; Revert "Replace eldoc-documentation-function with a hook"

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

@c -*- mode: texinfo; coding: utf-8 -*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2016 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Documentation
@chapter Documentation
@cindex documentation strings

  GNU Emacs has convenient built-in help facilities, most of which
derive their information from documentation strings associated with
functions and variables.  This chapter describes how to access
documentation strings in Lisp programs.

  The contents of a documentation string should follow certain
conventions.  In particular, its first line should be a complete
sentence (or two complete sentences) that briefly describes what the
function or variable does.  @xref{Documentation Tips}, for how to
write good documentation strings.

  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}.

@menu
* Documentation Basics::      Where doc strings are defined and stored.
* 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
@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.  It is, in fact, an
actual Lisp string.  When the string appears in the proper place in a
function or variable definition, it serves as the function's or
variable's documentation.

@cindex @code{function-documentation} property
  In a function definition (a @code{lambda} or @code{defun} form), the
documentation string is specified after the argument list, and is
normally stored directly in the function object.  @xref{Function
Documentation}.  You can also put function documentation in the
@code{function-documentation} property of a function name
(@pxref{Accessing Documentation}).

@cindex @code{variable-documentation} property
  In a variable definition (a @code{defvar} form), the documentation
string is specified after the initial value.  @xref{Defining
Variables}.  The string is stored in the variable's
@code{variable-documentation} property.

@cindex @file{DOC} (documentation) file
  Sometimes, Emacs does not keep documentation strings in memory.
There are two such circumstances.  Firstly, to save memory, the
documentation for preloaded functions and variables (including
primitives) is kept in a file named @file{DOC}, in the directory
specified by @code{doc-directory} (@pxref{Accessing Documentation}).
Secondly, when a function or variable is loaded from a byte-compiled
file, Emacs avoids loading its documentation string (@pxref{Docs and
Compilation}).  In both cases, Emacs looks up the documentation string
from the file only when needed, such as when the user calls @kbd{C-h
f} (@code{describe-function}) for a function.

  Documentation strings can contain special @dfn{key substitution
sequences}, referring to key bindings which are looked up only when
the user views the documentation.  This allows the help commands to
display the correct keys even if a user rearranges the default key
bindings.  @xref{Keys in Documentation}.

  In the documentation string of an autoloaded command
(@pxref{Autoload}), these key-substitution sequences have an
additional special effect: they cause @kbd{C-h f} on the command to
trigger autoloading.  (This is needed for correctly setting up the
hyperlinks in the @file{*Help*} buffer.)

@node Accessing Documentation
@section Access to Documentation Strings
@cindex accessing documentation strings

@defun documentation-property symbol property &optional verbatim
This function returns the documentation string recorded in
@var{symbol}'s property list under property @var{property}.  It is
most often used to look up the documentation strings of variables, for
which @var{property} is @code{variable-documentation}.  However, it
can also be used to look up other kinds of documentation, such as for
customization groups (but for function documentation, use the
@code{documentation} function, below).

If the property value refers to a documentation string stored in the
@file{DOC} file or a byte-compiled file, this function looks up that
string and returns it.

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 as a Lisp expression to
obtain a string.

Finally, this function passes the string through
@code{substitute-command-keys} to substitute key bindings (@pxref{Keys
in Documentation}).  It skips this step if @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}.  It
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}, this function calls
@code{substitute-command-keys}.  The result is the documentation
string to return.

The @code{documentation} function 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

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 @file{*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 (custom-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.}
    (help-setup-xref (list 'describe-symbols pattern) (interactive-p))
    (with-help-window (help-buffer)
      (mapcar describe-func (sort sym-list 'string<)))))
@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
minibuffer-temporary-goal-position      Variable
not documented
@end group

@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 ARG, 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’.

(fn ARG)
@end group

@group
temporary-goal-column   Variable
Current goal column for vertical motion.
It is the column where point was at the start of the current run
of vertical motion commands.

When moving by visual lines via the function ‘line-move-visual’, it is a cons
cell (COL . HSCROLL), where COL is the x-position, in pixels,
divided by the default column width, and HSCROLL is the number of
columns by which window is scrolled from left margin.

