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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
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@c See the file elisp.texi for copying conditions.
@setfilename ../info/commands
@node Command Loop, Keymaps, Minibuffers, Top
@chapter Command Loop
@cindex editor command loop
@cindex command loop

  When you run Emacs, it enters the @dfn{editor command loop} almost
immediately.  This loop reads key sequences, executes their definitions,
and displays the results.  In this chapter, we describe how these things
are done, and the subroutines that allow Lisp programs to do them.  

@menu
* Command Overview::    How the command loop reads commands.
* Defining Commands::   Specifying how a function should read arguments.
* Interactive Call::    Calling a command, so that it will read arguments.
* Command Loop Info::   Variables set by the command loop for you to examine.
* Input Events::	What input looks like when you read it.
* Reading Input::       How to read input events from the keyboard or mouse.
* Waiting::             Waiting for user input or elapsed time.
* Quitting::            How @kbd{C-g} works.  How to catch or defer quitting.
* Prefix Command Arguments::    How the commands to set prefix args work.
* Recursive Editing::   Entering a recursive edit,
                          and why you usually shouldn't.
* Disabling Commands::  How the command loop handles disabled commands.
* Command History::     How the command history is set up, and how accessed.
* Keyboard Macros::     How keyboard macros are implemented.
@end menu

@node Command Overview
@section Command Loop Overview

  The first thing the command loop must do is read a key sequence, which
is a sequence of events that translates into a command.  It does this by
calling the function @code{read-key-sequence}.  Your Lisp code can also
call this function (@pxref{Key Sequence Input}).  Lisp programs can also
do input at a lower level with @code{read-event} (@pxref{Reading One
Event}) or discard pending input with @code{discard-input}
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(@pxref{Event Input Misc}).
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  The key sequence is translated into a command through the currently
active keymaps.  @xref{Key Lookup}, for information on how this is done.
The result should be a keyboard macro or an interactively callable
function.  If the key is @kbd{M-x}, then it reads the name of another
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command, which it then calls.  This is done by the command
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@code{execute-extended-command} (@pxref{Interactive Call}).

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  To execute a command requires first reading the arguments for it.
This is done by calling @code{command-execute} (@pxref{Interactive
Call}).  For commands written in Lisp, the @code{interactive}
specification says how to read the arguments.  This may use the prefix
argument (@pxref{Prefix Command Arguments}) or may read with prompting
in the minibuffer (@pxref{Minibuffers}).  For example, the command
@code{find-file} has an @code{interactive} specification which says to
read a file name using the minibuffer.  The command's function body does
not use the minibuffer; if you call this command from Lisp code as a
function, you must supply the file name string as an ordinary Lisp
function argument.
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  If the command is a string or vector (i.e., a keyboard macro) then
@code{execute-kbd-macro} is used to execute it.  You can call this
function yourself (@pxref{Keyboard Macros}).

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  To terminate the execution of a running command, type @kbd{C-g}.  This
character causes @dfn{quitting} (@pxref{Quitting}).
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@defvar pre-command-hook
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The editor command loop runs this normal hook before each command.  At
that time, @code{this-command} contains the command that is about to
run, and @code{last-command} describes the previous command.
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@end defvar

@defvar post-command-hook
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The editor command loop runs this normal hook after each command
(including commands terminated prematurely by quitting or by errors),
and also when the command loop is first entered.  At that time,
@code{this-command} describes the command that just ran, and
@code{last-command} describes the command before that.
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@end defvar

@node Defining Commands
@section Defining Commands
@cindex defining commands
@cindex commands, defining
@cindex functions, making them interactive
@cindex interactive function

  A Lisp function becomes a command when its body contains, at top
level, a form which calls the special form @code{interactive}.  This
form does nothing when actually executed, but its presence serves as a
flag to indicate that interactive calling is permitted.  Its argument
controls the reading of arguments for an interactive call.

