help.texi 27.2 KB
Newer Older
Glenn Morris's avatar
Glenn Morris committed
1 2
@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
Paul Eggert's avatar
Paul Eggert committed
3
@c Copyright (C) 1990-1995, 1998-1999, 2001-2015 Free Software
4
@c Foundation, Inc.
Glenn Morris's avatar
Glenn Morris committed
5
@c See the file elisp.texi for copying conditions.
6
@node Documentation
Glenn Morris's avatar
Glenn Morris committed
7 8 9
@chapter Documentation
@cindex documentation strings

10 11 12
  GNU Emacs has convenient built-in help facilities, most of which
derive their information from documentation strings associated with
functions and variables.  This chapter describes how to access
13 14 15 16 17 18 19
documentation strings in Lisp programs.

  The contents of a documentation string should follow certain
conventions.  In particular, its first line should be a complete
sentence (or two complete sentences) that briefly describes what the
function or variable does.  @xref{Documentation Tips}, for how to
write good documentation strings.
Glenn Morris's avatar
Glenn Morris committed
20 21 22 23 24 25 26 27 28 29

  Note that the documentation strings for Emacs are not the same thing
as the Emacs manual.  Manuals have their own source files, written in
the Texinfo language; documentation strings are specified in the
definitions of the functions and variables they apply to.  A collection
of documentation strings is not sufficient as a manual because a good
manual is not organized in that fashion; it is organized in terms of
topics of discussion.

  For commands to display documentation strings, see @ref{Help, ,
30
Help, emacs, The GNU Emacs Manual}.
Glenn Morris's avatar
Glenn Morris committed
31 32

@menu
33
* Documentation Basics::      Where doc strings are defined and stored.
Glenn Morris's avatar
Glenn Morris committed
34 35 36 37 38 39 40 41 42 43 44 45 46 47
* Accessing Documentation::   How Lisp programs can access doc strings.
* Keys in Documentation::     Substituting current key bindings.
* Describing Characters::     Making printable descriptions of
                                non-printing characters and key sequences.
* Help Functions::            Subroutines used by Emacs help facilities.
@end menu

@node Documentation Basics
@section Documentation Basics
@cindex documentation conventions
@cindex writing a documentation string
@cindex string, writing a doc string

  A documentation string is written using the Lisp syntax for strings,
48 49 50 51 52 53 54 55 56 57 58 59 60 61
with double-quote characters surrounding the text.  It is, in fact, an
actual Lisp string.  When the string appears in the proper place in a
function or variable definition, it serves as the function's or
variable's documentation.

@cindex @code{function-documentation} property
  In a function definition (a @code{lambda} or @code{defun} form), the
documentation string is specified after the argument list, and is
normally stored directly in the function object.  @xref{Function
Documentation}.  You can also put function documentation in the
@code{function-documentation} property of a function name
(@pxref{Accessing Documentation}).

@cindex @code{variable-documentation} property
Paul Eggert's avatar
Paul Eggert committed
62
  In a variable definition (a @code{defvar} form), the documentation
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
string is specified after the initial value.  @xref{Defining
Variables}.  The string is stored in the variable's
@code{variable-documentation} property.

@cindex @file{DOC} (documentation) file
  Sometimes, Emacs does not keep documentation strings in memory.
There are two such circumstances.  Firstly, to save memory, the
documentation for preloaded functions and variables (including
primitives) is kept in a file named @file{DOC}, in the directory
specified by @code{doc-directory} (@pxref{Accessing Documentation}).
Secondly, when a function or variable is loaded from a byte-compiled
file, Emacs avoids loading its documentation string (@pxref{Docs and
Compilation}).  In both cases, Emacs looks up the documentation string
from the file only when needed, such as when the user calls @kbd{C-h
f} (@code{describe-function}) for a function.

  Documentation strings can contain special @dfn{key substitution
sequences}, referring to key bindings which are looked up only when
the user views the documentation.  This allows the help commands to
display the correct keys even if a user rearranges the default key
83 84 85
bindings.  @xref{Keys in Documentation}.

