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 you have one, because that looks odd when you use C-h f
(describe-function) or C-h v (describe-variable).
See section Tips for Documentation Strings.
Documentation strings may 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.)
Within the Lisp world, a documentation string is kept with the function or variable that it describes:
documentation knows how to extract it.
variable-documentation. The
function documentation-property knows how to extract 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'. The data structure inside
Emacs has an integer offset into the file, where the documentation
string ought to be. The functions documentation and
documentation-property read the documentation from the file
`emacs/etc/DOC-version' when they notice the integer there;
this is transparent to the user. Keeping the documentation strings out
of the Emacs core image saves a significant amount of space.
See section Building Emacs.
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.c' and `digest-doc.c'.
substitute-command-keys to substitute actual
key bindings. (This substitution is not done if verbatim is
non-nil; the verbatim argument exists only as of Emacs 19.)
(documentation-property 'command-line-processed
'variable-documentation)
=> "t once command line has been processed"
(symbol-plist 'command-line-processed)
=> (variable-documentation 188902)
In addition, documentation runs substitute-command-keys
on the resulting string, so the value contains the actual (current) key
bindings. (This is not done if verbatim is non-nil; the
verbatim argument exists only as of Emacs 19.)
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 C-x C-n, or nil. set-goal-column Command: 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 finds the file filename in the `emacs/etc' directory.
When the dumped Emacs is later executed, the same file is found 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
LFD 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 LFD 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"
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.
If predicate is non-nil, it should be a function to be
called on each symbol that has matched regexp. Only symbols for
which predicate returns a non-nil value are listed or
displayed.
In the first of the following examples, apropos finds all the
symbols with names containing `exec'. In the second example, it
finds and returns only those symbols that are also commands.
(We don't show 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)
(apropos "exec" nil 'commandp)
=> (Buffer-menu-execute execute-extended-command)
The command C-h a (command-apropos) calls apropos,
but specifies a predicate to restrict the output to symbols that
are commands. The call to apropos looks like this:
(apropos string t 'commandp)
apropos in that it searches
documentation strings as well as symbol names for matches for
regexp. By default, it searches the documentation strings only
for preloaded functions and variables. If do-all is
non-nil, it scans the names and documentation strings of all
functions and variables.
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
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's value 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.
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 exception is when C-h is meaningful input.)
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 found in the library `helper'.
They are for modes that want to provide help without relinquishing
control, such as the "electric" modes. You must load that library
with (require 'helper) in order to use them. 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.
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.