(Decoding Output): Document that null bytes force no-conversion for reading

[gnu-emacs] / doc / lispref / help.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/help
@node Documentation, Files, Modes, Top
@chapter Documentation
@cindex documentation strings

  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.

  For commands to display documentation strings, see @ref{Help, ,
Help, emacs, The GNU Emacs Manual}.  For the conventions for writing
documentation strings, see @ref{Documentation Tips}.

@menu
* Documentation Basics::      Good style for doc strings.
                                Where to put them.  How Emacs stores them.
* Accessing Documentation::   How Lisp programs can access doc strings.
* Keys in Documentation::     Substituting current key bindings.
* Describing Characters::     Making printable descriptions of
                                non-printing characters and key sequences.
* Help Functions::            Subroutines used by Emacs help facilities.
@end menu

@node Documentation Basics
@comment  node-name,  next,  previous,  up
@section Documentation Basics
@cindex documentation conventions
@cindex writing a documentation string
@cindex string, writing a doc string

  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 @code{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 @kbd{C-h f} (@code{describe-function}) or @kbd{C-h v}
(@code{describe-variable}) to view the documentation string.  There
are many other conventions for doc strings; see @ref{Documentation
Tips}.

  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.  (@xref{Keys in Documentation}.)

@vindex emacs-lisp-docstring-fill-column
  Emacs Lisp mode fills documentation strings to the width
specified by @code{emacs-lisp-docstring-fill-column}.

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

@itemize @bullet
@item
@kindex function-documentation
The documentation for a function is usually stored in the function
definition itself (@pxref{Lambda Expressions}).  The function
@code{documentation} knows how to extract it.  You can also put
function documentation in the @code{function-documentation} property
of the function name.  That is useful with definitions such as
keyboard macros that can't hold a documentation string.

@item
@kindex variable-documentation
The documentation for a variable is stored in the variable's property
list under the property name @code{variable-documentation}.  The
function @code{documentation-property} knows how to retrieve it.
@end itemize

@cindex @file{DOC-@var{version}} (documentation) file
To save space, the documentation for preloaded functions and variables
(including primitive functions and autoloaded functions) is stored in
the file @file{emacs/etc/DOC-@var{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
(@pxref{Docs 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 @code{documentation} and
@code{documentation-property} use that information to fetch the
documentation string from the appropriate file; this is transparent to
the user.

@c Wordy to prevent overfull hbox.  --rjc 15mar92
  The @file{emacs/lib-src} directory contains two utilities that you can
use to print nice-looking hardcopy for the file
@file{emacs/etc/DOC-@var{version}}.  These are @file{sorted-doc} and
@file{digest-doc}.

@node Accessing Documentation
@section Access to Documentation Strings

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

The last thing this function does is pass the string through
@code{substitute-command-keys} to substitute actual key bindings,
unless @var{verbatim} is non-@code{nil}.

@smallexample
@group
(documentation-property 'command-line-processed
   'variable-documentation)
     @result{} "Non-nil once command line has been processed"
@end group
@group
(symbol-plist 'command-line-processed)
     @result{} (variable-documentation 188902)
@end group
@group
(documentation-property 'emacs 'group-documentation)
     @result{} "Customization of the One True Editor."
@end group
@end smallexample
@end defun

@defun documentation function &optional verbatim
This function returns the documentation string of @var{function}.
@code{documentation} handles macros, named keyboard macros, and
special forms, as well as ordinary functions.

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

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

The function @code{documentation} signals a @code{void-function} error
if @var{function} has no function definition.  However, it is OK if
the function definition has no documentation string.  In that case,
@code{documentation} returns @code{nil}.
@end defun

@defun face-documentation face
This function returns the documentation string of @var{face} as a
face.
@end defun

@c Wordy to prevent overfull hboxes.  --rjc 15mar92
Here is an example of using the two functions, @code{documentation} and
@code{documentation-property}, to display the documentation strings for
several symbols in a @samp{*Help*} buffer.

