1;;; help-mode.el --- `help-mode' used by *Help* buffers
2
3;; Copyright (C) 1985, 1986, 1993, 1994, 1998, 1999, 2000, 2001, 2002,
4;;   2003, 2004, 2005, 2006, 2007 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., 51 Franklin Street, Fifth Floor,
24;; Boston, MA 02110-1301, 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(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-symbol)
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.
49Used by `help-follow' and `help-xref-go-back'.
50An element looks like (POSITION FUNCTION ARGS...).
51To 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'.
57The 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
68;; Button types used by help
69
70(define-button-type 'help-xref
71  'follow-link t
72  'action #'help-button-action)
73
74(defun help-button-action (button)
75  "Call BUTTON's help function."
76  (help-do-xref (button-start button)
77		(button-get button 'help-function)
78		(button-get button 'help-args)))
79
80;; These 6 calls to define-button-type were generated in a dolist
81;; loop, but that is bad because it means these button types don't
82;; have an easily found definition.
83
84(define-button-type 'help-function
85  :supertype 'help-xref
86  'help-function 'describe-function
87  'help-echo (purecopy "mouse-2, RET: describe this function"))
88
89(define-button-type 'help-variable
90  :supertype 'help-xref
91  'help-function 'describe-variable
92  'help-echo (purecopy "mouse-2, RET: describe this variable"))
93
94(define-button-type 'help-face
95  :supertype 'help-xref
96  'help-function 'describe-face
97  'help-echo (purecopy "mouse-2, RET: describe this face"))
98
99(define-button-type 'help-coding-system
100  :supertype 'help-xref
101  'help-function 'describe-coding-system
102  'help-echo (purecopy "mouse-2, RET: describe this coding system"))
103
104(define-button-type 'help-input-method
105  :supertype 'help-xref
106  'help-function 'describe-input-method
107  'help-echo (purecopy "mouse-2, RET: describe this input method"))
108
109(define-button-type 'help-character-set
110  :supertype 'help-xref
111  'help-function 'describe-character-set
112  'help-echo (purecopy "mouse-2, RET: describe this character set"))
113
114;; make some more ideosyncratic button types
115
116(define-button-type 'help-symbol
117  :supertype 'help-xref
118  'help-function #'help-xref-interned
119  'help-echo (purecopy "mouse-2, RET: describe this symbol"))
120
121(define-button-type 'help-back
122  :supertype 'help-xref
123  'help-function #'help-xref-go-back
124  'help-echo (purecopy "mouse-2, RET: go back to previous help buffer"))
125
126(define-button-type 'help-info
127  :supertype 'help-xref
128  'help-function #'info
129  'help-echo (purecopy "mouse-2, RET: read this Info node"))
130
131(define-button-type 'help-url
132  :supertype 'help-xref
133  'help-function #'browse-url
134  'help-echo (purecopy "mouse-2, RET: view this URL in a browser"))
135
136(define-button-type 'help-customize-variable
137  :supertype 'help-xref
138  'help-function (lambda (v)
139		   (customize-variable v))
140  'help-echo (purecopy "mouse-2, RET: customize variable"))
141
142(define-button-type 'help-customize-face
143  :supertype 'help-xref
144  'help-function (lambda (v)
145		   (customize-face v))
146  'help-echo (purecopy "mouse-2, RET: customize face"))
147
148(define-button-type 'help-function-def
149  :supertype 'help-xref
150  'help-function (lambda (fun file)
151		   (require 'find-func)
152		   (when (eq file 'C-source)
153		     (setq file
154			   (help-C-file-name (indirect-function fun) 'fun)))
155		   ;; Don't use find-function-noselect because it follows
156		   ;; aliases (which fails for built-in functions).
157		   (let ((location
158			  (find-function-search-for-symbol fun nil file)))
159		     (pop-to-buffer (car location))
160		     (if (cdr location)
161			 (goto-char (cdr location))
162		       (message "Unable to find location in file"))))
163  'help-echo (purecopy "mouse-2, RET: find function's definition"))
164
165(define-button-type 'help-variable-def
166  :supertype 'help-xref
167  'help-function (lambda (var &optional file)
168		   (when (eq file 'C-source)
169		     (setq file (help-C-file-name var 'var)))
170		   (let ((location (find-variable-noselect var file)))
171		     (pop-to-buffer (car location))
172		     (if (cdr location)
173		       (goto-char (cdr location))
174		       (message "Unable to find location in file"))))
175  'help-echo (purecopy "mouse-2, RET: find variable's definition"))
176
177(define-button-type 'help-face-def
178  :supertype 'help-xref
179  'help-function (lambda (fun file)
180		   (require 'find-func)
181		   ;; Don't use find-function-noselect because it follows
182		   ;; aliases (which fails for built-in functions).