@menu
* Using Interactive::     General rules for @code{interactive}.
* Interactive Codes::     The standard letter-codes for reading arguments
                             in various ways.
* Interactive Examples::  Examples of how to read interactive arguments.
@end menu

@node Using Interactive
@subsection Using @code{interactive}

  This section describes how to write the @code{interactive} form that
makes a Lisp function an interactively-callable command.

@defspec interactive arg-descriptor
@cindex argument descriptors
This special form declares that the function in which it appears is a
command, and that it may therefore be called interactively (via
@kbd{M-x} or by entering a key sequence bound to it).  The argument
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@var{arg-descriptor} declares how to compute the arguments to the
command when the command is called interactively.
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A command may be called from Lisp programs like any other function, but
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then the caller supplies the arguments and @var{arg-descriptor} has no
effect.
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The @code{interactive} form has its effect because the command loop
(actually, its subroutine @code{call-interactively}) scans through the
function definition looking for it, before calling the function.  Once
the function is called, all its body forms including the
@code{interactive} form are executed, but at this time
@code{interactive} simply returns @code{nil} without even evaluating its
argument.
@end defspec

There are three possibilities for the argument @var{arg-descriptor}:

@itemize @bullet
@item
It may be omitted or @code{nil}; then the command is called with no
arguments.  This leads quickly to an error if the command requires one
or more arguments.

@item
It may be a Lisp expression that is not a string; then it should be a
form that is evaluated to get a list of arguments to pass to the
command.
@cindex argument evaluation form

@item
@cindex argument prompt
It may be a string; then its contents should consist of a code character
followed by a prompt (which some code characters use and some ignore).
The prompt ends either with the end of the string or with a newline.
Here is a simple example:

@smallexample
(interactive "bFrobnicate buffer: ")
@end smallexample

@noindent
The code letter @samp{b} says to read the name of an existing buffer,
with completion.  The buffer name is the sole argument passed to the
command.  The rest of the string is a prompt.

If there is a newline character in the string, it terminates the prompt.
If the string does not end there, then the rest of the string should
contain another code character and prompt, specifying another argument.
You can specify any number of arguments in this way.

@c Emacs 19 feature
The prompt string can use @samp{%} to include previous argument values
in the prompt.  This is done using @code{format} (@pxref{Formatting
Strings}).  For example, here is how you could read the name of an
existing buffer followed by a new name to give to that buffer:

@smallexample
@group
(interactive "bBuffer to rename: \nsRename buffer %s to: ")
@end group
@end smallexample

@cindex @samp{*} in interactive
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@cindex read-only buffers in interactive
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If the first character in the string is @samp{*}, then an error is
signaled if the buffer is read-only.

@cindex @samp{@@} in interactive
@c Emacs 19 feature
If the first character in the string is @samp{@@}, and if the key
sequence used to invoke the command includes any mouse events, then
the window associated with the first of those events is selected
before the command is run.

You can use @samp{*} and @samp{@@} together; the order does not matter.
Actual reading of arguments is controlled by the rest of the prompt
string (starting with the first character that is not @samp{*} or
@samp{@@}).
@end itemize

@node Interactive Codes
@comment  node-name,  next,  previous,  up
@subsection Code Characters for @code{interactive}
@cindex interactive code description
@cindex description for interactive codes
@cindex codes, interactive, description of
@cindex characters for interactive codes

  The code character descriptions below contain a number of key words,
defined here as follows:

@table @b
@item Completion
@cindex interactive completion
Provide completion.  @key{TAB}, @key{SPC}, and @key{RET} perform name
completion because the argument is read using @code{completing-read}
(@pxref{Completion}).  @kbd{?} displays a list of possible completions.

@item Existing
Require the name of an existing object.  An invalid name is not
accepted; the commands to exit the minibuffer do not exit if the current
input is not valid.

@item Default
@cindex default argument string
A default value of some sort is used if the user enters no text in the
minibuffer.  The default depends on the code character.