@anchor{describe-symbols example}
@smallexample
@group
(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)
@end group
@group
            ;; @r{Print description of symbol.}
            (if (fboundp s)             ; @r{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")
@end group
@group
                   (or (documentation s)
                       "not documented"))))

            (if (boundp s)              ; @r{It is a variable.}
@end group
@group
                (princ
                 (format "%s\t%s\n%s\n\n" s
                   (if (user-variable-p s)
                       "Option " "Variable")
@end group
@group
                   (or (documentation-property
                         s 'variable-documentation)
                       "not documented")))))))
        sym-list)
@end group

@group
    ;; @r{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))))))
@end group

@group
    ;; @r{Display the data.}
    (with-output-to-temp-buffer "*Help*"
      (mapcar describe-func (sort sym-list 'string<))
      (print-help-return-message))))
@end group
@end smallexample

  The @code{describe-symbols} function works like @code{apropos},
but provides more information.

@smallexample
@group
(describe-symbols "goal")

---------- Buffer: *Help* ----------
goal-column     Option
Semipermanent goal column for vertical motion, as set by @dots{}
@end group
@c Do not blithely break or fill these lines.
@c That makes them incorrect.

@group
set-goal-column Keys: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
@end group
@c DO NOT put a blank line here!  That is factually inaccurate!
@group
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'.
@end group

@group
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* ----------
@end group
@end smallexample

@defun Snarf-documentation filename
@anchor{Definition of Snarf-documentation}
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 @var{filename}, and records
them in the in-core function definitions and variable property lists in
place of the actual strings.  @xref{Building Emacs}.

Emacs reads the file @var{filename} from the @file{emacs/etc} directory.
When the dumped Emacs is later executed, the same file will be looked
for in the directory @code{doc-directory}.  Usually @var{filename} is
@code{"DOC-@var{version}"}.
@end defun

@c Emacs 19 feature
@defvar doc-directory
This variable holds the name of the directory which should contain the
file @code{"DOC-@var{version}"} that contains documentation strings for
built-in and preloaded functions and variables.

In most cases, this is the same as @code{data-directory}.  They may be
different when you run Emacs from the directory where you built it,
without actually installing it.  @xref{Definition of data-directory}.

In older Emacs versions, @code{exec-directory} was used for this.
@end defvar

@node Keys in Documentation
@section Substituting Key Bindings in Documentation
@cindex documentation, keys in
@cindex keys in documentation strings
@cindex substituting keys 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 @code{substitute-command-keys}.  You
can also call that function yourself.

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

@table @code
@item \[@var{command}]
stands for a key sequence that will invoke @var{command}, or @samp{M-x
@var{command}} if @var{command} has no key bindings.

@item \@{@var{mapvar}@}
stands for a summary of the keymap which is the value of the variable
@var{mapvar}.  The summary is made using @code{describe-bindings}.

@item \<@var{mapvar}>
stands for no text itself.  It is used only for a side effect: it
specifies @var{mapvar}'s value as the keymap for any following
@samp{\[@var{command}]} sequences in this documentation string.

@item \=
quotes the following character and is discarded; thus, @samp{\=\[} puts
@samp{\[} into the output, and @samp{\=\=} puts @samp{\=} into the
output.
@end table

@strong{Please note:} Each @samp{\} must be doubled when written in a
string in Emacs Lisp.

@defun substitute-command-keys string
This function scans @var{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.
@end defun

  Here are examples of the special sequences:

@smallexample
@group
(substitute-command-keys
   "To abort recursive edit, type: \\[abort-recursive-edit]")
@result{} "To abort recursive edit, type: C-]"
@end group

@group
(substitute-command-keys
   "The keys that are defined for the minibuffer here are:
  \\@{minibuffer-local-must-match-map@}")
@result{} "The keys that are defined for the minibuffer here are:
@end group

?               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
"

@group
(substitute-command-keys
   "To abort a recursive edit from the minibuffer, type\
\\<minibuffer-local-must-match-map>\\[abort-recursive-edit].")
@result{} "To abort a recursive edit from the minibuffer, type C-g."
@end group
@end smallexample

  There are other special conventions for the text in documentation
strings---for instance, you can refer to functions, variables, and
sections of this manual.  @xref{Documentation Tips}, for details.

@node Describing Characters
@section Describing Characters for Help Messages
@cindex describe characters and events

  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.

