Help Functions
==============
Emacs provides a variety of on-line help functions, all accessible to
the user as subcommands of the prefix `C-h'. For more information
about them, see Note:Help. Here we describe some
program-level interfaces to the same information.
- Command: apropos regexp &optional do-all
This function finds all symbols whose names contain a match for the
regular expression REGEXP, and returns a list of them (Note:Regular Expressions). It also displays the symbols in a buffer
named `*Help*', each with a one-line description taken from the
beginning of its documentation string.
If DO-ALL is non-`nil', then `apropos' also shows key bindings for
the functions that are found; it also shows all symbols, even
those that are neither functions nor variables.
In the first of the following examples, `apropos' finds all the
symbols with names containing `exec'. (We don't show here the
output that results in the `*Help*' buffer.)
(apropos "exec")
=> (Buffer-menu-execute command-execute exec-directory
exec-path execute-extended-command execute-kbd-macro
executing-kbd-macro executing-macro)
- 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 "\C-h" 'help-command)
(fset 'help-command help-map)
- Function: print-help-return-message &optional function
This function builds a string that explains how to restore the
previous state of the windows after a help command. After
building the message, it applies FUNCTION to it if FUNCTION is
non-`nil'. Otherwise it calls `message' to display it in the echo
area.
This function expects to be called inside a
`with-output-to-temp-buffer' special form, and expects
`standard-output' to have the value bound by that special form.
For an example of its use, see the long example in Note:Accessing
Documentation.
- Variable: 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.
- Variable: 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' 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' (Note:Minibuffer Misc).
- 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'.
- Function: 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. In
older Emacs versions, `exec-directory' was used for this.
- 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.