@item No I/O
This code letter computes an argument without reading any input.
Therefore, it does not use a prompt string, and any prompt string you
supply is ignored.

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Even though the code letter doesn't use a prompt string, you must follow
it with a newline if it is not the last code character in the string.

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@item Prompt
A prompt immediately follows the code character.  The prompt ends either
with the end of the string or with a newline.

@item Special
This code character is meaningful only at the beginning of the
interactive string, and it does not look for a prompt or a newline.
It is a single, isolated character.
@end table

@cindex reading interactive arguments
  Here are the code character descriptions for use with @code{interactive}:

@table @samp
@item *
Signal an error if the current buffer is read-only.  Special.

@item @@
Select the window mentioned in the first mouse event in the key
sequence that invoked this command.  Special.

@item a
A function name (i.e., a symbol which is @code{fboundp}).  Existing,
Completion, Prompt.

@item b
The name of an existing buffer.  By default, uses the name of the
current buffer (@pxref{Buffers}).  Existing, Completion, Default,
Prompt.

@item B
A buffer name.  The buffer need not exist.  By default, uses the name of
a recently used buffer other than the current buffer.  Completion,
Prompt.

@item c
A character.  The cursor does not move into the echo area.  Prompt.

@item C
A command name (i.e., a symbol satisfying @code{commandp}).  Existing,
Completion, Prompt.

@item d
@cindex position argument
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The position of point, as an integer (@pxref{Point}).  No I/O.
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@item D
A directory name.  The default is the current default directory of the
current buffer, @code{default-directory} (@pxref{System Environment}).
Existing, Completion, Default, Prompt.

@item e
The first or next mouse event in the key sequence that invoked the command.
More precisely, @samp{e} gets events which are lists, so you can look at
the data in the lists.  @xref{Input Events}.  No I/O.

You can use @samp{e} more than once in a single command's interactive
specification.  If the key sequence which invoked the command has
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@var{n} events that are lists, the @var{n}th @samp{e} provides the
@var{n}th such event.  Events which are not lists, such as function keys
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and @sc{ASCII} characters, do not count where @samp{e} is concerned.

@item f
A file name of an existing file (@pxref{File Names}).  The default
directory is @code{default-directory}.  Existing, Completion, Default,
Prompt.

@item F
A file name.  The file need not exist.  Completion, Default, Prompt.

@item k
A key sequence (@pxref{Keymap Terminology}).  This keeps reading events
until a command (or undefined command) is found in the current key
maps.  The key sequence argument is represented as a string or vector.
The cursor does not move into the echo area.  Prompt.

This kind of input is used by commands such as @code{describe-key} and
@code{global-set-key}.

@item m
@cindex marker argument
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The position of the mark, as an integer.  No I/O.
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@item n
A number read with the minibuffer.  If the input is not a number, the
user is asked to try again.  The prefix argument, if any, is not used.
Prompt.

@item N
@cindex raw prefix argument usage
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The raw prefix argument.  If the prefix argument is @code{nil}, then
read a number as with @kbd{n}.  Requires a number.  Prompt.
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@item p
@cindex numeric prefix argument usage
The numeric prefix argument.  (Note that this @samp{p} is lower case.)
No I/O.@refill

@item P
The raw prefix argument.  (Note that this @samp{P} is upper case.)
@xref{Prefix Command Arguments}.  No I/O.@refill

@item r
@cindex region argument
Point and the mark, as two numeric arguments, smallest first.  This is
the only code letter that specifies two successive arguments rather than
one.  No I/O.

@item s
Arbitrary text, read in the minibuffer and returned as a string
(@pxref{Text from Minibuffer}).  Terminate the input with either
@key{LFD} or @key{RET}.  (@kbd{C-q} may be used to include either of
these characters in the input.)  Prompt.

@item S
An interned symbol whose name is read in the minibuffer.  Any whitespace
character terminates the input.  (Use @kbd{C-q} to include whitespace in
the string.)  Other characters that normally terminate a symbol (e.g.,
parentheses and brackets) do not do so here.  Prompt.