@defun key-description sequence &optional prefix
@cindex Emacs event standard notation
This function returns a string containing the Emacs standard notation
for the input events in @var{sequence}.  If @var{prefix} is
non-@code{nil}, it is a sequence of input events leading up to
@var{sequence} and is included in the return value.  Both arguments
may be strings, vectors or lists.  @xref{Input Events}, for more
information about valid events.

@smallexample
@group
(key-description [?\M-3 delete])
     @result{} "M-3 <delete>"
@end group
@group
(key-description [delete] "\M-3")
     @result{} "M-3 <delete>"
@end group
@end smallexample

  See also the examples for @code{single-key-description}, below.
@end defun

@defun single-key-description event &optional no-angles
@cindex event printing
@cindex character printing
@cindex control character printing
@cindex meta character printing
This function returns a string describing @var{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 @samp{C-}, a meta character turns into a string starting
with @samp{M-}, and space, tab, etc.@: appear as @samp{SPC},
@samp{TAB}, etc.  A function key symbol appears inside angle brackets
@samp{<@dots{}>}.  An event that is a list appears as the name of the
symbol in the @sc{car} of the list, inside angle brackets.

If the optional argument @var{no-angles} is non-@code{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.

@smallexample
@group
(single-key-description ?\C-x)
     @result{} "C-x"
@end group
@group
(key-description "\C-x \M-y \n \t \r \f123")
     @result{} "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3"
@end group
@group
(single-key-description 'delete)
     @result{} "<delete>"
@end group
@group
(single-key-description 'C-mouse-1)
     @result{} "<C-mouse-1>"
@end group
@group
(single-key-description 'C-mouse-1 t)
     @result{} "C-mouse-1"
@end group
@end smallexample
@end defun

@defun text-char-description character
This function returns a string describing @var{character} in the
standard Emacs notation for characters that appear in text---like
@code{single-key-description}, except that control characters are
represented with a leading caret (which is how control characters in
Emacs buffers are usually displayed).  Another difference is that
@code{text-char-description} recognizes the 2**7 bit as the Meta
character, whereas @code{single-key-description} uses the 2**27 bit
for Meta.

@smallexample
@group
(text-char-description ?\C-c)
     @result{} "^C"
@end group
@group
(text-char-description ?\M-m)
     @result{} "\xed"
@end group
@group
(text-char-description ?\C-\M-m)
     @result{} "\x8d"
@end group
@group
(text-char-description (+ 128 ?m))
     @result{} "M-m"
@end group
@group
(text-char-description (+ 128 ?\C-m))
     @result{} "M-^M"
@end group
@end smallexample
@end defun

@defun read-kbd-macro string &optional need-vector
This function is used mainly for operating on keyboard macros, but it
can also be used as a rough inverse for @code{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; @pxref{Key Sequences}.)  If @var{need-vector} is
non-@code{nil}, the return value is always a vector.
@end defun

@node Help Functions
@section Help Functions

  Emacs provides a variety of on-line help functions, all accessible to
the user as subcommands of the prefix @kbd{C-h}.  For more information
about them, see @ref{Help, , Help, emacs, The GNU Emacs Manual}.  Here
we describe some program-level interfaces to the same information.

@deffn Command apropos pattern &optional do-all
This function finds all ``meaningful'' symbols whose names contain a
match for the apropos pattern @var{pattern}.  An apropos pattern is
either a word to match, a space-separated list of words of which at
least two must match, or a regular expression (if any special regular
expression characters occur).  A symbol is ``meaningful'' if it has a
definition as a function, variable, or face, or has properties.

The function returns a list of elements that look like this:

@example
(@var{symbol} @var{score} @var{fn-doc} @var{var-doc}
 @var{plist-doc} @var{widget-doc} @var{face-doc} @var{group-doc})
@end example

Here, @var{score} is an integer measure of how important the symbol
seems to be as a match, and the remaining elements are documentation
strings for @var{symbol}'s various roles (or @code{nil}).

It also displays the symbols in a buffer named @samp{*Apropos*}, each
with a one-line description taken from the beginning of its
documentation string.

