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XEmacs 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 XEmacs are not the same thing as the XEmacs 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.
1.1 Documentation Basics | Good style for doc strings. Where to put them. How XEmacs stores them. | |
1.2 Access to Documentation Strings | How Lisp programs can access doc strings. | |
1.3 Substituting Key Bindings in Documentation | Substituting current key bindings. | |
1.4 Describing Characters for Help Messages | Making printable descriptions of non-printing characters and key sequences. | |
1.5 Help Functions | Subroutines used by XEmacs help facilities. |
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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
).
@xref{Documentation Tips}.
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 accessible through 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 documentation for functions and variables loaded during the Emacs session from byte-compiled files is stored in those files (@pxref{Docs and Compilation}).
The data structure inside Emacs has an integer offset into the file, or
a list containing a string and an integer, in place of the documentation
string. The functions documentation
and
documentation-property
use that information to read the
documentation from the appropriate file; this is transparent to the
user.
For information on the uses of documentation strings, see Help in The XEmacs Reference 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’.
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This function returns the documentation string that is recorded
symbol’s property list under property property. It
retrieves the text from a file if necessary, and runs
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)
This function returns the documentation string of function. It
reads the text from a file if necessary. Then (unless verbatim is
non-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 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* ----------
This function is used only during XEmacs initialization, just before the runnable XEmacs 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. @xref{Building XEmacs}.
XEmacs 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"
.
This variable holds the name of the directory which should contion 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 XEmacs from the directory where you built it,
without actually installing it. See data-directory
in Help Functions.
In older Emacs versions, exec-directory
was used for this.
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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]
stands for a key sequence that will invoke command, or ‘M-x command’ if command has no key bindings.
\{mapvar}
stands for a summary of the value of mapvar, which should be a
keymap. The summary is made by describe-bindings
.
\<mapvar>
stands for no text itself. It is used for a side effect: it specifies mapvar as the keymap for any following ‘\[command]’ sequences in this documentation string.
Please note: Each ‘\’ must be doubled when written in a string in Emacs Lisp.
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:
(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."
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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.
This function returns a string containing the XEmacs standard notation
for the input events in sequence. The argument sequence may
be a string, vector or list. @xref{Events}, for more information about
valid events. See also the examples for single-key-description
,
below.
This function returns a string describing event in the standard XEmacs 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, linefeed, etc. appear as ‘SPC’, ‘LFD’, etc. A function key symbol appears as itself. An event that is a list appears as the name of the symbol in the CAR of the list.
(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"
This function returns a string describing character in the
standard XEmacs 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"
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XEmacs 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 Help in The XEmacs Reference Manual. Here we describe some program-level interfaces to the same information.
This function finds all symbols whose names contain a match for the regular expression regexp, and returns a list of them (@pxref{Regular Expressions}). It also displays the symbols in a buffer named ‘*Help*’, each with a one-line description.
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)
This function differs from 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.
The value of this variable is a local keymap for characters following the Help key, C-h.
This symbol is not a function; its function definition is actually 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)
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 Access to Documentation Strings.
The value of this variable is the help character—the character that
XEmacs recognizes as meaning Help. By default, it is 8, which is
C-h. When Emacs reads this character, if 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.
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 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
(@pxref{Minibuffer Misc}).
This variable holds a function to print help for a prefix character.
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
.
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 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.
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
.
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
.
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.
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
.
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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