@item v
A variable declared to be a user option (i.e., satisfying the predicate
@code{user-variable-p}).  @xref{High-Level Completion}.  Existing,
Completion, Prompt.

@item x
A Lisp object specified in printed representation, terminated with a
@key{LFD} or @key{RET}.  The object is not evaluated.  @xref{Object from
Minibuffer}.  Prompt.

@item X
@cindex evaluated expression argument
A Lisp form is read as with @kbd{x}, but then evaluated so that its
value becomes the argument for the command.  Prompt.
@end table

@node Interactive Examples
@comment  node-name,  next,  previous,  up
@subsection Examples of Using @code{interactive}
@cindex examples of using @code{interactive}
@cindex @code{interactive}, examples of using 

  Here are some examples of @code{interactive}:

@example
@group
(defun foo1 ()              ; @r{@code{foo1} takes no arguments,}
    (interactive)           ;   @r{just moves forward two words.}
    (forward-word 2))
     @result{} foo1
@end group

@group
(defun foo2 (n)             ; @r{@code{foo2} takes one argument,}
    (interactive "p")       ;   @r{which is the numeric prefix.}
    (forward-word (* 2 n)))
     @result{} foo2
@end group

@group
(defun foo3 (n)             ; @r{@code{foo3} takes one argument,}
    (interactive "nCount:") ;   @r{which is read with the Minibuffer.}
    (forward-word (* 2 n)))
     @result{} foo3
@end group

@group
(defun three-b (b1 b2 b3)
  "Select three existing buffers.
Put them into three windows, selecting the last one."
@end group
    (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:")
    (delete-other-windows)
    (split-window (selected-window) 8)
    (switch-to-buffer b1)
    (other-window 1)
    (split-window (selected-window) 8)
    (switch-to-buffer b2)
    (other-window 1)
    (switch-to-buffer b3))
     @result{} three-b
@group
(three-b "*scratch*" "declarations.texi" "*mail*")
     @result{} nil
@end group
@end example

@node Interactive Call
@section Interactive Call
@cindex interactive call

  After the command loop has translated a key sequence into a
definition, it invokes that definition using the function
@code{command-execute}.  If the definition is a function that is a
command, @code{command-execute} calls @code{call-interactively}, which
reads the arguments and calls the command.  You can also call these
functions yourself.

@defun commandp object
Returns @code{t} if @var{object} is suitable for calling interactively;
that is, if @var{object} is a command.  Otherwise, returns @code{nil}.  

The interactively callable objects include strings and vectors (treated
as keyboard macros), lambda expressions that contain a top-level call to
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@code{interactive}, compiled function objects made from such lambda
expressions, autoload objects that are declared as interactive
(non-@code{nil} fourth argument to @code{autoload}), and some of the
primitive functions.
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A symbol is @code{commandp} if its function definition is
@code{commandp}.

Keys and keymaps are not commands.  Rather, they are used to look up
commands (@pxref{Keymaps}).

See @code{documentation} in @ref{Accessing Documentation}, for a
realistic example of using @code{commandp}.
@end defun

@defun call-interactively command &optional record-flag
This function calls the interactively callable function @var{command},
reading arguments according to its interactive calling specifications.
An error is signaled if @var{command} cannot be called interactively
(i.e., it is not a command).  Note that keyboard macros (strings and
vectors) are not accepted, even though they are considered commands.

@cindex record command history
If @var{record-flag} is non-@code{nil}, then this command and its
arguments are unconditionally added to the list @code{command-history}.
Otherwise, the command is added only if it uses the minibuffer to read
an argument.  @xref{Command History}.
@end defun

@defun command-execute command &optional record-flag
@cindex keyboard macro execution
This function executes @var{command} as an editing command.  The
argument @var{command} must satisfy the @code{commandp} predicate; i.e.,
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it must be an interactively callable function or a keyboard macro.
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A string or vector as @var{command} is executed with
@code{execute-kbd-macro}.  A function is passed to
@code{call-interactively}, along with the optional @var{record-flag}.

