emacs/lisp/help.el

;;; help.el --- help commands for Emacs

;; Copyright (C) 1985, 1986, 1993, 1994, 1998 Free Software Foundation, Inc.

;; Maintainer: FSF
;; Keywords: help, internal

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

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

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:

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

;;; Code:

;; Get the macro make-help-screen when this is compiled,
;; or run interpreted, but not when the compiled code is loaded.
(eval-when-compile (require 'help-macro))

(defvar help-map (make-sparse-keymap)
  "Keymap for characters following the Help key.")

(defvar help-mode-map (make-sparse-keymap)
  "Keymap for help mode.")

(define-key global-map (char-to-string help-char) 'help-command)
(define-key global-map [help] 'help-command)
(define-key global-map [f1] 'help-command)
(fset 'help-command help-map)

(define-key help-map (char-to-string help-char) 'help-for-help)
(define-key help-map [help] 'help-for-help)
(define-key help-map [f1] 'help-for-help)
(define-key help-map "?" 'help-for-help)

(define-key help-map "\C-c" 'describe-copying)
(define-key help-map "\C-d" 'describe-distribution)
(define-key help-map "\C-w" 'describe-no-warranty)
(define-key help-map "\C-p" 'describe-project)
(define-key help-map "a" 'apropos-command)

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

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

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

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

(define-key help-map "i" 'info)
(define-key help-map "\C-f" 'Info-goto-emacs-command-node)
(define-key help-map "\C-k" 'Info-goto-emacs-key-command-node)
(define-key help-map "\C-i" 'info-lookup-symbol)

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

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

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

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

(define-key help-map "s" 'describe-syntax)

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

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

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

(define-key help-map "q" 'help-quit)

(define-key help-mode-map [mouse-2] 'help-follow-mouse)
(define-key help-mode-map "\C-c\C-b" 'help-go-back)
(define-key help-mode-map "\C-c\C-c" 'help-follow)
(define-key help-mode-map "\t" 'help-next-ref)
(define-key help-mode-map [backtab] 'help-previous-ref)
(define-key help-mode-map [(shift tab)] 'help-previous-ref)
;; Documentation only, since we use minor-mode-overriding-map-alist.
(define-key help-mode-map "\r" 'help-follow)

;; Font-locking is incompatible with the new xref stuff.
;(defvar help-font-lock-keywords
;  (eval-when-compile
;    (let ((name-char "[-+a-zA-Z0-9_*]") (sym-char "[-+a-zA-Z0-9_:*]"))
;      (list
;       ;;
;       ;; The symbol itself.
;       (list (concat "\\`\\(" name-char "+\\)\\(\\(:\\)\\|\\('\\)\\)")
;	     '(1 (if (match-beginning 3)
;		     font-lock-function-name-face
;		   font-lock-variable-name-face)))
;       ;;
;       ;; Words inside `' which tend to be symbol names.
;       (list (concat "`\\(" sym-char sym-char "+\\)'")
;	     1 'font-lock-constant-face t)
;       ;;
;       ;; CLisp `:' keywords as references.
;       (list (concat "\\<:" sym-char "+\\>") 0 'font-lock-builtin-face t))))
;  "Default expressions to highlight in Help mode.")

(defvar help-xref-stack nil
  "A stack of ways by which to return to help buffers after following xrefs.
Used by `help-follow' and `help-xref-go-back'.
An element looks like (POSITION FUNCTION ARGS...).
To use the element, do (apply FUNCTION ARGS) then (goto-char POSITION).")
(put 'help-xref-stack 'permanent-local t)

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

(setq-default help-xref-stack nil help-xref-stack-item nil)

(defun help-mode ()
  "Major mode for viewing help text and navigating references in it.
Entry to this mode runs the normal hook `help-mode-hook'.
Commands:
\\{help-mode-map}"
  (interactive)
  (kill-all-local-variables)
  (use-local-map help-mode-map)
  (setq mode-name "Help")
  (setq major-mode 'help-mode)
  (make-local-variable 'font-lock-defaults)
  (setq font-lock-defaults nil)         ; font-lock would defeat xref
  (view-mode)
  (make-local-variable 'view-no-disable-on-exit)
  (setq view-no-disable-on-exit t)
  ;; `help-make-xrefs' would be run here if not invoked from
  ;; `help-mode-maybe'.
  (run-hooks 'help-mode-hook))

