XEmacs 21.2-b1

[chise/xemacs-chise.git.1] / man / lispref / help.texi

@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/help.info
@node Documentation, Files, Modes, Top
@chapter Documentation
@cindex documentation strings

  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.

@menu
* Documentation Basics::      Good style for doc strings.
                                Where to put them.  How XEmacs 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 XEmacs help facilities.
* Obsoleteness::              Upgrading Lisp functionality over time.
@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 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 you have one, because that looks odd when you use @kbd{C-h f}
(@code{describe-function}) or @kbd{C-h v} (@code{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.  (@xref{Accessing Documentation}.)

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

@itemize @bullet
@item
The documentation for a function is stored in the function definition
itself (@pxref{Lambda Expressions}).  The function
@code{documentation} knows how to extract it.

@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 extract it.
@end itemize

@cindex @file{DOC} (documentation) file
To save space, the documentation for preloaded functions and variables
(including primitive functions and autoloaded functions) is stored in
the @dfn{internal doc file} @file{DOC}.  The documentation for functions
and variables loaded during the XEmacs session from byte-compiled files
is stored in those very same byte-compiled files (@pxref{Docs and
Compilation}).

XEmacs does not keep documentation strings in memory unless necessary.
Instead, XEmacs maintains, for preloaded symbols, an integer offset into
the internal doc file, and for symbols loaded from byte-compiled files,
a list containing the filename of the byte-compiled file and an integer
offset, in place of the documentation string.  The functions
@code{documentation} and @code{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 @ref{Help, ,
Help, emacs, The XEmacs Reference Manual}.

@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.c} and
@file{digest-doc.c}.

@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 necessary, and runs
@code{substitute-command-keys} to substitute actual key bindings.  (This
substitution is not done if @var{verbatim} is non-@code{nil}; the
@var{verbatim} argument exists only as of Emacs 19.)

@smallexample
@group
(documentation-property 'command-line-processed
   'variable-documentation)
     @result{} "t once command line has been processed"
@end group
@group
(symbol-plist 'command-line-processed)
     @result{} (variable-documentation 188902)
@end group
@end smallexample
@end defun

@defun documentation function &optional verbatim
This function returns the documentation string of @var{function}.  It
reads the text from a file if necessary.  Then (unless @var{verbatim} is
non-@code{nil}) it calls @code{substitute-command-keys}, 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

@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.

@smallexample
@group
(defun describe-symbols (pattern)
  "Describe the XEmacs 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 C-x C-n, or nil.
@end group
@c Do not blithely break or fill these lines.
@c That makes them incorrect.

@group
set-goal-column Command: 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
  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 @var{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 @var{filename} in the @file{lib-src} directory.
When the dumped XEmacs is later executed, the same file is found in the
directory @code{doc-directory}.  The usual value for @var{filename} is
@file{DOC}, but this can be changed by modifying the variable
@code{internal-doc-file-name}.
@end defun

@defvar internal-doc-file-name
This variable holds the name of the file containing documentation
strings of built-in symbols, usually @file{DOC}.  The full pathname of
the internal doc file is @samp{(concat doc-directory internal-doc-file-name)}.
@end defvar

@defvar doc-directory
This variable holds the name of the directory which contains the
@dfn{internal doc file} that contains documentation strings for built-in
and preloaded functions and variables.

In most cases, this is the same as @code{exec-directory}.  They may be
different when you run XEmacs from the directory where you built it,
without actually installing it.  See @code{exec-directory} in @ref{Help
Functions}.

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

@defvar data-directory
This variable holds the name of the directory in which XEmacs finds
certain system independent documentation and text files that come
with XEmacs.  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 value of @var{mapvar}, which should be a
keymap.  The summary is made by @code{describe-bindings}.