  In the documentation string of an autoloaded command
86 87 88 89
(@pxref{Autoload}), these key-substitution sequences have an
additional special effect: they cause @kbd{C-h f} on the command to
trigger autoloading.  (This is needed for correctly setting up the
hyperlinks in the @file{*Help*} buffer.)
Glenn Morris's avatar
Glenn Morris committed
90 91 92

@node Accessing Documentation
@section Access to Documentation Strings
93
@cindex accessing documentation strings
Glenn Morris's avatar
Glenn Morris committed
94 95

@defun documentation-property symbol property &optional verbatim
96 97 98 99 100 101
This function returns the documentation string recorded in
@var{symbol}'s property list under property @var{property}.  It is
most often used to look up the documentation strings of variables, for
which @var{property} is @code{variable-documentation}.  However, it
can also be used to look up other kinds of documentation, such as for
customization groups (but for function documentation, use the
102 103 104 105 106
@code{documentation} function, below).

If the property value refers to a documentation string stored in the
@file{DOC} file or a byte-compiled file, this function looks up that
string and returns it.
107

108 109 110
If the property value isn't @code{nil}, isn't a string, and doesn't
refer to text in a file, then it is evaluated as a Lisp expression to
obtain a string.
Glenn Morris's avatar
Glenn Morris committed
111

112 113 114 115
Finally, this function passes the string through
@code{substitute-command-keys} to substitute key bindings (@pxref{Keys
in Documentation}).  It skips this step if @var{verbatim} is
non-@code{nil}.
Glenn Morris's avatar
Glenn Morris committed
116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134

@smallexample
@group
(documentation-property 'command-line-processed
   'variable-documentation)
     @result{} "Non-nil once command line has been processed"
@end group
@group
(symbol-plist 'command-line-processed)
     @result{} (variable-documentation 188902)
@end group
@group
(documentation-property 'emacs 'group-documentation)
     @result{} "Customization of the One True Editor."
@end group
@end smallexample
@end defun

@defun documentation function &optional verbatim
135 136 137
This function returns the documentation string of @var{function}.  It
handles macros, named keyboard macros, and special forms, as well as
ordinary functions.
Glenn Morris's avatar
Glenn Morris committed
138 139 140 141

If @var{function} is a symbol, this function first looks for the
@code{function-documentation} property of that symbol; if that has a
non-@code{nil} value, the documentation comes from that value (if the
142 143 144 145 146 147
value is not a string, it is evaluated).

If @var{function} is not a symbol, or if it has no
@code{function-documentation} property, then @code{documentation}
extracts the documentation string from the actual function definition,
reading it from a file if called for.
Glenn Morris's avatar
Glenn Morris committed
148

149 150 151
Finally, unless @var{verbatim} is non-@code{nil}, this function calls
@code{substitute-command-keys}.  The result is the documentation
string to return.
Glenn Morris's avatar
Glenn Morris committed
152

153
The @code{documentation} function signals a @code{void-function} error
Glenn Morris's avatar
Glenn Morris committed
154 155 156 157 158 159 160 161 162 163 164 165
if @var{function} has no function definition.  However, it is OK if
the function definition has no documentation string.  In that case,
@code{documentation} returns @code{nil}.
@end defun

@defun face-documentation face
This function returns the documentation string of @var{face} as a
face.
@end defun

Here is an example of using the two functions, @code{documentation} and
@code{documentation-property}, to display the documentation strings for
166
several symbols in a @file{*Help*} buffer.
Glenn Morris's avatar
Glenn Morris committed
167 168 169 170 171 172 173

@anchor{describe-symbols example}
@smallexample
@group
(defun describe-symbols (pattern)
  "Describe the Emacs Lisp symbols matching PATTERN.
All symbols that have PATTERN in their name are described
174
in the *Help* buffer."
Glenn Morris's avatar
Glenn Morris committed
175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203
  (interactive "sDescribe symbols matching: ")
  (let ((describe-func
         (function
          (lambda (s)
@end group
@group
            ;; @r{Print description of symbol.}
            (if (fboundp s)             ; @r{It is a function.}
                (princ
                 (format "%s\t%s\n%s\n\n" s
                   (if (commandp s)
                       (let ((keys (where-is-internal s)))
                         (if keys
                             (concat
                              "Keys: "
                              (mapconcat 'key-description
                                         keys " "))
                           "Keys: none"))
                     "Function")
@end group
@group
                   (or (documentation s)
                       "not documented"))))