(defun help-mode-maybe ()
  (if (eq major-mode 'fundamental-mode)
      (help-mode))
  (when (eq major-mode 'help-mode) 
    ;; View mode's read-only status of existing *Help* buffer is lost
    ;; by with-output-to-temp-buffer.
    (toggle-read-only 1)
    (help-make-xrefs (current-buffer)))
  (setq view-return-to-alist
	(list (cons (selected-window) help-return-method))))

(add-hook 'temp-buffer-show-hook 'help-mode-maybe)

(defun help-quit ()
  (interactive)
  nil)

(defun help-with-tutorial (&optional arg)
  "Select the Emacs learn-by-doing tutorial.
If there is a tutorial version written in the language
of the selected language environment, that version is used.
If there's no tutorial in that language, `TUTORIAL' is selected.
With arg, you are asked to choose which language."
  (interactive "P")
  (let ((lang (if arg
		  (read-language-name 'tutorial "Language: " "English")
		(if (get-language-info current-language-environment 'tutorial)
		    current-language-environment
		  "English")))
	file filename)
    (setq filename (get-language-info lang 'tutorial))
    (setq file (expand-file-name (concat "~/" filename)))
    (delete-other-windows)
    (if (get-file-buffer file)
	(switch-to-buffer (get-file-buffer file))
      (switch-to-buffer (create-file-buffer file))
      (setq buffer-file-name file)
      (setq default-directory (expand-file-name "~/"))
      (setq buffer-auto-save-file-name nil)
      (insert-file-contents (expand-file-name filename data-directory))
      (goto-char (point-min))
      (search-forward "\n<<")
      (beginning-of-line)
      (delete-region (point) (progn (end-of-line) (point)))
      (let ((n (- (window-height (selected-window))
		  (count-lines (point-min) (point))
		  6)))
	(if (< n 12)
	    (newline n)
	  ;; Some people get confused by the large gap.
	  (newline (/ n 2))
	  (insert "[Middle of page left blank for didactic purposes.  "
		  "Text continues below]")
	  (newline (- n (/ n 2)))))
      (goto-char (point-min))
      (set-buffer-modified-p nil))))

(defun describe-key-briefly (key &optional insert)
  "Print the name of the function KEY invokes.  KEY is a string.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
  (interactive "kDescribe key briefly: \nP")
  ;; If this key seq ends with a down event, discard the
  ;; following click or drag event.  Otherwise that would
  ;; erase the message.
  (let ((type (aref key (1- (length key)))))
    (if (listp type) (setq type (car type)))
    (and (symbolp type)
	 (memq 'down (event-modifiers type))
	 (read-event)))
  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
	  (standard-output (if insert (current-buffer) t))
	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
      ;; Ok, now look up the key and name the command.
      (let ((defn (key-binding key))
	    (key-desc (key-description key)))
	(if (or (null defn) (integerp defn))
	    (princ (format "%s is undefined" key-desc))
	  (princ (format (if insert
			     "`%s' (`%s')"
			   (if (windowp window)
			       "%s at that spot runs the command %s"
			     "%s runs the command %s"))
			 key-desc
			 (if (symbolp defn) defn (prin1-to-string defn)))))))))

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

(defun print-help-return-message (&optional function)
  "Display or return message saying how to restore windows after help command.
Computes a message and applies the optional argument FUNCTION to it.
If FUNCTION is nil, applies `message' to it, thus printing it."
  (and (not (get-buffer-window standard-output))
       (let ((first-message
	      (cond ((special-display-p (buffer-name standard-output))
		     (setq help-return-method (cons (selected-window) t))
		     ;; If the help output buffer is a special display buffer,
		     ;; don't say anything about how to get rid of it.
		     ;; First of all, the user will do that with the window
		     ;; manager, not with Emacs.
		     ;; Secondly, the buffer has not been displayed yet,
		     ;; so we don't know whether its frame will be selected.
		     nil)
		    ((not (one-window-p t))
		     (setq help-return-method
			   (cons (selected-window) 'quit-window))
		     "Type \\[switch-to-buffer-other-window] RET to restore the other window.")
		    (pop-up-windows
		     (setq help-return-method (cons (selected-window) t))
		     "Type \\[delete-other-windows] to remove help window.")
		    (t
		     (setq help-return-method
			   (list (selected-window) (window-buffer)
				 (window-start) (window-point)))
		     "Type \\[switch-to-buffer] RET to remove help window."))))
	 (funcall (or function 'message)
		  (concat
		   (if first-message
		       (substitute-command-keys first-message)
		     "")
		   (if first-message "  " "")
		   ;; If the help buffer will go in a separate frame,
		   ;; it's no use mentioning a command to scroll, so don't.
		   (if (special-display-p (buffer-name standard-output))
		       nil
		     (if (same-window-p (buffer-name standard-output))
			 ;; Say how to scroll this window.
			 (substitute-command-keys
			  "\\[scroll-up] to scroll the help.")
		       ;; Say how to scroll some other window.
		       (substitute-command-keys
			"\\[scroll-other-window] to scroll the help."))))))))