183		   (let ((location
184			  (find-function-search-for-symbol fun 'defface file)))
185		     (pop-to-buffer (car location))
186		     (if (cdr location)
187			 (goto-char (cdr location))
188		       (message "Unable to find location in file"))))
189  'help-echo (purecopy "mouse-2, RET: find face's definition"))
190
191
192;;;###autoload
193(defun help-mode ()
194  "Major mode for viewing help text and navigating references in it.
195Entry to this mode runs the normal hook `help-mode-hook'.
196Commands:
197\\{help-mode-map}"
198  (interactive)
199  (kill-all-local-variables)
200  (use-local-map help-mode-map)
201  (setq mode-name "Help")
202  (setq major-mode 'help-mode)
203  (view-mode)
204  (make-local-variable 'view-no-disable-on-exit)
205  (setq view-no-disable-on-exit t)
206  (setq view-exit-action (lambda (buffer)
207			   (or (window-minibuffer-p (selected-window))
208			       (one-window-p t)
209			       (delete-window))))
210  (run-mode-hooks 'help-mode-hook))
211
212;;;###autoload
213(defun help-mode-setup ()
214  (help-mode)
215  (setq buffer-read-only nil))
216
217;;;###autoload
218(defun help-mode-finish ()
219  (let ((entry (assq (selected-window) view-return-to-alist)))
220	(if entry
221	    ;; When entering Help mode from the Help window,
222	    ;; such as by following a link, preserve the same
223	    ;; meaning for the q command.
224	    ;; (setcdr entry (cons (selected-window) help-return-method))
225	    nil
226	  (setq view-return-to-alist
227		(cons (cons (selected-window) help-return-method)
228		      view-return-to-alist))))
229  (when (eq major-mode 'help-mode)
230    ;; View mode's read-only status of existing *Help* buffer is lost
231    ;; by with-output-to-temp-buffer.
232    (toggle-read-only 1)
233    (help-make-xrefs (current-buffer))))
234
235;; Grokking cross-reference information in doc strings and
236;; hyperlinking it.
237
238;; This may have some scope for extension and the same or something
239;; similar should be done for widget doc strings, which currently use
240;; another mechanism.
241
242(defvar help-back-label (purecopy "[back]")
243  "Label to use by `help-make-xrefs' for the go-back reference.")
244
245(defconst help-xref-symbol-regexp
246  (purecopy (concat "\\(\\<\\(\\(variable\\|option\\)\\|"  ; Link to var
247 		    "\\(function\\|command\\)\\|"          ; Link to function
248 		    "\\(face\\)\\|"			   ; Link to face
249 		    "\\(symbol\\|program\\|property\\)\\|" ; Don't link
250		    "\\(source \\(?:code \\)?\\(?:of\\|for\\)\\)\\)"
251		    "[ \t\n]+\\)?"
252		    ;; Note starting with word-syntax character:
253		    "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'"))
254  "Regexp matching doc string references to symbols.
255
256The words preceding the quoted symbol can be used in doc strings to
257distinguish references to variables, functions and symbols.")
258
259(defvar help-xref-mule-regexp nil
260  "Regexp matching doc string references to MULE-related keywords.
261
262It is usually nil, and is temporarily bound to an appropriate regexp
263when help commands related to multilingual environment (e.g.,
264`describe-coding-system') are invoked.")
265
266
267(defconst help-xref-info-regexp
268  (purecopy "\\<[Ii]nfo[ \t\n]+\\(node\\|anchor\\)[ \t\n]+`\\([^']+\\)'")
269  "Regexp matching doc string references to an Info node.")
270
271(defconst help-xref-url-regexp
272  (purecopy "\\<[Uu][Rr][Ll][ \t\n]+`\\([^']+\\)'")
273  "Regexp matching doc string references to a URL.")