A symbol is handled by using its function definition in its place.  A
symbol with an @code{autoload} definition counts as a command if it was
declared to stand for an interactively callable function.  Such a
definition is handled by loading the specified library and then
rechecking the definition of the symbol.
@end defun

@deffn Command execute-extended-command prefix-argument
@cindex read command name
This function reads a command name from the minibuffer using
@code{completing-read} (@pxref{Completion}).  Then it uses
@code{command-execute} to call the specified command.  Whatever that
command returns becomes the value of @code{execute-extended-command}.

@cindex execute with prefix argument
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If the command asks for a prefix argument, it receives the value
@var{prefix-argument}.  If @code{execute-extended-command} is called
interactively, the current raw prefix argument is used for
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@var{prefix-argument}, and thus passed on to whatever command is run.

@c !!! Should this be @kindex?
@cindex @kbd{M-x}
@code{execute-extended-command} is the normal definition of @kbd{M-x},
so it uses the string @w{@samp{M-x }} as a prompt.  (It would be better
to take the prompt from the events used to invoke
@code{execute-extended-command}, but that is painful to implement.)  A
description of the value of the prefix argument, if any, also becomes
part of the prompt.

@example
@group
(execute-extended-command 1)
---------- Buffer: Minibuffer ----------
M-x forward-word RET
---------- Buffer: Minibuffer ----------
     @result{} t
@end group
@end example
@end deffn

@defun interactive-p
This function returns @code{t} if the containing function (the one that
called @code{interactive-p}) was called interactively, with the function
@code{call-interactively}.  (It makes no difference whether
@code{call-interactively} was called from Lisp or directly from the
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editor command loop.)  If the containing function was called by Lisp
evaluation (or with @code{apply} or @code{funcall}), then it was not
called interactively.
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The most common use of @code{interactive-p} is for deciding whether to
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print an informative message.  As a special exception,
@code{interactive-p} returns @code{nil} whenever a keyboard macro is
being run.  This is to suppress the informative messages and speed
execution of the macro.

For example:

@example
@group
(defun foo ()
  (interactive)
  (and (interactive-p)
       (message "foo")))
     @result{} foo
@end group

@group
(defun bar ()
  (interactive)
  (setq foobar (list (foo) (interactive-p))))
     @result{} bar
@end group

@group
;; @r{Type @kbd{M-x foo}.}
     @print{} foo
@end group

@group
;; @r{Type @kbd{M-x bar}.}
;; @r{This does not print anything.}
@end group

@group
foobar
     @result{} (nil t)
@end group
@end example
@end defun

@node Command Loop Info
@comment  node-name,  next,  previous,  up
@section Information from the Command Loop

The editor command loop sets several Lisp variables to keep status
records for itself and for commands that are run.  

@defvar last-command
This variable records the name of the previous command executed by the
command loop (the one before the current command).  Normally the value
is a symbol with a function definition, but this is not guaranteed.

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The value is copied from @code{this-command} when a command returns to
the command loop, except when the command specifies a prefix argument
for the following command.
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@end defvar

@defvar this-command
@cindex current command
This variable records the name of the command now being executed by
the editor command loop.  Like @code{last-command}, it is normally a symbol
with a function definition.