(defun describe-key (key)
  "Display documentation of the function invoked by KEY.  KEY is a string."
  (interactive "kDescribe key: ")
  ;; If this key seq ends with a down event, discard the
  ;; following click or drag event.  Otherwise that would
  ;; erase the message.
  (let ((type (aref key (1- (length key)))))
    (if (listp type) (setq type (car type)))
    (and (symbolp type)
	 (memq 'down (event-modifiers type))
	 (read-event)))
  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
      (let ((defn (key-binding key)))
	(if (or (null defn) (integerp defn))
	    (message "%s is undefined" (key-description key))
	  (with-output-to-temp-buffer "*Help*"
	    (princ (key-description key))
	    (if (windowp window)
		(princ " at that spot"))
	    (princ " runs the command ")
	    (prin1 defn)
	    (princ "\n   which is ")
	    (describe-function-1 defn nil)
	    (print-help-return-message)))))))

(defun describe-mode ()
  "Display documentation of current major mode and minor modes.
For this to work correctly for a minor mode, the mode's indicator variable
\(listed in `minor-mode-alist') must also be a function whose documentation
describes the minor mode."
  (interactive)
  (with-output-to-temp-buffer "*Help*"
    (let ((minor-modes minor-mode-alist)
	  (first t))
      (while minor-modes
	(let* ((minor-mode (car (car minor-modes)))
	       (indicator (car (cdr (car minor-modes)))))
	  ;; Document a minor mode if it is listed in minor-mode-alist,
	  ;; bound locally in this buffer, non-nil, and has a function
	  ;; definition.
	  (if (and (symbol-value minor-mode)
		   (fboundp minor-mode))
	      (let ((pretty-minor-mode minor-mode))
		(if (string-match "-mode$" (symbol-name minor-mode))
		    (setq pretty-minor-mode
			  (capitalize
			   (substring (symbol-name minor-mode)
				      0 (match-beginning 0)))))
		(while (and indicator (symbolp indicator))
		  (setq indicator (symbol-value indicator)))
		(if first
		    (princ "The minor modes are described first,
followed by the major mode, which is described on the last page.\n\f\n"))
		(setq first nil)
		(princ (format "%s minor mode (%s):\n"
			       pretty-minor-mode
			       (if indicator
				   (format "indicator%s" indicator)
				 "no indicator")))
		(princ (documentation minor-mode))
		(princ "\n\f\n"))))
	(setq minor-modes (cdr minor-modes))))
    (princ mode-name)
    (princ " mode:\n")
    (princ (documentation major-mode))
    (help-setup-xref (list #'help-xref-mode (current-buffer)) (interactive-p))
    (print-help-return-message)))

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

(defun describe-distribution ()
  "Display info on how to obtain the latest version of GNU Emacs."
  (interactive)
  (find-file-read-only
   (expand-file-name "DISTRIB" data-directory)))

(defun describe-copying ()
  "Display info on how you may redistribute copies of GNU Emacs."
  (interactive)
  (find-file-read-only
   (expand-file-name "COPYING" data-directory))
  (goto-char (point-min)))

(defun describe-project ()
  "Display info on the GNU project."
  (interactive)
  (find-file-read-only
   (expand-file-name "GNU" data-directory))
  (goto-char (point-min)))

(defun describe-no-warranty ()
  "Display info on all the kinds of warranty Emacs does NOT have."
  (interactive)
  (describe-copying)
  (let (case-fold-search)
    (search-forward "NO WARRANTY")
    (recenter 0)))

(defun describe-prefix-bindings ()
  "Describe the bindings of the prefix used to reach this command.
The prefix described consists of all but the last event
of the key sequence that ran this command."
  (interactive)
  (let* ((key (this-command-keys)))
    (describe-bindings
     (if (stringp key)
	 (substring key 0 (1- (length key)))
       (let ((prefix (make-vector (1- (length key)) nil))
	     (i 0))
	 (while (< i (length prefix))
	   (aset prefix i (aref key i))
	   (setq i (1+ i)))
	 prefix)))))
;; Make C-h after a prefix, when not specifically bound, 
;; run describe-prefix-bindings.
(setq prefix-help-command 'describe-prefix-bindings)

