restriction
 648           (narrow-to-region from to)
 649           (goto-char (point-min))
 650           (condition-case nil
 651               (while (not (eobp))
 652                 (cond
 653                  ((looking-at "\"") (forward-sexp 1))
 654                  ((looking-at "#<") (search-forward ">" nil 'move))
 655                  ((looking-at "\\(\\(\\sw\\|\\s_\\)+\\)")
 656                   (let* ((sym (intern-soft (match-string 1)))
 657                          (type (cond ((fboundp sym) 'help-function)
 658                                      ((or (memq sym '(t nil))
 659                                           (keywordp sym))
 660                                       nil)
 661                                      ((and sym
 662                                            (or (boundp sym)
 663                                                (get sym
 664                                                     'variable-documentation)))
 665                                       'help-variable))))
 666                     (when type (help-xref-button 1 type sym)))
 667                   (goto-char (match-end 1)))
 668                  (t (forward-char 1))))
 669             (error nil)))))))
 670
 671 \f
 672 ;; Additional functions for (re-)creating types of help buffers.
 673 (defun help-xref-interned (symbol)
 674   "Follow a hyperlink which appeared to be an arbitrary interned SYMBOL.
 675 Both variable, function and face documentation are extracted into a single
 676 help buffer."
 677   (with-current-buffer (help-buffer)
 678     ;; Push the previous item on the stack before clobbering the output buffer.
 679     (help-setup-xref nil nil)
 680     (let ((facedoc (when (facep symbol)
 681                      ;; Don't record the current entry in the stack.
 682                      (setq help-xref-stack-item nil)
 683                      (describe-face symbol)))
 684           (fdoc (when (fboundp symbol)
 685                   ;; Don't record the current entry in the stack.
 686                   (setq help-xref-stack-item nil)
 687                   (describe-function symbol)))
 688           (sdoc (when (or (boundp symbol)
 689                           (get symbol 'variable-documentation))
 690                   ;; Don't record the current entry in the stack.
 691                   (setq help-xref-stack-item nil)
 692                   (describe-variable symbol))))
 693       (cond
 694        (sdoc
 695         ;; We now have a help buffer on the variable.
 696         ;; Insert the function and face text before it.
 697         (when (or fdoc facedoc)
 698           (goto-char (point-min))
 699           (let ((inhibit-read-only t))
 700             (when fdoc
 701               (insert fdoc "\n\n")
 702               (when facedoc
 703                 (insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
 704                         " is also a " "face." "\n\n")))
 705             (when facedoc
 706               (insert facedoc "\n\n"))
 707             (insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
 708                     " is also a " "variable." "\n\n"))
 709           ;; Don't record the `describe-variable' item in the stack.
 710           (setq help-xref-stack-item nil)
 711           (help-setup-xref (list #'help-xref-interned symbol) nil)))
 712        (fdoc
 713         ;; We now have a help buffer on the function.
 714         ;; Insert face text before it.
 715         (when facedoc
 716           (goto-char (point-max))
 717           (let ((inhibit-read-only t))
 718             (insert "\n\n" (make-string 30 ?-) "\n\n" (symbol-name symbol)
 719                     " is also a " "face." "\n\n" facedoc))
 720           ;; Don't record the `describe-function' item in the stack.
 721           (setq help-xref-stack-item nil)
 722           (help-setup-xref (list #'help-xref-interned symbol) nil)))))))
 723
 724 \f
 725 ;; Navigation/hyperlinking with xrefs
 726
 727 (defun help-xref-go-back (buffer)
 728   "From BUFFER, go back to previous help buffer text using `help-xref-stack'."
 729   (let (item position method args)
 730     (with-current-buffer buffer
 731       (push (cons (point) help-xref-stack-item) help-xref-forward-stack)
 732       (when help-xref-stack
 733         (setq item (pop help-xref-stack)
 734               ;; Clear the current item so that it won't get pushed
 735               ;; by the function we're about to call.  TODO: We could also
 736               ;; push it onto a "forward" stack and add a `forw' button.