274
275;;;###autoload
276(defun help-setup-xref (item interactive-p)
277  "Invoked from commands using the \"*Help*\" buffer to install some xref info.
278
279ITEM is a (FUNCTION . ARGS) pair appropriate for recreating the help
280buffer after following a reference.  INTERACTIVE-P is non-nil if the
281calling command was invoked interactively.  In this case the stack of
282items for help buffer \"back\" buttons is cleared.
283
284This should be called very early, before the output buffer is cleared,
285because we want to record the \"previous\" position of point so we can
286restore it properly when going back."
287  (with-current-buffer (help-buffer)
288    (when help-xref-stack-item
289      (push (cons (point) help-xref-stack-item) help-xref-stack))
290    (when interactive-p
291      (let ((tail (nthcdr 10 help-xref-stack)))
292	;; Truncate the stack.
293	(if tail (setcdr tail nil))))
294    (setq help-xref-stack-item item)))
295
296(defvar help-xref-following nil
297  "Non-nil when following a help cross-reference.")
298
299(defun help-buffer ()
300  (buffer-name				;for with-output-to-temp-buffer
301   (if help-xref-following
302       (current-buffer)
303     (get-buffer-create "*Help*"))))
304
305(defvar help-xref-override-view-map
306  (let ((map (make-sparse-keymap)))
307    (set-keymap-parent map view-mode-map)
308    (define-key map "\r" nil)
309    map)
310  "Replacement keymap for `view-mode' in help buffers.")
311
312;;;###autoload
313(defun help-make-xrefs (&optional buffer)
314  "Parse and hyperlink documentation cross-references in the given BUFFER.
315
316Find cross-reference information in a buffer and activate such cross
317references for selection with `help-follow'.  Cross-references have
318the canonical form `...'  and the type of reference may be
319disambiguated by the preceding word(s) used in
320`help-xref-symbol-regexp'.  Faces only get cross-referenced if
321preceded or followed by the word `face'.  Variables without
322variable documentation do not get cross-referenced, unless
323preceded by the word `variable' or `option'.
324
325If the variable `help-xref-mule-regexp' is non-nil, find also
326cross-reference information related to multilingual environment
327\(e.g., coding-systems).  This variable is also used to disambiguate
328the type of reference as the same way as `help-xref-symbol-regexp'.
329
330A special reference `back' is made to return back through a stack of
331help buffers.  Variable `help-back-label' specifies the text for
332that."
333  (interactive "b")
334  (save-excursion
335    (set-buffer (or buffer (current-buffer)))
336    (goto-char (point-min))
337    ;; Skip the header-type info, though it might be useful to parse
338    ;; it at some stage (e.g. "function in `library'").
339    (forward-paragraph)
340    (let ((old-modified (buffer-modified-p)))
341      (let ((stab (syntax-table))
342            (case-fold-search t)
343            (inhibit-read-only t))
344        (set-syntax-table emacs-lisp-mode-syntax-table)
345        ;; The following should probably be abstracted out.
346        (unwind-protect
347            (progn
348              ;; Info references
349              (save-excursion
350                (while (re-search-forward help-xref-info-regexp nil t)
351                  (let ((data (match-string 2)))
352		    (save-match-data
353		      (unless (string-match "^([^)]+)" data)
354			(setq data (concat "(emacs)" data))))
355		    (help-xref-button 2 'help-info data))))
356              ;; URLs
357              (save-excursion
358                (while (re-search-forward help-xref-url-regexp nil t)
359                  (let ((data (match-string 1)))
360		    (help-xref-button 1 'help-url data))))
361	      ;; Mule related keywords.  Do this before trying
362	      ;; `help-xref-symbol-regexp' because some of Mule
363	      ;; keywords have variable or function definitions.