(defun view-emacs-news (&optional arg)
  "Display info on recent changes to Emacs.
With numeric argument display information on correspondingly older changes."
  (interactive "P")
  (let* ((arg (if arg (prefix-numeric-value arg) 0)))
    (find-file-read-only
     (expand-file-name (concat (make-string arg ?O) "NEWS")
		       data-directory))))

(defun view-emacs-FAQ ()
  "Display the Emacs Frequently Asked Questions (FAQ) file."
  (interactive)
  (find-file-read-only (expand-file-name "FAQ" data-directory)))

(defun view-lossage ()
  "Display last 100 input keystrokes."
  (interactive)
  (with-output-to-temp-buffer "*Help*"
    (princ (mapconcat (function (lambda (key)
				  (if (or (integerp key)
					  (symbolp key)
					  (listp key))
				      (single-key-description key)
				    (prin1-to-string key nil))))
		      (recent-keys)
		      " "))
    (save-excursion
      (set-buffer standard-output)
      (goto-char (point-min))
      (while (progn (move-to-column 50) (not (eobp)))
	(search-forward " " nil t)
	(insert "\n"))
      (setq help-xref-stack nil
	    help-xref-stack-item nil))
    (print-help-return-message)))

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

a  command-apropos.  Give a substring, and see a list of commands
	(functions interactively callable) that contain
	that substring.  See also the  apropos  command.
b  describe-bindings.  Display table of all key bindings.
c  describe-key-briefly.  Type a command key sequence;
	it prints the function name that sequence runs.
C  describe-coding-system.  This describes either a specific coding system
        (if you type its name) or the coding systems currently in use
	(if you type just RET).
f  describe-function.  Type a function name and get documentation of it.
C-f Info-goto-emacs-command-node.  Type a function name;
	it takes you to the Info node for that command.
i  info. The  info  documentation reader.
I  describe-input-method.  Describe a specific input method (if you type
	its name) or the current input method (if you type just RET).
k  describe-key.  Type a command key sequence;
	it displays the full documentation.
C-k Info-goto-emacs-key-command-node.  Type a command key sequence;
	it takes you to the Info node for the command bound to that key.
l  view-lossage.  Shows last 100 characters you typed.
L  describe-language-environment.  This describes either the a
	specific language environment (if you type its name)
	or the current language environment (if you type just RET).
m  describe-mode.  Print documentation of current minor modes,
	and the current major mode, including their special commands.
n  view-emacs-news.  Shows emacs news file.
p  finder-by-keyword. Find packages matching a given topic keyword.
s  describe-syntax.  Display contents of syntax table, plus explanations
t  help-with-tutorial.  Select the Emacs learn-by-doing tutorial.
v  describe-variable.  Type name of a variable;
	it displays the variable's documentation and value.
w  where-is.  Type command name; it prints which keystrokes
	invoke that command.

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

;; Return a function which is called by the list containing point.
;; If that gives no function, return a function whose name is around point.
;; If that doesn't give a function, return nil.
(defun function-called-at-point ()
  (let ((stab (syntax-table)))
    (set-syntax-table emacs-lisp-mode-syntax-table)
    (unwind-protect
	(or (condition-case ()
		(save-excursion
		  (save-restriction
		    (narrow-to-region (max (point-min) (- (point) 1000)) (point-max))
		    ;; Move up to surrounding paren, then after the open.
		    (backward-up-list 1)
		    (forward-char 1)
		    ;; If there is space here, this is probably something
		    ;; other than a real Lisp function call, so ignore it.
		    (if (looking-at "[ \t]")
			(error "Probably not a Lisp function call"))
		    (let (obj)
		      (setq obj (read (current-buffer)))
		      (and (symbolp obj) (fboundp obj) obj))))
	      (error nil))
	    (condition-case ()
		(save-excursion
		  (or (not (zerop (skip-syntax-backward "_w")))
		      (eq (char-syntax (following-char)) ?w)
		      (eq (char-syntax (following-char)) ?_)
		      (forward-sexp -1))
		  (skip-chars-forward "'")
		  (let ((obj (read (current-buffer))))
		    (and (symbolp obj) (fboundp obj) obj)))
	      (error nil)))
      (set-syntax-table stab))))

(defun describe-function-find-file (function)
  (let ((files load-history)
	file functions)
    (while files
      (if (memq function (cdr (car files)))
	  (setq file (car (car files)) files nil))
      (setq files (cdr files)))
    file))

