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24. Documentation

GNU Emacs Lisp has convenient on-line help facilities, most of which derive their information from the documentation strings associated with functions and variables. This chapter describes how to write good documentation strings for your Lisp programs, as well as how to write programs to access documentation.

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.

24.1 Documentation Basics  Good style for doc strings. Where to put them. How Emacs stores them.
24.2 Access to Documentation Strings  How Lisp programs can access doc strings.
24.3 Substituting Key Bindings in Documentation  Substituting current key bindings.
24.4 Describing Characters for Help Messages  Making printable descriptions of non-printing characters and key sequences.
24.5 Help Functions  Subroutines used by Emacs help facilities.

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24.1 Documentation Basics

A documentation string is written using the Lisp syntax for strings, with double-quote characters surrounding the text of the string. This is because it really is a Lisp string object. The string serves as documentation when it is written in the proper place in the definition of a function or variable. In a function definition, the documentation string follows the argument list. In a variable definition, the documentation string follows the initial value of the variable.

When you write a documentation string, make the first line a complete sentence (or two complete sentences) since some commands, such as apropos, show only the first line of a multi-line documentation string. Also, you should not indent the second line of a documentation string, if it has one, because that looks odd when you use C-h f (describe-function) or C-h v (describe-variable) to view the documentation string. See section D.3 Tips for Documentation Strings.

Documentation strings can contain several special substrings, which stand for key bindings to be looked up in the current keymaps when the documentation is displayed. This allows documentation strings to refer to the keys for related commands and be accurate even when a user rearranges the key bindings. (See section 24.2 Access to Documentation Strings.)

In Emacs Lisp, a documentation string is accessible through the function or variable that it describes:

To save space, the documentation for preloaded functions and variables (including primitive functions and autoloaded functions) is stored in the file `emacs/etc/DOC-version'---not inside Emacs. The documentation strings for functions and variables loaded during the Emacs session from byte-compiled files are stored in those files (see section 16.3 Documentation Strings and Compilation).

The data structure inside Emacs has an integer offset into the file, or a list containing a file name and an integer, in place of the documentation string. The functions documentation and documentation-property use that information to fetch the documentation string from the appropriate file; this is transparent to the user.

For information on the uses of documentation strings, see section `Help' in The GNU Emacs Manual.

The `emacs/lib-src' directory contains two utilities that you can use to print nice-looking hardcopy for the file `emacs/etc/DOC-version'. These are `sorted-doc' and `digest-doc'.

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24.2 Access to Documentation Strings

Function: documentation-property symbol property &optional verbatim
This function returns the documentation string that is recorded in symbol's property list under property property. It retrieves the text from a file if the value calls for that. If the property value isn't nil, isn't a string, and doesn't refer to text in a file, then it is evaluated to obtain a string.

Finally, documentation-property passes the string through substitute-command-keys to substitute actual key bindings, unless verbatim is non-nil.

(documentation-property 'command-line-processed
     => "Non-nil once command line has been processed"
(symbol-plist 'command-line-processed)
     => (variable-documentation 188902)

Function: documentation function &optional verbatim
This function returns the documentation string of function.

If function is a symbol, this function first looks for the function-documentation property of that symbol; if that has a non-nil value, the documentation comes from that value (if the value is not a string, it is evaluated). If function is not a symbol, or if it has no function-documentation property, then documentation extracts the documentation string from the actual function definition, reading it from a file if called for.

Finally, unless verbatim is non-nil, it calls substitute-command-keys so as to return a value containing the actual (current) key bindings.

The function documentation signals a void-function error if function has no function definition. However, it is OK if the function definition has no documentation string. In that case, documentation returns nil.

Here is an example of using the two functions, documentation and documentation-property, to display the documentation strings for several symbols in a `*Help*' buffer.