            (if (boundp s)              ; @r{It is a variable.}
@end group
@group
                (princ
                 (format "%s\t%s\n%s\n\n" s
204
                   (if (custom-variable-p s)
Glenn Morris's avatar
Glenn Morris committed
205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223
                       "Option " "Variable")
@end group
@group
                   (or (documentation-property
                         s 'variable-documentation)
                       "not documented")))))))
        sym-list)
@end group

@group
    ;; @r{Build a list of symbols that match pattern.}
    (mapatoms (function
               (lambda (sym)
                 (if (string-match pattern (symbol-name sym))
                     (setq sym-list (cons sym sym-list))))))
@end group

@group
    ;; @r{Display the data.}
224 225 226
    (help-setup-xref (list 'describe-symbols pattern) (interactive-p))
    (with-help-window (help-buffer)
      (mapcar describe-func (sort sym-list 'string<)))))
Glenn Morris's avatar
Glenn Morris committed
227 228 229 230 231 232 233 234 235 236 237 238
@end group
@end smallexample

  The @code{describe-symbols} function works like @code{apropos},
but provides more information.

@smallexample
@group
(describe-symbols "goal")

---------- Buffer: *Help* ----------
goal-column     Option
239
Semipermanent goal column for vertical motion, as set by @dots{}
Glenn Morris's avatar
Glenn Morris committed
240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267
@end group
@c Do not blithely break or fill these lines.
@c That makes them incorrect.

@group
set-goal-column Keys: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
@end group
@c DO NOT put a blank line here!  That is factually inaccurate!
@group
Those commands will move to this position in the line moved to
rather than trying to keep the same horizontal position.
With a non-nil argument, clears out the goal column
so that C-n and C-p resume vertical motion.
The goal column is stored in the variable `goal-column'.
@end group

@group
temporary-goal-column   Variable
Current goal column for vertical motion.
It is the column where point was
at the start of current run of vertical motion commands.
When the `track-eol' feature is doing its job, the value is 9999.
---------- Buffer: *Help* ----------
@end group
@end smallexample

@anchor{Definition of Snarf-documentation}
268 269 270 271 272 273
@defun Snarf-documentation filename
This function is used when building Emacs, just before the runnable
Emacs is dumped.  It finds the positions of the documentation strings
stored in the file @var{filename}, and records those positions into
memory in the function definitions and variable property lists.
@xref{Building Emacs}.
Glenn Morris's avatar
Glenn Morris committed
274 275 276 277

Emacs reads the file @var{filename} from the @file{emacs/etc} directory.
When the dumped Emacs is later executed, the same file will be looked
for in the directory @code{doc-directory}.  Usually @var{filename} is
278
@code{"DOC"}.
Glenn Morris's avatar
Glenn Morris committed
279 280 281 282
@end defun

@defvar doc-directory
This variable holds the name of the directory which should contain the
283
file @code{"DOC"} that contains documentation strings for
Glenn Morris's avatar
Glenn Morris committed
284 285 286 287 288 289 290 291 292 293 294 295
built-in and preloaded functions and variables.

In most cases, this is the same as @code{data-directory}.  They may be
different when you run Emacs from the directory where you built it,
without actually installing it.  @xref{Definition of data-directory}.
@end defvar

@node Keys in Documentation
@section Substituting Key Bindings in Documentation
@cindex documentation, keys in
@cindex keys in documentation strings
@cindex substituting keys in documentation
296
@cindex key substitution sequence
Glenn Morris's avatar
Glenn Morris committed
297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321

  When documentation strings refer to key sequences, they should use the
current, actual key bindings.  They can do so using certain special text
sequences described below.  Accessing documentation strings in the usual
way substitutes current key binding information for these special
sequences.  This works by calling @code{substitute-command-keys}.  You
can also call that function yourself.