364	      (if help-xref-mule-regexp
365		  (save-excursion
366		    (while (re-search-forward help-xref-mule-regexp nil t)
367		      (let* ((data (match-string 7))
368			     (sym (intern-soft data)))
369			(cond
370			 ((match-string 3) ; coding system
371			  (and sym (coding-system-p sym)
372			       (help-xref-button 6 'help-coding-system sym)))
373			 ((match-string 4) ; input method
374			  (and (assoc data input-method-alist)
375			       (help-xref-button 7 'help-input-method data)))
376			 ((or (match-string 5) (match-string 6)) ; charset
377			  (and sym (charsetp sym)
378			       (help-xref-button 7 'help-character-set sym)))
379			 ((assoc data input-method-alist)
380			  (help-xref-button 7 'help-character-set data))
381			 ((and sym (coding-system-p sym))
382			  (help-xref-button 7 'help-coding-system sym))
383			 ((and sym (charsetp sym))
384			  (help-xref-button 7 'help-character-set sym)))))))
385              ;; Quoted symbols
386              (save-excursion
387                (while (re-search-forward help-xref-symbol-regexp nil t)
388                  (let* ((data (match-string 8))
389                         (sym (intern-soft data)))
390                    (if sym
391                        (cond
392                         ((match-string 3)  ; `variable' &c
393                          (and (or (boundp sym) ; `variable' doesn't ensure
394                                        ; it's actually bound
395				   (get sym 'variable-documentation))
396                               (help-xref-button 8 'help-variable sym)))
397                         ((match-string 4)   ; `function' &c
398                          (and (fboundp sym) ; similarly
399                               (help-xref-button 8 'help-function sym)))
400			 ((match-string 5) ; `face'
401			  (and (facep sym)
402			       (help-xref-button 8 'help-face sym)))
403                         ((match-string 6)) ; nothing for `symbol'
404			 ((match-string 7)
405;;;  this used:
406;;; 			  #'(lambda (arg)
407;;; 			      (let ((location
408;;; 				     (find-function-noselect arg)))
409;;; 				(pop-to-buffer (car location))
410;;; 				(goto-char (cdr location))))
411			  (help-xref-button 8 'help-function-def sym))
412			 ((and
413                           (facep sym)
414                           (save-match-data (looking-at "[ \t\n]+face\\W")))
415                          (help-xref-button 8 'help-face sym))
416                         ((and (or (boundp sym)
417				   (get sym 'variable-documentation))
418			       (fboundp sym))
419                          ;; We can't intuit whether to use the
420                          ;; variable or function doc -- supply both.
421                          (help-xref-button 8 'help-symbol sym))
422                         ((and
423			   (or (boundp sym)
424			       (get sym 'variable-documentation))
425 			   (or
426 			    (documentation-property
427 			     sym 'variable-documentation)
428 			    (condition-case nil
429 				(documentation-property
430 				 (indirect-variable sym)
431 				 'variable-documentation)
432 			      (cyclic-variable-indirection nil))))
433			  (help-xref-button 8 'help-variable sym))
434			 ((fboundp sym)
435			  (help-xref-button 8 'help-function sym)))))))
436              ;; An obvious case of a key substitution:
437              (save-excursion
438                (while (re-search-forward
439			;; Assume command name is only word characters
440			;; and dashes to get things like `use M-x foo.'.
441                        "\\<M-x\\s-+\\(\\sw\\(\\sw\\|-\\)+\\)" nil t)
442                  (let ((sym (intern-soft (match-string 1))))
443                    (if (fboundp sym)
444                        (help-xref-button 1 'help-function sym)))))
445              ;; Look for commands in whole keymap substitutions:
446              (save-excursion
447		;; Make sure to find the first keymap.
448		(goto-char (point-min))
449                ;; Find a header and the column at which the command
450                ;; name will be found.
451
452		;; If the keymap substitution isn't the last thing in
453		;; the doc string, and if there is anything on the
454		;; same line after it, this code won't recognize the end of it.
455                (while (re-search-forward "^key +binding\n\\(-+ +\\)-+\n\n"
456                                          nil t)
457                  (let ((col (- (match-end 1) (match-beginning 1))))
458                    (while
459                        (and (not (eobp))
460			     ;; Stop at a pair of blank lines.
461			     (not (looking-at "\n\\s-*\n")))
462		      ;; Skip a single blank line.
