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.
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 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 Access to Documentation Strings.)
In Emacs Lisp, a documentation string is accessible through the function or variable that it describes:
documentation knows how
to extract it.
variable-documentation. The function
documentation-property knows how to retrieve it.
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 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'.
substitute-command-keys to substitute actual key
bindings. (This substitution is not done if verbatim is
non-nil.)
(documentation-property 'command-line-processed
'variable-documentation)
=> "Non-nil once command line has been processed"
(symbol-plist 'command-line-processed)
=> (variable-documentation 188902)
nil) it calls
substitute-command-keys, 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
(function
(lambda (s)
;; Print description of symbol.
(if (fboundp s) ; 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")
(or (documentation s)
"not documented"))))
(if (boundp s) ; It is a variable.
(princ
(format "%s\t%s\n%s\n\n" s
(if (user-variable-p s)
"Option " "Variable")
(or (documentation-property
s 'variable-documentation)
"not documented")))))))
sym-list)
;; 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<))
(print-help-return-message))))
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* ----------
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".
"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 section Help Functions.
In older Emacs versions, exec-directory was used
for this.
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:
\[command]\{mapvar}describe-bindings.
\<mapvar>\=Please note: Each `\' must be doubled when written in a string in Emacs Lisp.
Here are examples of the special sequences:
(substitute-command-keys
"To abort recursive edit, type: \\[abort-recursive-edit]")
=> "To abort recursive edit, type: C-]"
(substitute-command-keys
"The keys that are defined for the minibuffer here are:
\\{minibuffer-local-must-match-map}")
=> "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
"
(substitute-command-keys
"To abort a recursive edit from the minibuffer, type\
\\<minibuffer-local-must-match-map>\\[abort-recursive-edit].")
=> "To abort a recursive edit from the minibuffer, type C-g."
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.
single-key-description, below.
(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 'C-mouse-1)
=> "C-mouse-1"
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"
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 Keymap
Terminology.)
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.
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)
help-map. It is defined in `help.el' as
follows: (define-key global-map "\C-h" 'help-command) (fset 'help-command help-map)
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
section Access to Documentation
Strings.
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.
help-char.
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 Minibuffer Miscellany).
describe-prefix-bindings.
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.
describe-bindings.
nil.
This can be customized by changing the map
Helper-help-map.
exec-directory was used for this.
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.
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.