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The command loop sets this variable just before running a command, and
copies its value into @code{last-command} when the command finishes
(unless the command specifies a prefix argument for the following
command).
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@cindex kill command repetition
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Some commands set this variable during their execution, as a flag for
whatever command runs next.  In particular, the functions that kill text
set @code{this-command} to @code{kill-region} so that any kill commands
immediately following will know to append the killed text to the
previous kill.
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@end defvar

If you do not want a particular command to be recognized as the previous
command in the case where it got an error, you must code that command to
prevent this.  One way is to set @code{this-command} to @code{t} at the
beginning of the command, and set @code{this-command} back to its proper
value at the end, like this:

@example
(defun foo (args@dots{})
  (interactive @dots{})
  (let ((old-this-command this-command))
    (setq this-command t)
    @r{@dots{}do the work@dots{}}
    (setq this-command old-this-command)))
@end example

@defun this-command-keys
This function returns a string or vector containing the key sequence
that invoked the present command, plus any previous commands that
generated the prefix argument for this command.  The value is a string
if all those events were characters.  @xref{Input Events}.

@example
@group
(this-command-keys)
;; @r{Now type @kbd{C-u C-x C-e}.}
     @result{} "^U^X^E"
@end group
@end example
@end defun

@defvar last-nonmenu-event
This variable holds the last input event read as part of a key
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sequence, not counting events resulting from mouse menus.
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One use of this variable is to figure out a good default location to
pop up another menu.
@end defvar

@defvar last-command-event
@defvarx last-command-char
This variable is set to the last input event that was read by the
command loop as part of a command.  The principal use of this variable
is in @code{self-insert-command}, which uses it to decide which
character to insert.

@example
@group
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last-command-event
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;; @r{Now type @kbd{C-u C-x C-e}.}
     @result{} 5
@end group
@end example

@noindent
The value is 5 because that is the @sc{ASCII} code for @kbd{C-e}.

The alias @code{last-command-char} exists for compatibility with
Emacs version 18.
@end defvar

@c Emacs 19 feature
@defvar last-event-frame
This variable records which frame the last input event was directed to.
Usually this is the frame that was selected when the event was
generated, but if that frame has redirected input focus to another
frame, the value is the frame to which the event was redirected.
@xref{Input Focus}.
@end defvar

@defvar echo-keystrokes
This variable determines how much time should elapse before command
characters echo.  Its value must be an integer, which specifies the
number of seconds to wait before echoing.  If the user types a prefix
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key (such as @kbd{C-x}) and then delays this many seconds before
continuing, the prefix key is echoed in the echo area.  Any subsequent
characters in the same command will be echoed as well.
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If the value is zero, then command input is not echoed.
@end defvar

@node Input Events
@section Input Events
@cindex events
@cindex input events

The Emacs command loop reads a sequence of @dfn{input events} that
represent keyboard or mouse activity.  The events for keyboard activity
are characters or symbols; mouse events are always lists.  This section
describes the representation and meaning of input events in detail.

@defun eventp object
This function returns non-@code{nil} if @var{event} is an input event.
@end defun

@menu
* Keyboard Events::		Ordinary characters--keys with symbols on them.
* Function Keys::		Function keys--keys with names, not symbols.
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* Mouse Events::                Overview of mouse events.
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* Click Events::		Pushing and releasing a mouse button.
* Drag Events::			Moving the mouse before releasing the button.
* Button-Down Events::		A button was pushed and not yet released.
* Repeat Events::               Double and triple click (or drag, or down).
* Motion Events::		Just moving the mouse, not pushing a button.
* Focus Events::		Moving the mouse between frames.
* Event Examples::		Examples of the lists for mouse events.
* Classifying Events::		Finding the modifier keys in an event symbol.
				Event types.
* Accessing Events::		Functions to extract info from events.
* Strings of Events::           Special considerations for putting
				  keyboard character events in a string.
@end menu

@node Keyboard Events
@subsection Keyboard Events

There are two kinds of input you can get from the keyboard: ordinary
keys, and function keys.  Ordinary keys correspond to characters; the
events they generate are represented in Lisp as characters.  In Emacs
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versions 18 and earlier, characters were the only events.  The event
type of a character event is the character itself (an integer); 
see @ref{Classifying Events}.
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@cindex modifier bits (of input character)
@cindex basic code (of input character)
An input character event consists of a @dfn{basic code} between 0 and
255, plus any or all of these @dfn{modifier bits}:

@table @asis
@item meta
The 2**23 bit in the character code indicates a character
typed with the meta key held down.