  Here is a list of the special sequences and what they mean:

@table @code
@item \[@var{command}]
stands for a key sequence that will invoke @var{command}, or @samp{M-x
@var{command}} if @var{command} has no key bindings.

@item \@{@var{mapvar}@}
stands for a summary of the keymap which is the value of the variable
@var{mapvar}.  The summary is made using @code{describe-bindings}.

@item \<@var{mapvar}>
stands for no text itself.  It is used only for a side effect: it
specifies @var{mapvar}'s value as the keymap for any following
@samp{\[@var{command}]} sequences in this documentation string.

@item \=
322 323 324
quotes the following character and is discarded; thus, @samp{\=\[} puts
@samp{\[} into the output, and @samp{\=\=} puts @samp{\=} into the
output.
Glenn Morris's avatar
Glenn Morris committed
325 326 327 328 329 330 331 332 333 334
@end table

@strong{Please note:} Each @samp{\} must be doubled when written in a
string in Emacs Lisp.

@defun substitute-command-keys string
This function scans @var{string} for the above special sequences and
replaces them by what they stand for, returning the result as a string.
This permits display of documentation that refers accurately to the
user's own customized key bindings.
335

336 337 338 339 340
In the returned string, replacement characters that should not be
further transformed have the text property @code{escaped}.  For
example, when @samp{\=`} is replaced by @samp{`} in the output, the
replacement character has the text property @code{escaped}.

341 342 343 344 345 346 347 348 349 350 351 352 353 354
@cindex advertised binding
If a command has multiple bindings, this function normally uses the
first one it finds.  You can specify one particular key binding by
assigning an @code{:advertised-binding} symbol property to the
command, like this:

@smallexample
(put 'undo :advertised-binding [?\C-/])
@end smallexample

@noindent
The @code{:advertised-binding} property also affects the binding shown
in menu items (@pxref{Menu Bar}).  The property is ignored if it
specifies a key binding that the command does not actually have.
Glenn Morris's avatar
Glenn Morris committed
355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435
@end defun

  Here are examples of the special sequences:

@smallexample
@group
(substitute-command-keys
   "To abort recursive edit, type: \\[abort-recursive-edit]")
@result{} "To abort recursive edit, type: C-]"
@end group

@group
(substitute-command-keys
   "The keys that are defined for the minibuffer here are:
  \\@{minibuffer-local-must-match-map@}")
@result{} "The keys that are defined for the minibuffer here are:
@end group

?               minibuffer-completion-help
SPC             minibuffer-complete-word
TAB             minibuffer-complete
C-j             minibuffer-complete-and-exit
RET             minibuffer-complete-and-exit
C-g             abort-recursive-edit
"

@group
(substitute-command-keys
   "To abort a recursive edit from the minibuffer, type\
\\<minibuffer-local-must-match-map>\\[abort-recursive-edit].")
@result{} "To abort a recursive edit from the minibuffer, type C-g."
@end group
@end smallexample

  There are other special conventions for the text in documentation
strings---for instance, you can refer to functions, variables, and
sections of this manual.  @xref{Documentation Tips}, for details.

@node Describing Characters
@section Describing Characters for Help Messages
@cindex describe characters and events

  These functions convert events, key sequences, or characters to
textual descriptions.  These descriptions are useful for including
arbitrary text characters or key sequences in messages, because they
convert non-printing and whitespace characters to sequences of printing
characters.  The description of a non-whitespace printing character is
the character itself.

@defun key-description sequence &optional prefix
@cindex Emacs event standard notation
This function returns a string containing the Emacs standard notation
for the input events in @var{sequence}.  If @var{prefix} is
non-@code{nil}, it is a sequence of input events leading up to
@var{sequence} and is included in the return value.  Both arguments
may be strings, vectors or lists.  @xref{Input Events}, for more
information about valid events.