(defun describe-function (function)
  "Display the full documentation of FUNCTION (a symbol)."
  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Describe function (default %s): " fn)
				  "Describe function: ")
				obarray 'fboundp t nil nil (symbol-name fn)))
     (list (if (equal val "")
	       fn (intern val)))))
  (if function
      (with-output-to-temp-buffer "*Help*"
	(prin1 function)
	;; Use " is " instead of a colon so that
	;; it is easier to get out the function name using forward-sexp.
	(princ " is ")
	(describe-function-1 function nil)
	(print-help-return-message)
	(save-excursion
	  (set-buffer standard-output)
	  ;; Return the text we displayed.
	  (buffer-string)))
    (message "You didn't specify a function")))

(defun describe-function-1 (function parens)
  (let* ((def (symbol-function function))
	 file-name string need-close
	 (beg (if (commandp def) "an interactive " "a ")))
    (setq string
	  (cond ((or (stringp def)
		     (vectorp def))
		 "a keyboard macro")
		((subrp def)
		 (concat beg "built-in function"))
		((byte-code-function-p def)
		 (concat beg "compiled Lisp function"))
		((symbolp def)
		 (format "alias for `%s'" def))
		((eq (car-safe def) 'lambda)
		 (concat beg "Lisp function"))
		((eq (car-safe def) 'macro)
		 "a Lisp macro")
		((eq (car-safe def) 'mocklisp)
		 "a mocklisp function")
		((eq (car-safe def) 'autoload)
		 (setq file-name (nth 1 def))
		 (format "%s autoloaded Lisp %s"
			 (if (commandp def) "an interactive" "an")
			 (if (nth 4 def) "macro" "function")
			 ))
		(t "")))
    (when (and parens (not (equal string "")))
      (setq need-close t)
      (princ "("))
    (princ string)
    (or file-name
	(setq file-name (describe-function-find-file function)))
    (if file-name
	(progn
	  (princ " in `")
	  ;; We used to add .el to the file name,
	  ;; but that's completely wrong when the user used load-file.
	  (princ file-name)
	  (princ "'")
	  ;; Make a hyperlink to the library.
	  (with-current-buffer "*Help*"
	    (save-excursion
	      (re-search-backward "`\\([^`']+\\)'" nil t)
	      (help-xref-button 1 #'(lambda (arg)
				      (let ((location
					     (find-function-noselect arg)))
					(display-buffer (nth 0 location))
					(goto-char (nth 1 location))))
				function)))))
    (if need-close (princ ")"))
    (princ ".")
    (terpri)
    (let* ((inner-function (if (and (listp def) 'macro)
			       (cdr def)
			     def))
	   (arglist (cond ((byte-code-function-p inner-function)
			   (car (append inner-function nil)))
			  ((eq (car-safe inner-function) 'lambda)
			   (nth 1 inner-function))
			  (t t))))
      (if (listp arglist)
	  (progn
	    (princ (cons function
			 (mapcar (lambda (arg)
				   (if (memq arg '(&optional &rest))
				       arg
				     (intern (upcase (symbol-name arg)))))
				 arglist)))
	    (terpri))))
    (let ((doc (documentation function)))
      (if doc
	  (progn (terpri)
		 (princ doc)
		 (help-setup-xref (list #'describe-function function) (interactive-p)))
	(princ "not documented")))))

;; We return 0 if we can't find a variable to return.
(defun variable-at-point ()
  (condition-case ()
      (let ((stab (syntax-table)))
	(unwind-protect
	    (save-excursion
	      (set-syntax-table emacs-lisp-mode-syntax-table)
	      (or (not (zerop (skip-syntax-backward "_w")))
		  (eq (char-syntax (following-char)) ?w)
		  (eq (char-syntax (following-char)) ?_)
		  (forward-sexp -1))
	      (skip-chars-forward "'")
	      (let ((obj (read (current-buffer))))
		(or (and (symbolp obj) (boundp obj) obj)
		    0)))
	  (set-syntax-table stab)))
    (error 0)))