 737               help-xref-stack-item nil
 738               position (car item)
 739               method (cadr item)
 740               args (cddr item))))
 741     (apply method args)
 742     (with-current-buffer buffer
 743       (if (get-buffer-window buffer)
 744           (set-window-point (get-buffer-window buffer) position)
 745         (goto-char position)))))
 746
 747 (defun help-xref-go-forward (buffer)
 748   "From BUFFER, go forward to next help buffer."
 749   (let (item position method args)
 750     (with-current-buffer buffer
 751       (push (cons (point) help-xref-stack-item) help-xref-stack)
 752       (when help-xref-forward-stack
 753         (setq item (pop help-xref-forward-stack)
 754               ;; Clear the current item so that it won't get pushed
 755               ;; by the function we're about to call.  TODO: We could also
 756               ;; push it onto a "forward" stack and add a `forw' button.
 757               help-xref-stack-item nil
 758               position (car item)
 759               method (cadr item)
 760               args (cddr item))))
 761     (apply method args)
 762     (with-current-buffer buffer
 763       (if (get-buffer-window buffer)
 764           (set-window-point (get-buffer-window buffer) position)
 765         (goto-char position)))))
 766
 767 (defun help-go-back ()
 768   "Go back to previous topic in this help buffer."
 769   (interactive)
 770   (if help-xref-stack
 771       (help-xref-go-back (current-buffer))
 772     (error "No previous help buffer")))
 773
 774 (defun help-go-forward ()
 775   "Go back to next topic in this help buffer."
 776   (interactive)
 777   (if help-xref-forward-stack
 778       (help-xref-go-forward (current-buffer))
 779     (error "No next help buffer")))
 780
 781 (defun help-do-xref (_pos function args)
 782   "Call the help cross-reference function FUNCTION with args ARGS.
 783 Things are set up properly so that the resulting help-buffer has
 784 a proper [back] button."
 785   ;; There is a reference at point.  Follow it.
 786   (let ((help-xref-following t))
 787     (apply function args)))
 788
 789 ;; The doc string is meant to explain what buttons do.
 790 (defun help-follow-mouse ()
 791   "Follow the cross-reference that you click on."
 792   (interactive)
 793   (error "No cross-reference here"))
 794
 795 ;; The doc string is meant to explain what buttons do.
 796 (defun help-follow ()
 797   "Follow cross-reference at point.
 798
 799 For the cross-reference format, see `help-make-xrefs'."
 800   (interactive)
 801   (error "No cross-reference here"))
 802
 803 (defun help-follow-symbol (&optional pos)
 804   "In help buffer, show docs for symbol at POS, defaulting to point.
 805 Show all docs for that symbol as either a variable, function or face."
 806   (interactive "d")
 807   (unless pos
 808     (setq pos (point)))
 809   ;; check if the symbol under point is a function, variable or face
 810   (let ((sym
 811          (intern
 812           (save-excursion
 813             (goto-char pos) (skip-syntax-backward "w_")
 814             (buffer-substring (point)
 815                               (progn (skip-syntax-forward "w_")
 816                                      (point)))))))
 817     (when (or (boundp sym)
 818               (get sym 'variable-documentation)
 819               (fboundp sym) (facep sym))
 820       (help-do-xref pos #'help-xref-interned (list sym)))))
 821
 822 (defun help-mode-revert-buffer (_ignore-auto noconfirm)
 823   (when (or noconfirm (yes-or-no-p "Revert help buffer? "))
 824     (let ((pos (point))
 825           (item help-xref-stack-item)
 826           ;; Pretend there is no current item to add to the history.
 827           (help-xref-stack-item nil)
 828           ;; Use the current buffer.
 829           (help-xref-following t))
 830       (apply (car item) (cdr item))
 831       (goto-char pos))))
 832
 833 (defun help-insert-string (string)
 834   "Insert STRING to the help buffer and install xref info for it.
 835 This function can be used to restore the old contents of the help buffer
 836 when going back to the previous topic in the xref stack.  It is needed
 837 in case when it is impossible to recompute the old contents of the
 838 help buffer by other means."
 839   (setq help-xref-stack-item (list #'help-insert-string string))
 840   (with-output-to-temp-buffer (help-buffer)
 841     (insert string)))
 842
 843 (provide 'help-mode)
 844
 845 ;;; help-mode.el ends here