@smallexample
@group
(key-description [?\M-3 delete])
     @result{} "M-3 <delete>"
@end group
@group
(key-description [delete] "\M-3")
     @result{} "M-3 <delete>"
@end group
@end smallexample

  See also the examples for @code{single-key-description}, below.
@end defun

@defun single-key-description event &optional no-angles
@cindex event printing
@cindex character printing
@cindex control character printing
@cindex meta character printing
This function returns a string describing @var{event} in the standard
Emacs notation for keyboard input.  A normal printing character
appears as itself, but a control character turns into a string
starting with @samp{C-}, a meta character turns into a string starting
436
with @samp{M-}, and space, tab, etc., appear as @samp{SPC},
Glenn Morris's avatar
Glenn Morris committed
437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503
@samp{TAB}, etc.  A function key symbol appears inside angle brackets
@samp{<@dots{}>}.  An event that is a list appears as the name of the
symbol in the @sc{car} of the list, inside angle brackets.

If the optional argument @var{no-angles} is non-@code{nil}, the angle
brackets around function keys and event symbols are omitted; this is
for compatibility with old versions of Emacs which didn't use the
brackets.

@smallexample
@group
(single-key-description ?\C-x)
     @result{} "C-x"
@end group
@group
(key-description "\C-x \M-y \n \t \r \f123")
     @result{} "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3"
@end group
@group
(single-key-description 'delete)
     @result{} "<delete>"
@end group
@group
(single-key-description 'C-mouse-1)
     @result{} "<C-mouse-1>"
@end group
@group
(single-key-description 'C-mouse-1 t)
     @result{} "C-mouse-1"
@end group
@end smallexample
@end defun

@defun text-char-description character
This function returns a string describing @var{character} in the
standard Emacs notation for characters that appear in text---like
@code{single-key-description}, except that control characters are
represented with a leading caret (which is how control characters in
Emacs buffers are usually displayed).  Another difference is that
@code{text-char-description} recognizes the 2**7 bit as the Meta
character, whereas @code{single-key-description} uses the 2**27 bit
for Meta.

@smallexample
@group
(text-char-description ?\C-c)
     @result{} "^C"
@end group
@group
(text-char-description ?\M-m)
     @result{} "\xed"
@end group
@group
(text-char-description ?\C-\M-m)
     @result{} "\x8d"
@end group
@group
(text-char-description (+ 128 ?m))
     @result{} "M-m"
@end group
@group
(text-char-description (+ 128 ?\C-m))
     @result{} "M-^M"
@end group
@end smallexample
@end defun

504
@deffn Command read-kbd-macro string &optional need-vector
Glenn Morris's avatar
Glenn Morris committed
505 506 507 508 509 510 511
This function is used mainly for operating on keyboard macros, but it
can also be used as a rough inverse for @code{key-description}.  You
call it with a string containing key descriptions, separated by spaces;
it returns a string or vector containing the corresponding events.
(This may or may not be a single valid key sequence, depending on what
events you use; @pxref{Key Sequences}.)  If @var{need-vector} is
non-@code{nil}, the return value is always a vector.
512
@end deffn
Glenn Morris's avatar
Glenn Morris committed
513 514 515

@node Help Functions
@section Help Functions
516
@cindex help functions
Glenn Morris's avatar
Glenn Morris committed
517

518
  Emacs provides a variety of built-in help functions, all accessible to
Glenn Morris's avatar
Glenn Morris committed
519 520 521 522 523 524 525 526 527 528 529 530 531 532 533
the user as subcommands of the prefix @kbd{C-h}.  For more information
about them, see @ref{Help, , Help, emacs, The GNU Emacs Manual}.  Here
we describe some program-level interfaces to the same information.

@deffn Command apropos pattern &optional do-all
This function finds all ``meaningful'' symbols whose names contain a
match for the apropos pattern @var{pattern}.  An apropos pattern is
either a word to match, a space-separated list of words of which at
least two must match, or a regular expression (if any special regular
expression characters occur).  A symbol is ``meaningful'' if it has a
definition as a function, variable, or face, or has properties.