(defun describe-variable (variable)
  "Display the full documentation of VARIABLE (a symbol).
Returns the documentation as a string, also."
  (interactive 
   (let ((v (variable-at-point))
	 (enable-recursive-minibuffers t)
	 val)
     (setq val (completing-read (if (symbolp v)
				    (format "Describe variable (default %s): " v)
				  "Describe variable: ")
				obarray 'boundp t nil nil
				(if (symbolp v) (symbol-name v))))
     (list (if (equal val "")
	       v (intern val)))))
  (if (symbolp variable)
      (let (valvoid)
	(with-output-to-temp-buffer "*Help*"
	  (prin1 variable)
	  (if (not (boundp variable))
	      (progn
		(princ " is void")
		(terpri)
		(setq valvoid t))
	    (princ "'s value is ")
	    (terpri)
	    (pp (symbol-value variable))
	    (terpri))
	  (if (local-variable-p variable)
	      (progn
		(princ (format "Local in buffer %s; " (buffer-name)))
		(if (not (default-boundp variable))
		    (princ "globally void")
		  (princ "global value is ")
		  (terpri)
		  (pp (default-value variable)))
		(terpri)))
	  (terpri)
	  (save-current-buffer
	    (set-buffer standard-output)
	    (if (> (count-lines (point-min) (point-max)) 10)
		(progn
		  (goto-char (point-min))
		  (if valvoid
		      (forward-line 1)
		    (forward-sexp 1)
		    (delete-region (point) (progn (end-of-line) (point)))
		    (insert "'s value is shown below.\n\n")
		    (save-excursion
		      (insert "\n\nValue:"))))))
	  (princ "Documentation:")
	  (terpri)
	  (let ((doc (documentation-property variable 'variable-documentation)))
	    (princ (or doc "not documented as a variable.")))
          (help-setup-xref (list #'describe-variable variable) (interactive-p))

	  ;; Make a link to customize if this variable can be customized.
	  (if (or (get variable 'custom-type)
		  (user-variable-p variable))
	      (let ((customize-label "customize"))
		(terpri)
		(terpri)
		(princ (concat "You can " customize-label " this variable."))
		(with-current-buffer "*Help*"
		  (save-excursion
		    (re-search-backward 
		     (concat "\\(" customize-label "\\)") nil t)
		    (help-xref-button 1 #'(lambda (v)
					    (customize-variable v)) variable)
		    ))))

	  (print-help-return-message)
	  (save-excursion
	    (set-buffer standard-output)
	    ;; Return the text we displayed.
	    (buffer-string))))
    (message "You did not specify a variable")))

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

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

(defun where-is (definition &optional insert)
  "Print message listing key sequences that invoke specified command.
Argument is a command definition, usually a symbol with a function definition.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Where is command (default %s): " fn)
				  "Where is command: ")
				obarray 'fboundp t))
     (list (if (equal val "")
	       fn (intern val))
	   current-prefix-arg)))
  (let* ((keys (where-is-internal definition overriding-local-map nil nil))
	 (keys1 (mapconcat 'key-description keys ", "))
	 (standard-output (if insert (current-buffer) t)))
    (if insert
	(if (> (length keys1) 0)
	    (princ (format "%s (%s)" keys1 definition))
	  (princ (format "M-x %s RET" definition)))
      (if (> (length keys1) 0)
	  (princ (format "%s is on %s" definition keys1))
	(princ (format "%s is not on any key" definition)))))
  nil)

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

If the optional third arg PATH is specified, that list of directories
is used instead of `load-path'."
  (interactive (list (read-string "Locate library: ")
		     nil nil
		     t))
  (let (result)
    (catch 'answer
      (mapcar
       (lambda (dir)
	 (mapcar
	  (lambda (suf)
	    (let ((try (expand-file-name (concat library suf) dir)))
	      (and (file-readable-p try)
		   (null (file-directory-p try))
		   (progn
		     (setq result try)
		     (throw 'answer try)))))
	  (if nosuffix
	      '("")
	    '(".elc" ".el" "")
;;; load doesn't handle this yet.
;;;	    (let ((basic '(".elc" ".el" ""))
;;;		  (compressed '(".Z" ".gz" "")))
;;;	      ;; If autocompression mode is on,
;;;	      ;; consider all combinations of library suffixes
;;;	      ;; and compression suffixes.
;;;	      (if (rassq 'jka-compr-handler file-name-handler-alist)
;;;		  (apply 'nconc
;;;			 (mapcar (lambda (compelt)
;;;				   (mapcar (lambda (baselt)
;;;					     (concat baselt compelt))
;;;					   basic))
;;;				 compressed))
;;;		basic))
	    )))
       (or path load-path)))
    (and interactive-call
	 (if result
	     (message "Library is file %s" result)
	   (message "No library %s in search path" library)))
    result))


;;; Grokking cross-reference information in doc strings and
;;; hyperlinking it.

