24.5 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 section `Help' in The GNU Emacs Manual. 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 (see section 34.2 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 24.2 Access to Documentation Strings.

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 (see section 20.9 Minibuffer Miscellany).

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.