When the ‘track-eol’ feature is doing its job, the value is
‘most-positive-fixnum’.
---------- Buffer: *Help* ----------
@end group
@end smallexample

@anchor{Definition of Snarf-documentation}
@defun Snarf-documentation filename
This function is used when building Emacs, just before the runnable
Emacs is dumped.  It finds the positions of the documentation strings
stored in the file @var{filename}, and records those positions into
memory in the function definitions and variable property lists.
@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"}.
@end defun

@defvar doc-directory
This variable holds the name of the directory which should contain the
file @code{"DOC"} 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}.
@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
@cindex key substitution sequence

  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 `
(grave accent) stands for a left quote.
This generates a left single quotation mark, an apostrophe, or a grave
accent depending on the value of @code{text-quoting-style}.

@item '
(apostrophe) stands for a right quote.
This generates a right single quotation mark or an apostrophe
depending on the value of @code{text-quoting-style}.

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

@defvar text-quoting-style
@cindex curved quotes
@cindex curly quotes
The value of this variable is a symbol that specifies the style Emacs
should use for single quotes in the wording of help and messages.
If the variable's value is @code{curve}, the style is
@t{‘like this’} with curved single quotes.  If the value is
@code{straight}, the style is @t{'like this'} with straight
apostrophes.  If the value is @code{grave},
quotes are not translated and the style is @t{`like
this'} with grave accent and apostrophe, the standard style
before Emacs version 25.  The default value @code{nil}
acts like @code{curve} if curved single quotes are displayable, and
like @code{grave} otherwise.

This variable can be used by experts on platforms that have problems
with curved quotes.  As it is not intended for casual use, it is not a
user option.
@end defvar

@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.

@cindex advertised binding
If a command has multiple bindings, this function normally uses the
first one it finds.  You can specify one particular key binding by
assigning an @code{:advertised-binding} symbol property to the
command, like this:

@smallexample
(put 'undo :advertised-binding [?\C-/])
@end smallexample

@noindent
The @code{:advertised-binding} property also affects the binding shown
in menu items (@pxref{Menu Bar}).  The property is ignored if it
specifies a key binding that the command does not actually have.
@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

@deffn Command 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 deffn

@node Help Functions
@section Help Functions
@cindex help functions

  Emacs provides a variety of built-in 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{function-doc} @var{variable-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.  Each of the remaining elements is a
documentation string, or @code{nil}, for @var{symbol} as a function,
variable, etc.

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

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 (string help-char) 'help-command)
(fset 'help-command help-map)
@end group
@end smallexample
@end deffn

@defopt 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 defopt

@defopt 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 defopt

@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}, @code{read-char-choice}, 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

@deffn Command 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 deffn

  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}.

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

@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.
@end defvar

@defun help-buffer
This function returns the name of the help buffer, which is normally
@file{*Help*}; if such a buffer does not exist, it is first created.
@end defun

@vindex help-window-select
@defmac with-help-window buffer-name body@dots{}
This macro evaluates @var{body} like @code{with-output-to-temp-buffer}
(@pxref{Temporary Displays}), inserting any output produced by its forms
into a buffer named @var{buffer-name}.  (Usually, @var{buffer-name}
should be the value returned by the function @code{help-buffer}.)  It
also puts the specified buffer into Help mode and displays a message
telling the user how to quit and scroll the help window.  It selects the
help window if the current value of the user option
@code{help-window-select} has been set accordingly.  It returns the last
value in @var{body}.
@end defmac

@defun help-setup-xref item interactive-p
This function updates the cross reference data in the @file{*Help*}
buffer, which is used to regenerate the help information when the user
clicks on the @samp{Back} or @samp{Forward} buttons.  Most commands
that use the @file{*Help*} buffer should invoke this function before
clearing the buffer.  The @var{item} argument should have the form
@code{(@var{function} . @var{args})}, where @var{function} is a function
to call, with argument list @var{args}, to regenerate the help buffer.
The @var{interactive-p} argument is non-@code{nil} if the calling
command was invoked interactively; in that case, the stack of items
for the @file{*Help*} buffer's @samp{Back} buttons is cleared.
@end defun

@xref{describe-symbols example}, for an example of using
@code{help-buffer}, @code{with-help-window}, and
@code{help-setup-xref}.

@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

@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