463		      (and (eolp) (forward-line))
464		      (end-of-line)
465		      (skip-chars-backward "^\t\n")
466		      (if (and (>= (current-column) col)
467			       (looking-at "\\(\\sw\\|-\\)+$"))
468			  (let ((sym (intern-soft (match-string 0))))
469			    (if (fboundp sym)
470				(help-xref-button 0 'help-function sym))))
471		      (forward-line))))))
472	  (set-syntax-table stab))
473	;; Delete extraneous newlines at the end of the docstring
474	(goto-char (point-max))
475	(while (and (not (bobp)) (bolp))
476	  (delete-char -1))
477        (insert "\n")
478        ;; Make a back-reference in this buffer if appropriate.
479        (when help-xref-stack
480	  (insert "\n")
481	  (help-insert-xref-button help-back-label 'help-back
482				   (current-buffer))
483          (insert "\n")))
484      ;; View mode steals RET from us.
485      (set (make-local-variable 'minor-mode-overriding-map-alist)
486           (list (cons 'view-mode help-xref-override-view-map)))
487      (set-buffer-modified-p old-modified))))
488
489;;;###autoload
490(defun help-xref-button (match-number type &rest args)
491  "Make a hyperlink for cross-reference text previously matched.
492MATCH-NUMBER is the subexpression of interest in the last matched
493regexp.  TYPE is the type of button to use.  Any remaining arguments are
494passed to the button's help-function when it is invoked.
495See `help-make-xrefs'."
496  ;; Don't mung properties we've added specially in some instances.
497  (unless (button-at (match-beginning match-number))
498    (make-text-button (match-beginning match-number)
499		      (match-end match-number)
500		      'type type 'help-args args)))
501
502;;;###autoload
503(defun help-insert-xref-button (string type &rest args)
504  "Insert STRING and make a hyperlink from cross-reference text on it.
505TYPE is the type of button to use.  Any remaining arguments are passed
506to the button's help-function when it is invoked.
507See `help-make-xrefs'."
508  (unless (button-at (point))
509    (insert-text-button string 'type type 'help-args args)))
510
511;;;###autoload
512(defun help-xref-on-pp (from to)
513  "Add xrefs for symbols in `pp's output between FROM and TO."
514  (if (> (- to from) 5000) nil
515    (with-syntax-table emacs-lisp-mode-syntax-table
516      (save-excursion
517	(save-restriction
518	  (narrow-to-region from to)
519	  (goto-char (point-min))
520	  (condition-case nil
521	      (while (not (eobp))
522		(cond
523		 ((looking-at "\"") (forward-sexp 1))
524		 ((looking-at "#<") (search-forward ">" nil 'move))
525		 ((looking-at "\\(\\(\\sw\\|\\s_\\)+\\)")
526		  (let* ((sym (intern-soft (match-string 1)))
527			 (type (cond ((fboundp sym) 'help-function)
528				     ((or (memq sym '(t nil))
529					  (keywordp sym))
530				      nil)
531				     ((and sym
532					   (or (boundp sym)
533					       (get sym
534						    'variable-documentation)))
535				      'help-variable))))
536		    (when type (help-xref-button 1 type sym)))
537		  (goto-char (match-end 1)))
538		 (t (forward-char 1))))
539	    (error nil)))))))
540
541
542;; Additional functions for (re-)creating types of help buffers.
543(defun help-xref-interned (symbol)
544  "Follow a hyperlink which appeared to be an arbitrary interned SYMBOL.
545Both variable, function and face documentation are extracted into a single
546help buffer."
547  (with-current-buffer (help-buffer)
548    ;; Push the previous item on the stack before clobbering the output buffer.
549    (help-setup-xref nil nil)
550    (let ((facedoc (when (facep symbol)
551		     ;; Don't record the current entry in the stack.
552		     (setq help-xref-stack-item nil)
553		     (describe-face symbol)))
554	  (fdoc (when (fboundp symbol)
555		  ;; Don't record the current entry in the stack.
556		  (setq help-xref-stack-item nil)
557		  (describe-function symbol)))
558	  (sdoc (when (or (boundp symbol)
559			  (get symbol 'variable-documentation))
560		  ;; Don't record the current entry in the stack.
561		  (setq help-xref-stack-item nil)
562		  (describe-variable symbol))))
563      (cond
564       (sdoc
565	;; We now have a help buffer on the variable.
566	;; Insert the function and face text before it.