The function returns a list of elements that look like this:

@example
Chong Yidong's avatar
Chong Yidong committed
534
(@var{symbol} @var{score} @var{function-doc} @var{variable-doc}
Glenn Morris's avatar
Glenn Morris committed
535 536 537 538
 @var{plist-doc} @var{widget-doc} @var{face-doc} @var{group-doc})
@end example

Here, @var{score} is an integer measure of how important the symbol
539 540 541
seems to be as a match.  Each of the remaining elements is a
documentation string, or @code{nil}, for @var{symbol} as a function,
variable, etc.
Glenn Morris's avatar
Glenn Morris committed
542

543
It also displays the symbols in a buffer named @file{*Apropos*}, each
Glenn Morris's avatar
Glenn Morris committed
544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565
with a one-line description taken from the beginning of its
documentation string.

If @var{do-all} is non-@code{nil}, or if the user option
@code{apropos-do-all} is non-@code{nil}, then @code{apropos} also
shows key bindings for the functions that are found; it also shows
@emph{all} interned symbols, not just meaningful ones (and it lists
them in the return value as well).
@end deffn

@defvar help-map
The value of this variable is a local keymap for characters following the
Help key, @kbd{C-h}.
@end defvar

@deffn {Prefix Command} help-command
This symbol is not a function; its function definition cell holds the
keymap known as @code{help-map}.  It is defined in @file{help.el} as
follows:

@smallexample
@group
566
(define-key global-map (string help-char) 'help-command)
Glenn Morris's avatar
Glenn Morris committed
567 568 569 570 571
(fset 'help-command help-map)
@end group
@end smallexample
@end deffn

572
@defopt help-char
Glenn Morris's avatar
Glenn Morris committed
573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588
The value of this variable is the help character---the character that
Emacs recognizes as meaning Help.  By default, its value is 8, which
stands for @kbd{C-h}.  When Emacs reads this character, if
@code{help-form} is a non-@code{nil} Lisp expression, it evaluates that
expression, and displays the result in a window if it is a string.

Usually the value of @code{help-form} is @code{nil}.  Then the
help character has no special meaning at the level of command input, and
it becomes part of a key sequence in the normal way.  The standard key
binding of @kbd{C-h} is a prefix key for several general-purpose help
features.

The help character is special after prefix keys, too.  If it has no
binding as a subcommand of the prefix key, it runs
@code{describe-prefix-bindings}, which displays a list of all the
subcommands of the prefix key.
589
@end defopt
Glenn Morris's avatar
Glenn Morris committed
590

591
@defopt help-event-list
Glenn Morris's avatar
Glenn Morris committed
592
The value of this variable is a list of event types that serve as
593
alternative ``help characters''.  These events are handled just like the
Glenn Morris's avatar
Glenn Morris committed
594
event specified by @code{help-char}.
595
@end defopt
Glenn Morris's avatar
Glenn Morris committed
596 597 598 599 600 601

@defvar help-form
If this variable is non-@code{nil}, its value is a form to evaluate
whenever the character @code{help-char} is read.  If evaluating the form
produces a string, that string is displayed.

Glenn Morris's avatar
Glenn Morris committed
602 603 604 605 606 607
A command that calls @code{read-event}, @code{read-char-choice}, or
@code{read-char} probably should bind @code{help-form} to a
non-@code{nil} expression while it does input.  (The time when you
should not do this is when @kbd{C-h} has some other meaning.)
Evaluating this expression should result in a string that explains
what the input is for and how to enter it properly.
Glenn Morris's avatar
Glenn Morris committed
608 609 610 611 612 613 614 615 616 617 618 619

Entry to the minibuffer binds this variable to the value of
@code{minibuffer-help-form} (@pxref{Definition of minibuffer-help-form}).
@end defvar

@defvar prefix-help-command
This variable holds a function to print help for a prefix key.  The
function is called when the user types a prefix key followed by the help
character, and the help character has no binding after that prefix.  The
variable's default value is @code{describe-prefix-bindings}.
@end defvar

620
@deffn Command describe-prefix-bindings
Glenn Morris's avatar
Glenn Morris committed
621 622 623 624
This function calls @code{describe-bindings} to display a list of all
the subcommands of the prefix key of the most recent key sequence.  The
prefix described consists of all but the last event of that key
sequence.  (The last event is, presumably, the help character.)
625
@end deffn
Glenn Morris's avatar
Glenn Morris committed
626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643

  The following two functions are meant for modes that want to provide
help without relinquishing control, such as the ``electric'' modes.
Their names begin with @samp{Helper} to distinguish them from the
ordinary help functions.