(defun describe-symbols (pattern)
  "Describe the Emacs Lisp symbols matching PATTERN.
All symbols that have PATTERN in their name are described
in the `*Help*' buffer."
  (interactive "sDescribe symbols matching: ")
  (let ((describe-func
          (lambda (s)
            ;; Print description of symbol.
            (if (fboundp s)             ; It is a function.
                 (format "%s\t%s\n%s\n\n" s
                   (if (commandp s) 
                       (let ((keys (where-is-internal s)))
                         (if keys
                              "Keys: "
                              (mapconcat 'key-description 
                                         keys " "))
                           "Keys: none"))
                   (or (documentation s) 
                       "not documented"))))
            (if (boundp s)              ; It is a variable.
                 (format "%s\t%s\n%s\n\n" s
                   (if (user-variable-p s) 
                       "Option " "Variable")
                   (or (documentation-property 
                         s 'variable-documentation)
                       "not documented")))))))

    ;; 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))))))

    ;; Display the data.
    (with-output-to-temp-buffer "*Help*"
      (mapcar describe-func (sort sym-list 'string<))

The describe-symbols function works like apropos, but provides more information.

(describe-symbols "goal")

---------- Buffer: *Help* ----------
goal-column     Option 
*Semipermanent goal column for vertical motion, as set by ...

set-goal-column Keys: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
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'.

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* ----------

The asterisk `*' as the first character of a variable's doc string, as shown above for the goal-column variable, means that it is a user option; see the description of defvar in 11.5 Defining Global Variables.

Function: Snarf-documentation filename
This function is used only during Emacs initialization, just before the runnable Emacs is dumped. It finds the file offsets of the documentation strings stored in the file filename, and records them in the in-core function definitions and variable property lists in place of the actual strings. See section E.1 Building Emacs.

Emacs reads the file filename from the `emacs/etc' directory. When the dumped Emacs is later executed, the same file will be looked for in the directory doc-directory. Usually filename is "DOC-version".

Variable: doc-directory
This variable holds the name of the directory which should contain the file "DOC-version" that contains documentation strings for built-in and preloaded functions and variables.

In most cases, this is the same as data-directory. They may be different when you run Emacs from the directory where you built it, without actually installing it. See data-directory in 24.5 Help Functions.

In older Emacs versions, exec-directory was used for this.

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24.3 Substituting Key Bindings in Documentation

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 substitute-command-keys. You can also call that function yourself.

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

stands for a key sequence that will invoke command, or `M-x command' if command has no key bindings.

stands for a summary of the keymap which is the value of the variable mapvar. The summary is made using describe-bindings.

stands for no text itself. It is used only for a side effect: it specifies mapvar's value as the keymap for any following `\[command]' sequences in this documentation string.

quotes the following character and is discarded; thus, `\=\[' puts `\[' into the output, and `\=\=' puts `\=' into the output.

Please note: Each `\' must be doubled when written in a string in Emacs Lisp.

Function: substitute-command-keys string
This function scans 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.

Here are examples of the special sequences:

   "To abort recursive edit, type: \\[abort-recursive-edit]")
=> "To abort recursive edit, type: C-]"

   "The keys that are defined for the minibuffer here are:
=> "The keys that are defined for the minibuffer here are:

?               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

   "To abort a recursive edit from the minibuffer, type\
=> "To abort a recursive edit from the minibuffer, type C-g."

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24.4 Describing Characters for Help Messages

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.

Function: key-description sequence
This function returns a string containing the Emacs standard notation for the input events in sequence. The argument sequence may be a string, vector or list. See section 21.6 Input Events, for more information about valid events. See also the examples for single-key-description, below.

Function: single-key-description event &optional no-angles
This function returns a string describing 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 `C-', a meta character turns into a string starting with `M-', and space, tab, etc. appear as `SPC', `TAB', etc. A function key symbol appears inside angle brackets `<...>'. An event that is a list appears as the name of the symbol in the CAR of the list, inside angle brackets.

If the optional argument no-angles is non-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.

(single-key-description ?\C-x)
     => "C-x"
(key-description "\C-x \M-y \n \t \r \f123")
     => "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3"
(single-key-description 'delete)
     => "<delete>"
(single-key-description 'C-mouse-1)
     => "<C-mouse-1>"
(single-key-description 'C-mouse-1 t)
     => "C-mouse-1"

Function: text-char-description character
This function returns a string describing character in the standard Emacs notation for characters that appear in text--like single-key-description, except that control characters are represented with a leading caret (which is how control characters in Emacs buffers are usually displayed).

(text-char-description ?\C-c)
     => "^C"
(text-char-description ?\M-m)
     => "M-m"
(text-char-description ?\C-\M-m)
     => "M-^M"

Function: read-kbd-macro string
This function is used mainly for operating on keyboard macros, but it can also be used as a rough inverse for 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; see section 22.1 Keymap Terminology.)

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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.

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