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

(defcustom help-highlight-p t
  "*If non-nil, `help-make-xrefs' highlight cross-references.
Under a window system it highlights them with face defined by
`help-highlight-face'.  On a character terminal highlighted
references look like cross-references in info mode."
 :group 'help
 :version "20.3"
 :type 'boolean)

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

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

(defvar help-xref-symbol-regexp
  (concat "\\(\\<\\(\\(variable\\|option\\)\\|"
          "\\(function\\|command\\)\\|"
          "\\(symbol\\)\\)\\s-+\\)?"
          ;; Note starting with word-syntax character:
          "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'")
  "Regexp matching doc string references to symbols.

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

(defvar help-xref-info-regexp
  "\\<info\\s-+node\\s-`\\([^']+\\)'"
  "Regexp matching doc string references to an Info node.")

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

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

(defun help-make-xrefs (&optional buffer)
  "Parse and hyperlink documentation cross-references in the given BUFFER.

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

A special reference `back' is made to return back through a stack of
help buffers.  Variable `help-back-label' specifies the text for
that."
  (interactive "b")
  (save-excursion
    (set-buffer (or buffer (current-buffer)))
    (goto-char (point-min))
    ;; Skip the header-type info, though it might be useful to parse
    ;; it at some stage (e.g. "function in `library'").
    (forward-paragraph)
    (let ((old-modified (buffer-modified-p)))
      (let ((stab (syntax-table))
            (case-fold-search t)
            (inhibit-read-only t))
        (set-syntax-table emacs-lisp-mode-syntax-table)
        ;; The following should probably be abstracted out.
        (unwind-protect
            (progn
              ;; Quoted symbols
              (save-excursion
                (while (re-search-forward help-xref-symbol-regexp nil t)
                  (let* ((data (match-string 6))
                         (sym (intern-soft data)))
                    (if sym
                        (cond
                         ((match-string 3) ; `variable' &c
                          (and (boundp sym) ; `variable' doesn't ensure
                                        ; it's actually bound
                               (help-xref-button 6 #'describe-variable sym)))
                         ((match-string 4) ; `function' &c
                          (and (fboundp sym) ; similarly
                               (help-xref-button 6 #'describe-function sym)))
                         ((match-string 5)) ; nothing for symbol
                         ((and (boundp sym) (fboundp sym))
                          ;; We can't intuit whether to use the
                          ;; variable or function doc -- supply both.
                          (help-xref-button 6 #'help-xref-interned sym))
                         ((boundp sym)
                          (help-xref-button 6 #'describe-variable sym))
                         ((fboundp sym)
                          (help-xref-button 6 #'describe-function sym)))))))
              ;; Info references
              (save-excursion
                (while (re-search-forward help-xref-info-regexp nil t)
                  (help-xref-button 1 #'Info-goto-node (list (match-data 1)))))
              ;; An obvious case of a key substitution:
              (save-excursion              
                (while (re-search-forward 
                        "\\<M-x\\s-+\\(\\sw\\(\\sw\\|\\s_\\)+\\)" nil t)
                  (let ((sym (intern-soft (match-string 1))))
                    (if (fboundp sym)
                        (help-xref-button 1 #'describe-function sym)))))
              ;; Look for commands in whole keymap substitutions:
              (save-excursion
                ;; Find a header and the column at which the command
                ;; name will be found.
                (while (re-search-forward "^key +binding\n\\(-+ +\\)-+\n\n" 
                                          nil t)
                  (let ((col (- (match-end 1) (match-beginning 1))))
                    (while
                        ;; Ignore single blank lines in table, but not
                        ;; double ones, which should terminate it.
                        (and (looking-at "^\n?[^\n]")
                             (progn
                               (if (and (> (move-to-column col) 0)
                                        (looking-at "\\(\\sw\\|\\s_\\)+$"))
                                   ;; 
                                   (let ((sym (intern-soft (match-string 0))))
                                     (if (fboundp sym)
                                         (help-xref-button 
                                          0 #'describe-function sym))))
                               t)
                             (zerop (forward-line))
                             (move-to-column 0)))))))
          (set-syntax-table stab))
        ;; Make a back-reference in this buffer if appropriate.
        (when help-xref-stack
          (goto-char (point-max))
          (save-excursion
            (insert "\n\n" help-back-label))
          ;; Just to provide the match data:
          (looking-at (concat "\n\n\\(" (regexp-quote help-back-label) "\\)"))
          (help-xref-button 1 #'help-xref-go-back (current-buffer))))
      ;; View mode steals RET from us.
      (set (make-local-variable 'minor-mode-overriding-map-alist)
           (list (cons 'view-mode
                       (let ((map (make-sparse-keymap)))
                         (set-keymap-parent map view-mode-map)
                         (define-key map "\r" 'help-follow)
                         map))))
      (set-buffer-modified-p old-modified))))

(defun help-xref-button (match-number function data)
  "Make a hyperlink for cross-reference text previously matched.