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

@item \=
quotes the following character and is discarded; this @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 XEmacs 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
LFD             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

@group
(substitute-command-keys
  "Substrings of the form \\=\\@{MAPVAR@} are replaced by summaries
\(made by describe-bindings) of the value of MAPVAR, taken as a keymap.
Substrings of the form \\=\\<MAPVAR> specify to use the value of MAPVAR
as the keymap for future \\=\\[COMMAND] substrings.
\\=\\= quotes the following character and is discarded;
thus, \\=\\=\\=\\= puts \\=\\= into the output,
and \\=\\=\\=\\[ puts \\=\\[ into the output.")
@result{} "Substrings of the form \@{MAPVAR@} are replaced by summaries
(made by describe-bindings) of the value of MAPVAR, taken as a keymap.
Substrings of the form \<MAPVAR> specify to use the value of MAPVAR
as the keymap for future \[COMMAND] substrings.
\= quotes the following character and is discarded;
thus, \=\= puts \= into the output,
and \=\[ puts \[ into the output."
@end group
@end smallexample

@node Describing Characters
@section Describing Characters for Help Messages

  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
@cindex XEmacs event standard notation
This function returns a string containing the XEmacs standard notation
for the input events in @var{sequence}.  The argument @var{sequence} may
be a string, vector or list.  @xref{Events}, for more information about
valid events.  See also the examples for @code{single-key-description},
below.
@end defun

@defun single-key-description key
@cindex event printing
@cindex character printing
@cindex control character printing
@cindex meta character printing
This function returns a string describing @var{key} 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
@samp{C-}, a meta character turns into a string starting with @samp{M-},
and space, linefeed, etc.@: appear as @samp{SPC}, @samp{LFD}, etc.  A
symbol appears as the name of the symbol.  An event that is a list
appears as the name of the symbol in the @sc{car} of the list.

@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 LFD SPC TAB SPC RET SPC C-l 1 2 3"
@end group
@group
(single-key-description 'kp_next)
     @result{} "kp_next"
@end group
@group
(single-key-description '(shift button1))
     @result{} "Sh-button1"
@end group
@end smallexample
@end defun

@defun text-char-description character
This function returns a string describing @var{character} in the
standard XEmacs 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
XEmacs buffers are usually displayed).

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

@node Help Functions
@section Help Functions

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

@deffn Command apropos regexp &optional do-all predicate
This function finds all symbols whose names contain a match for the
regular expression @var{regexp}, and returns a list of them
(@pxref{Regular Expressions}).  It also displays the symbols in a buffer
named @samp{*Help*}, each with a one-line description.

@c Emacs 19 feature
If @var{do-all} is non-@code{nil}, then @code{apropos} also shows
key bindings for the functions that are found.

If @var{predicate} is non-@code{nil}, it should be a function to be
called on each symbol that has matched @var{regexp}.  Only symbols for
which @var{predicate} returns a non-@code{nil} value are listed or
displayed.

In the first of the following examples, @code{apropos} finds all the
symbols with names containing @samp{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 @samp{*Help*} buffer.)

@smallexample
@group
(apropos "exec")
     @result{} (Buffer-menu-execute command-execute exec-directory
    exec-path execute-extended-command execute-kbd-macro
    executing-kbd-macro executing-macro)
@end group

@group
(apropos "exec" nil 'commandp)
     @result{} (Buffer-menu-execute execute-extended-command)
@end group
@ignore
@group
---------- Buffer: *Help* ----------
Buffer-menu-execute
  Function: Save and/or delete buffers marked with
  M-x Buffer-menu-save or M-x Buffer-menu-delete commands.
execute-extended-command      ESC x
  Function: Read function name, then read its
  arguments and call it.
---------- Buffer: *Help* ----------
@end group
@end ignore
@end smallexample

@code{apropos} is used by various user-level commands, such as @kbd{C-h
a} (@code{hyper-apropos}), a graphical front-end to @code{apropos}; and
@kbd{C-h A} (@code{command-apropos}), which does an apropos over only
those functions which are user commands.  @code{command-apropos} calls
@code{apropos}, specifying a @var{predicate} to restrict the output to
symbols that are commands.  The call to @code{apropos} looks like this:

@smallexample
(apropos string t 'commandp)
@end smallexample
@end deffn

@c Emacs 19 feature
@c super-apropos is obsolete - function absorbed by apropos --mrb
@ignore
@deffn Command super-apropos regexp &optional do-all
This function differs from @code{apropos} in that it searches
documentation strings as well as symbol names for matches for
@var{regexp}.  By default, it searches the documentation strings only
for preloaded functions and variables.  If @var{do-all} is
non-@code{nil}, it scans the names and documentation strings of all
functions and variables.
@end deffn
@end ignore

@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 is actually the
keymap known as @code{help-map}.  It is defined in @file{help.el} as
follows:

@smallexample
@group
(define-key global-map "\C-h" '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
XEmacs recognizes as meaning Help.  By default, it is the character
@samp{?\^H} (ASCII 8), which is @kbd{C-h}.  When XEmacs reads this
character, if @code{help-form} is non-@code{nil} Lisp expression, it
evaluates that expression, and displays the result in a window if it is
a string.

@code{help-char} can be a character or a key description such as
@code{help} or @code{(meta h)}.

Usually the value of @code{help-form}'s value 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-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{next-command-event} or @code{next-event}
probably should bind @code{help-form} to a non-@code{nil} expression
while it does input.  (The exception is when @kbd{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
@code{minibuffer-help-form} (@pxref{Minibuffer Misc}).
@end defvar

@defvar prefix-help-command
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 @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 found in the library @file{helper}.
They are for modes that want to provide help without relinquishing
control, such as the ``electric'' modes.  You must load that library
with @code{(require 'helper)} in order to use them.  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

@ignore @c Not in XEmacs currently
@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}.
@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
@end ignore

@node Obsoleteness
@section Obsoleteness

As you add functionality to a package, you may at times want to
replace an older function with a new one.  To preserve compatibility
with existing code, the older function needs to still exist; but
users of that function should be told to use the newer one instead.
XEmacs Lisp lets you mark a function or variable as @dfn{obsolete},
and indicate what should be used instead.

@defun make-obsolete function new
This function indicates that @var{function} is an obsolete function,
and the function @var{new} should be used instead.  The byte compiler
will issue a warning to this effect when it encounters a usage of the
older function, and the help system will also note this in the function's
documentation.  @var{new} can also be a string (if there is not a single
function with the same functionality any more), and should be a descriptive
statement, such as "use @var{foo} or @var{bar} instead" or "this function is
unnecessary".
@end defun

@defun make-obsolete-variable variable new
This is like @code{make-obsolete} but is for variables instead of functions.
@end defun

@defun define-obsolete-function-alias oldfun newfun
This function combines @code{make-obsolete} and @code{define-function},
declaring @var{oldfun} to be an obsolete variant of @var{newfun} and
defining @var{oldfun} as an alias for @var{newfun}.
@end defun

@defun define-obsolete-variable-alias oldvar newvar
This is like @code{define-obsolete-function-alias} but for variables.
@end defun

Note that you should not normally put obsoleteness information
explicitly in a function or variable's doc string.  The obsoleteness
information that you specify using the above functions will be displayed
whenever the doc string is displayed, and by adding it explicitly the
result is redundancy.

Also, if an obsolete function is substantially the same as a newer one
but is not actually an alias, you should consider omitting the doc
string entirely (use a null string @samp{""} as the doc string).  That
way, the user is told about the obsoleteness and is forced to look at
the documentation of the new function, making it more likely that he
will use the new function.

@defun function-obsoleteness-doc function
If @var{function} is obsolete, this function returns a string describing
this.  This is the message that is printed out during byte compilation
or in the function's documentation.  If @var{function} is not obsolete,
@code{nil} is returned.
@end defun

@defun variable-obsoleteness-doc variable
This is like @code{function-obsoleteness-doc} but for variables.
@end defun

The obsoleteness information is stored internally by putting a property
@code{byte-obsolete-info} (for functions) or
@code{byte-obsolete-variable} (for variables) on the symbol that
specifies the obsolete function or variable.  For more information, see
the implementation of @code{make-obsolete} and
@code{make-obsolete-variable} in
@file{lisp/bytecomp/bytecomp-runtime.el}.