@c Emacs 19 feature
If @var{do-all} is non-@code{nil}, or if the user option
@code{apropos-do-all} is non-@code{nil}, then @code{apropos} also
shows key bindings for the functions that are found; it also shows
@emph{all} interned symbols, not just meaningful ones (and it lists
them in the return value as well).
@end deffn

@defvar help-map
The value of this variable is a local keymap for characters following the
Help key, @kbd{C-h}.
@end defvar

@deffn {Prefix Command} help-command
This symbol is not a function; its function definition cell holds the
keymap known as @code{help-map}.  It is defined in @file{help.el} as
follows:

@smallexample
@group
(define-key global-map (char-to-string help-char) 'help-command)
(fset 'help-command help-map)
@end group
@end smallexample
@end deffn

@defun 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 @var{function} to it if @var{function} is non-@code{nil}.
Otherwise it calls @code{message} to display it in the echo area.

This function expects to be called inside a
@code{with-output-to-temp-buffer} special form, and expects
@code{standard-output} to have the value bound by that special form.
For an example of its use, see the long example in @ref{Accessing
Documentation}.
@end defun

@defvar 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 @kbd{C-h}.  When Emacs reads this character, if
@code{help-form} is a non-@code{nil} Lisp expression, it evaluates that
expression, and displays the result in a window if it is a string.

Usually the value of @code{help-form} is @code{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 @kbd{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
@code{describe-prefix-bindings}, which displays a list of all the
subcommands of the prefix key.
@end defvar

@defvar 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 @code{help-char}.
@end defvar

@defvar help-form
If this variable is non-@code{nil}, its value is a form to evaluate
whenever the character @code{help-char} is read.  If evaluating the form
produces a string, that string is displayed.

A command that calls @code{read-event} or @code{read-char} probably
should bind @code{help-form} to a non-@code{nil} expression while it
does input.  (The time when you should not do this is when @kbd{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
@code{minibuffer-help-form} (@pxref{Definition of minibuffer-help-form}).
@end defvar

@defvar 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 @code{describe-prefix-bindings}.
@end defvar

@defun describe-prefix-bindings
This function calls @code{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.)
@end defun

  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 @samp{Helper} to distinguish them from the
ordinary help functions.

@deffn 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 @code{describe-bindings}.
@end deffn

@deffn Command Helper-help
This command provides help for the current mode.  It prompts the user
in the minibuffer with the message @samp{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 @code{nil}.

This can be customized by changing the map @code{Helper-help-map}.
@end deffn

@c Emacs 19 feature
@defvar data-directory
@anchor{Definition of 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, @code{exec-directory} was used for this.
@end defvar

@c Emacs 19 feature
@defmac make-help-screen fname help-line help-text help-map
This macro defines a help command named @var{fname} that acts like a
prefix key that shows a list of the subcommands it offers.

When invoked, @var{fname} displays @var{help-text} in a window, then
reads and executes a key sequence according to @var{help-map}.  The
string @var{help-text} should describe the bindings available in
@var{help-map}.

The command @var{fname} is defined to handle a few events itself, by
scrolling the display of @var{help-text}.  When @var{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 @var{help-map}, it executes that key's binding and
then returns.

The argument @var{help-line} should be a single-line summary of the
alternatives in @var{help-map}.  In the current version of Emacs, this
argument is used only if you set the option @code{three-step-help} to
@code{t}.

This macro is used in the command @code{help-for-help} which is the
binding of @kbd{C-h C-h}.
@end defmac

@defmac with-help-window buffer-name body@dots{}
This macro evaluates the @var{body} forms, inserting any output they
produce into a buffer named @var{buffer-name} like
@code{with-output-to-temp-buffer} (@pxref{Temporary Displays}).  It
also puts that buffer in Help mode, displays a message telling the
user how to quit and scroll the help window, and does various other
things that make a help window work better.

Don't use @code{print-help-return-message} in the body of this macro;
it would cause bad results.
@end defmac

@defopt three-step-help
If this variable is non-@code{nil}, commands defined with
@code{make-help-screen} display their @var{help-line} strings in the
echo area at first, and display the longer @var{help-text} strings only
if the user types the help character again.
@end defopt

@ignore
   arch-tag: ba36b4c2-e60f-49e2-bc25-61158fdcd815
@end ignore