567	(when (or fdoc facedoc)
568	  (goto-char (point-min))
569	  (let ((inhibit-read-only t))
570	    (when fdoc
571	      (insert fdoc "\n\n")
572	      (when facedoc
573		(insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
574			" is also a " "face." "\n\n")))
575	    (when facedoc
576	      (insert facedoc "\n\n"))
577	    (insert (make-string 30 ?-) "\n\n" (symbol-name symbol)
578		    " is also a " "variable." "\n\n"))
579	  ;; Don't record the `describe-variable' item in the stack.
580	  (setq help-xref-stack-item nil)
581	  (help-setup-xref (list #'help-xref-interned symbol) nil)))
582       (fdoc
583	;; We now have a help buffer on the function.
584	;; Insert face text before it.
585	(when facedoc
586	  (goto-char (point-max))
587	  (let ((inhibit-read-only t))
588	    (insert "\n\n" (make-string 30 ?-) "\n\n" (symbol-name symbol)
589		    " is also a " "face." "\n\n" facedoc))
590	  ;; Don't record the `describe-function' item in the stack.
591	  (setq help-xref-stack-item nil)
592	  (help-setup-xref (list #'help-xref-interned symbol) nil)))))))
593
594
595;; Navigation/hyperlinking with xrefs
596
597(defun help-xref-go-back (buffer)
598  "From BUFFER, go back to previous help buffer text using `help-xref-stack'."
599  (let (item position method args)
600    (with-current-buffer buffer
601      (when help-xref-stack
602	(setq item (pop help-xref-stack)
603	      ;; Clear the current item so that it won't get pushed
604	      ;; by the function we're about to call.  TODO: We could also
605	      ;; push it onto a "forward" stack and add a `forw' button.
606	      help-xref-stack-item nil
607	      position (car item)
608	      method (cadr item)
609	      args (cddr item))))
610    (apply method args)
611    (with-current-buffer buffer
612      (if (get-buffer-window buffer)
613	  (set-window-point (get-buffer-window buffer) position)
614	(goto-char position)))))
615
616(defun help-go-back ()
617  "Go back to previous topic in this help buffer."
618  (interactive)
619  (if help-xref-stack
620      (help-xref-go-back (current-buffer))
621    (error "No previous help buffer")))
622
623(defun help-do-xref (pos function args)
624  "Call the help cross-reference function FUNCTION with args ARGS.
625Things are set up properly so that the resulting help-buffer has
626a proper [back] button."
627  ;; There is a reference at point.  Follow it.
628  (let ((help-xref-following t))
629    (apply function args)))
630
631;; The doc string is meant to explain what buttons do.
632(defun help-follow-mouse ()
633  "Follow the cross-reference that you click on."
634  (interactive)
635  (error "No cross-reference here"))
636
637;; The doc string is meant to explain what buttons do.
638(defun help-follow ()
639  "Follow cross-reference at point.
640
641For the cross-reference format, see `help-make-xrefs'."
642  (interactive)
643  (error "No cross-reference here"))
644
645(defun help-follow-symbol (&optional pos)
646  "In help buffer, show docs for symbol at POS, defaulting to point.
647Show all docs for that symbol as either a variable, function or face."
648  (interactive "d")
649  (unless pos
650    (setq pos (point)))
651  ;; check if the symbol under point is a function, variable or face
652  (let ((sym
653	 (intern
654	  (save-excursion
655	    (goto-char pos) (skip-syntax-backward "w_")
656	    (buffer-substring (point)
657			      (progn (skip-syntax-forward "w_")
658				     (point)))))))
659    (when (or (boundp sym)
660	      (get sym 'variable-documentation)
661	      (fboundp sym) (facep sym))
662      (help-do-xref pos #'help-xref-interned (list sym)))))
663
664(defun help-insert-string (string)
665  "Insert STRING to the help buffer and install xref info for it.
666This function can be used to restore the old contents of the help buffer
667when going back to the previous topic in the xref stack.  It is needed
668in case when it is impossible to recompute the old contents of the
669help buffer by other means."
670  (setq help-xref-stack-item (list #'help-insert-string string))
671  (with-output-to-temp-buffer (help-buffer)
672    (insert string)))
673
674(provide 'help-mode)
675
676;; arch-tag: 850954ae-3725-4cb4-8e91-0bf6d52d6b0b
677;;; help-mode.el ends here
678