elisp: Help Functions
23.5 Help Functions
===================
Emacs provides a variety of built-in help functions, all accessible to
the user as subcommands of the prefix ‘C-h’. For more information about
them, see 
Help (emacs)Help. Here we describe some program-level
interfaces to the same information.
-- Command: apropos pattern &optional do-all
This function finds all meaningful symbols whose names contain a
match for the apropos pattern 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:
(SYMBOL SCORE FUNCTION-DOC VARIABLE-DOC
PLIST-DOC WIDGET-DOC FACE-DOC GROUP-DOC)
Here, SCORE is an integer measure of how important the symbol seems
to be as a match. Each of the remaining elements is a
documentation string, or ‘nil’, for SYMBOL as a function, variable,
etc.
It also displays the symbols in a buffer named ‘*Apropos*’, each
with a one-line description taken from the beginning of its
documentation string.
If DO-ALL is non-‘nil’, or if the user option ‘apropos-do-all’ is
non-‘nil’, then ‘apropos’ also shows key bindings for the functions
that are found; it also shows _all_ interned symbols, not just
meaningful ones (and it lists them in the return value as well).
-- Variable: help-map
The value of this variable is a local keymap for characters
following the Help key, ‘C-h’.
-- Prefix Command: help-command
This symbol is not a function; its function definition cell holds
the keymap known as ‘help-map’. It is defined in ‘help.el’ as
follows:
(define-key global-map (string help-char) 'help-command)
(fset 'help-command help-map)
-- User Option: help-char
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 ‘C-h’. When Emacs reads this character, if
‘help-form’ is a non-‘nil’ Lisp expression, it evaluates that
expression, and displays the result in a window if it is a string.
Usually the value of ‘help-form’ is ‘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 ‘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
‘describe-prefix-bindings’, which displays a list of all the
subcommands of the prefix key.
-- User Option: help-event-list
The value of this variable is a list of event types that serve as
alternative help characters. These events are handled just like
the event specified by ‘help-char’.
-- Variable: help-form
If this variable is non-‘nil’, its value is a form to evaluate
whenever the character ‘help-char’ is read. If evaluating the form
produces a string, that string is displayed.
A command that calls ‘read-event’, ‘read-char-choice’, or
‘read-char’ probably should bind ‘help-form’ to a non-‘nil’
expression while it does input. (The time when you should not do
this is when ‘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.
Entry to the minibuffer binds this variable to the value of
‘minibuffer-help-form’ (Definition of
minibuffer-help-form).
-- Variable: 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
‘describe-prefix-bindings’.
-- Command: describe-prefix-bindings
This function calls ‘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.)
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 ‘Helper’ to distinguish them from the ordinary help
functions.
-- 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 ‘describe-bindings’.
-- Command: Helper-help
This command provides help for the current mode. It prompts the
user in the minibuffer with the message ‘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 ‘nil’.
This can be customized by changing the map ‘Helper-help-map’.
-- Variable: data-directory
This variable holds the name of the directory in which Emacs finds
certain documentation and text files that come with Emacs.
-- Function: help-buffer
This function returns the name of the help buffer, which is
normally ‘*Help*’; if such a buffer does not exist, it is first
created.
-- Macro: with-help-window buffer-name body...
This macro evaluates BODY like ‘with-output-to-temp-buffer’ (
Temporary Displays), inserting any output produced by its forms
into a buffer named BUFFER-NAME. (Usually, BUFFER-NAME should be
the value returned by the function ‘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
‘help-window-select’ has been set accordingly. It returns the last
value in BODY.
-- Function: help-setup-xref item interactive-p
This function updates the cross reference data in the ‘*Help*’
buffer, which is used to regenerate the help information when the
user clicks on the ‘Back’ or ‘Forward’ buttons. Most commands that
use the ‘*Help*’ buffer should invoke this function before clearing
the buffer. The ITEM argument should have the form ‘(FUNCTION .
ARGS)’, where FUNCTION is a function to call, with argument list
ARGS, to regenerate the help buffer. The INTERACTIVE-P argument is
non-‘nil’ if the calling command was invoked interactively; in that
case, the stack of items for the ‘*Help*’ buffer’s ‘Back’ buttons
is cleared.
describe-symbols example, for an example of using
‘help-buffer’, ‘with-help-window’, and ‘help-setup-xref’.
-- Macro: make-help-screen fname help-line help-text help-map
This macro defines a help command named FNAME that acts like a
prefix key that shows a list of the subcommands it offers.
When invoked, FNAME displays HELP-TEXT in a window, then reads and
executes a key sequence according to HELP-MAP. The string
HELP-TEXT should describe the bindings available in HELP-MAP.
The command FNAME is defined to handle a few events itself, by
scrolling the display of HELP-TEXT. When 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 HELP-MAP, it executes that key’s binding and then
returns.
The argument HELP-LINE should be a single-line summary of the
alternatives in HELP-MAP. In the current version of Emacs, this
argument is used only if you set the option ‘three-step-help’ to
‘t’.
This macro is used in the command ‘help-for-help’ which is the
binding of ‘C-h C-h’.
-- User Option: three-step-help
If this variable is non-‘nil’, commands defined with
‘make-help-screen’ display their HELP-LINE strings in the echo area
at first, and display the longer HELP-TEXT strings only if the user
types the help character again.
Info Catalog
elisp: Describing Characters
elisp: Documentation