@item control
The 2**22 bit in the character code indicates a non-@sc{ASCII}
control character.

@sc{ASCII} control characters such as @kbd{C-a} have special basic
codes of their own, so Emacs needs no special bit to indicate them.
Thus, the code for @kbd{C-a} is just 1.

But if you type a control combination not in @sc{ASCII}, such as
@kbd{%} with the control key, the numeric value you get is the code
for @kbd{%} plus 2**22 (assuming the terminal supports non-@sc{ASCII}
control characters).

@item shift
The 2**21 bit in the character code indicates an @sc{ASCII} control
character typed with the shift key held down.

For letters, the basic code indicates upper versus lower case; for
digits and punctuation, the shift key selects an entirely different
character with a different basic code.  In order to keep within
the @sc{ASCII} character set whenever possible, Emacs avoids using
the 2**21 bit for those characters.

However, @sc{ASCII} provides no way to distinguish @kbd{C-A} from
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@kbd{C-a}, so Emacs uses the 2**21 bit in @kbd{C-A} and not in
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@kbd{C-a}.

@item hyper
The 2**20 bit in the character code indicates a character
typed with the hyper key held down.

@item super
The 2**19 bit in the character code indicates a character
typed with the super key held down.

@item alt
The 2**18 bit in the character code indicates a character typed with
the alt key held down.  (On some terminals, the key labeled @key{ALT}
is actually the meta key.)
@end table

  In the future, Emacs may support a larger range of basic codes.  We
may also move the modifier bits to larger bit numbers.  Therefore, you
should avoid mentioning specific bit numbers in your program.
Instead, the way to test the modifier bits of a character is with the
function @code{event-modifiers} (@pxref{Classifying Events}).

@node Function Keys
@subsection Function Keys

@cindex function keys
Most keyboards also have @dfn{function keys}---keys which have names or
symbols that are not characters.  Function keys are represented in Lisp
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as symbols; the symbol's name is the function key's label, in lower
case.  For example, pressing a key labeled @key{F1} places the symbol
@code{f1} in the input stream.
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The event type of a function key event is the event symbol itself.
@xref{Classifying Events}.
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Here are a few special cases in the symbol naming convention for
function keys:

@table @asis
@item @code{backspace}, @code{tab}, @code{newline}, @code{return}, @code{delete}
These keys correspond to common @sc{ASCII} control characters that have
special keys on most keyboards.

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In @sc{ASCII}, @kbd{C-i} and @key{TAB} are the same character.  If the
terminal can distinguish between them, Emacs conveys the distinction to
Lisp programs by representing the former as the integer 9, and the
latter as the symbol @code{tab}.
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Most of the time, it's not useful to distinguish the two.  So normally
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@code{function-key-map} is set up to map @code{tab} into 9.  Thus, a key
binding for character code 9 (the character @kbd{C-i}) also applies to
@code{tab}.  Likewise for the other symbols in this group.  The function
@code{read-char} likewise converts these events into characters.
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In @sc{ASCII}, @key{BS} is really @kbd{C-h}.  But @code{backspace}
converts into the character code 127 (@key{DEL}), not into code 8
(@key{BS}).  This is what most users prefer.