@deffn Command Helper-describe-bindings
This command pops up a window displaying a help buffer containing a
listing of all of the key bindings from both the local and global keymaps.
It works by calling @code{describe-bindings}.
@end deffn

@deffn Command Helper-help
This command provides help for the current mode.  It prompts the user
in the minibuffer with the message @samp{Help (Type ? for further
options)}, and then provides assistance in finding out what the key
bindings are, and what the mode is intended for.  It returns @code{nil}.

Glenn Morris's avatar
Glenn Morris committed
644
@vindex Helper-help-map
Glenn Morris's avatar
Glenn Morris committed
645 646 647 648 649 650
This can be customized by changing the map @code{Helper-help-map}.
@end deffn

@defvar data-directory
@anchor{Definition of data-directory}
This variable holds the name of the directory in which Emacs finds
651
certain documentation and text files that come with Emacs.
Glenn Morris's avatar
Glenn Morris committed
652 653
@end defvar

654
@defun help-buffer
655
This function returns the name of the help buffer, which is normally
656
@file{*Help*}; if such a buffer does not exist, it is first created.
657
@end defun
658

659
@vindex help-window-select
660
@defmac with-help-window buffer-name body@dots{}
661 662 663 664 665 666 667 668 669
This macro evaluates @var{body} like @code{with-output-to-temp-buffer}
(@pxref{Temporary Displays}), inserting any output produced by its forms
into a buffer named @var{buffer-name}.  (Usually, @var{buffer-name}
should be the value returned by the function @code{help-buffer}.)  It
also puts the specified buffer into Help mode and displays a message
telling the user how to quit and scroll the help window.  It selects the
help window if the current value of the user option
@code{help-window-select} has been set accordingly.  It returns the last
value in @var{body}.
670 671
@end defmac

672
@defun help-setup-xref item interactive-p
673
This function updates the cross reference data in the @file{*Help*}
674 675
buffer, which is used to regenerate the help information when the user
clicks on the @samp{Back} or @samp{Forward} buttons.  Most commands
676
that use the @file{*Help*} buffer should invoke this function before
677
clearing the buffer.  The @var{item} argument should have the form
Juanma Barranquero's avatar
Juanma Barranquero committed
678
@code{(@var{function} . @var{args})}, where @var{function} is a function
679 680 681
to call, with argument list @var{args}, to regenerate the help buffer.
The @var{interactive-p} argument is non-@code{nil} if the calling
command was invoked interactively; in that case, the stack of items
682
for the @file{*Help*} buffer's @samp{Back} buttons is cleared.
683
@end defun
684 685 686 687 688

@xref{describe-symbols example}, for an example of using
@code{help-buffer}, @code{with-help-window}, and
@code{help-setup-xref}.

Glenn Morris's avatar
Glenn Morris committed
689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719
@defmac make-help-screen fname help-line help-text help-map
This macro defines a help command named @var{fname} that acts like a
prefix key that shows a list of the subcommands it offers.

When invoked, @var{fname} displays @var{help-text} in a window, then
reads and executes a key sequence according to @var{help-map}.  The
string @var{help-text} should describe the bindings available in
@var{help-map}.

The command @var{fname} is defined to handle a few events itself, by
scrolling the display of @var{help-text}.  When @var{fname} reads one of
those special events, it does the scrolling and then reads another
event.  When it reads an event that is not one of those few, and which
has a binding in @var{help-map}, it executes that key's binding and
then returns.

The argument @var{help-line} should be a single-line summary of the
alternatives in @var{help-map}.  In the current version of Emacs, this
argument is used only if you set the option @code{three-step-help} to
@code{t}.

This macro is used in the command @code{help-for-help} which is the
binding of @kbd{C-h C-h}.
@end defmac

@defopt three-step-help
If this variable is non-@code{nil}, commands defined with
@code{make-help-screen} display their @var{help-line} strings in the
echo area at first, and display the longer @var{help-text} strings only
if the user types the help character again.
@end defopt