MATCH-NUMBER is the subexpression of interest in the last matched
regexp.  FUNCTION is a function to invoke when the button is
activated, applied to DATA.  DATA may be a single value or a list.
See `help-make-xrefs'."
  (add-text-properties (match-beginning match-number)
                     (match-end match-number)
                       (list 'mouse-face 'highlight  
                     'help-xref (cons function
                                      (if (listp data)
                                          data
                                        (list data)))))
  (if help-highlight-p
      (put-text-property (match-beginning match-number)
                         (match-end match-number)
                         'face help-highlight-face)))


;; Additional functions for (re-)creating types of help buffers.
(defun help-xref-interned (symbol)
  "Follow a hyperlink which appeared to be an arbitrary interned SYMBOL.

Both variable and function documentation are extracted into a single
help buffer."
  (let ((fdoc (describe-function symbol)))
    (describe-variable symbol)
    ;; We now have a help buffer on the variable.  Insert the function
    ;; text after it.
    (goto-char (point-max))
    (insert "\n\n" fdoc))
  (goto-char (point-min))
  (help-setup-xref (list #'help-xref-interned symbol) nil))

(defun help-xref-mode (buffer)
  "Do a `describe-mode' for the specified BUFFER."
  (save-excursion
    (set-buffer buffer)
    (describe-mode)))

;;; Navigation/hyperlinking with xrefs

(defun help-follow-mouse (click)
  "Follow the cross-reference that you click on."
  (interactive "e")
  (save-excursion
    (let* ((start (event-start click))
	   (window (car start))
	   (pos (car (cdr start))))
      (set-buffer (window-buffer window))
      (help-follow pos))))

(defun help-xref-go-back (buffer)
  "Go back to the previous help buffer text using info on `help-xref-stack'."
  (interactive)
  (let (item position method args)
    (with-current-buffer buffer
      (when help-xref-stack
	(setq help-xref-stack (cdr help-xref-stack)) ; due to help-follow
	(setq item (car help-xref-stack)
	      position (car item)
	      method (cadr item)
	      args (cddr item))
	(setq help-xref-stack (cdr help-xref-stack))))
    (apply method args)
    (goto-char position)))

(defun help-go-back ()
  (interactive)
  (help-follow (1- (point-max))))

(defun help-follow (&optional pos)
  "Follow cross-reference at POS, defaulting to point.

For the cross-reference format, see `help-make-xrefs'."
  (interactive "d")
  (let* ((help-data (or (and (not (= pos (point-max)))
			     (get-text-property pos 'help-xref))
			(and (not (= pos (point-min)))
			     (get-text-property (1- pos) 'help-xref))))
         (method (car help-data))
         (args (cdr help-data)))
    (setq help-xref-stack (cons (cons (point) help-xref-stack-item)
				help-xref-stack))
    (setq help-xref-stack-item nil)
    (when help-data
      ;; There is a reference at point.  Follow it.
      (apply method args))))

;; For tabbing through buffer.
(defun help-next-ref ()
  "Find the next help cross-reference in the buffer."
  (interactive)
  (let (pos)
    (while (not pos) 
      (if (get-text-property (point) 'help-xref) ; move off reference
	   (goto-char (or (next-single-property-change (point) 'help-xref)
                          (point))))
      (cond ((setq pos (next-single-property-change (point) 'help-xref))
	     (if pos (goto-char pos)))
	    ((bobp)
	     (message "No cross references in the buffer.")
	     (setq pos t))
	    (t				; be circular
	     (goto-char (point-min)))))))

(defun help-previous-ref ()
  "Find the previous help cross-reference in the buffer."
  (interactive)
  (let (pos)
    (while (not pos) 
      (if (get-text-property (point) 'help-xref) ; move off reference
	  (goto-char (or (previous-single-property-change (point) 'help-xref)
                         (point))))
      (cond ((setq pos (previous-single-property-change (point) 'help-xref))
	     (if pos (goto-char pos)))
	    ((bobp)
	     (message "No cross references in the buffer.")
	     (setq pos t))
	    (t				; be circular
	     (goto-char (point-max)))))))

;;; help.el ends here