@item @code{kp-add}, @code{kp-decimal}, @code{kp-divide}, @dots{}
Keypad keys (to the right of the regular keyboard).
@item @code{kp-0}, @code{kp-1}, @dots{}
Keypad keys with digits.
@item @code{kp-f1}, @code{kp-f2}, @code{kp-f3}, @code{kp-f4}
Keypad PF keys.
@item @code{left}, @code{up}, @code{right}, @code{down}
Cursor arrow keys
@end table

You can use the modifier keys @key{CTRL}, @key{META}, @key{HYPER},
@key{SUPER}, @key{ALT} and @key{SHIFT} with function keys.  The way
to represent them is with prefixes in the symbol name:

@table @samp
@item A-
The alt modifier.
@item C-
The control modifier.
@item H-
The hyper modifier.
@item M-
The meta modifier.
@item S-
The shift modifier.
@item s-
The super modifier.
@end table

Thus, the symbol for the key @key{F3} with @key{META} held down is
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@kbd{M-@key{f3}}.  When you use more than one prefix, we recommend you
write them in alphabetical order; but the order does not matter in
arguments to the key-binding lookup and modification functions.

@node Mouse Events
@subsection Mouse Events

Emacs supports four kinds of mouse events: click events, drag events,
button-down events, and motion events.  All mouse events are represented
as lists.  The @sc{car} of the list is the event type; this says which
mouse button was involved, and which modifier keys were used with it.
The event type can also distinguish double or triple button presses
(@pxref{Repeat Events}).  The rest of the list elements give position
and time information.

For key lookup, only the event type matters: two events of the same type
necessarily run the same command.  The command can access the full
values of these events using the @samp{e} interactive code.
@xref{Interactive Codes}.

A key sequence that starts with a mouse event is read using the keymaps
of the buffer in the window that the mouse was in, not the current
buffer.  This does not imply that clicking in a window selects that
window or its buffer---that is entirely under the control of the command
binding of the key sequence.
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@node Click Events
@subsection Click Events
@cindex click event
@cindex mouse click event

When the user presses a mouse button and releases it at the same
location, that generates a @dfn{click} event.  Mouse click events have
this form:

@example
(@var{event-type}
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 (@var{window} @var{buffer-pos} (@var{x} . @var{y}) @var{timestamp})
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 @var{click-count})
@end example

Here is what the elements normally mean:

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@table @asis
@item @var{event-type}
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This is a symbol that indicates which mouse button was used.  It is
one of the symbols @code{mouse-1}, @code{mouse-2}, @dots{}, where the
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buttons are numbered left to right.
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You can also use prefixes @samp{A-}, @samp{C-}, @samp{H-}, @samp{M-},
@samp{S-} and @samp{s-} for modifiers alt, control, hyper, meta, shift
and super, just as you would with function keys.

This symbol also serves as the event type of the event.  Key bindings
describe events by their types; thus, if there is a key binding for
@code{mouse-1}, that binding would apply to all events whose
@var{event-type} is @code{mouse-1}.

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@item @var{window}
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This is the window in which the click occurred.

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@item @var{x}, @var{y}
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These are the pixel-based coordinates of the click, relative to the top
left corner of @var{window}, which is @code{(0 . 0)}.

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@item @var{buffer-pos}
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This is the buffer position of the character clicked on.

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@item @var{timestamp}
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This is the time at which the event occurred, in milliseconds.  (Since
this value wraps around the entire range of Emacs Lisp integers in about
five hours, it is useful only for relating the times of nearby events.)

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@item @var{click-count}
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This is the number of rapid repeated presses so far of the same mouse
button.  @xref{Repeat Events}.
@end table

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The meanings of @var{buffer-pos}, @var{x} and @var{y} are somewhat
different when the event location is in a special part of the screen,
such as the mode line or a scroll bar.
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If the location is in a scroll bar, then @var{buffer-pos} is the symbol
@code{vertical-scroll-bar} or @code{horizontal-scroll-bar}, and the pair
@code{(@var{x} . @var{y})} is replaced with a pair @code{(@var{portion}
. @var{whole})}, where @var{portion} is the distance of the click from
the top or left end of the scroll bar, and @var{whole} is the length of
the entire scroll bar.

If the position is on a mode line or the vertical line separating
@var{window} from its neighbor to the right, then @var{buffer-pos} is
the symbol @code{mode-line} or @code{vertical-line}.  For the mode line,
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@var{y} does not have meaningful data.  For the vertical line, @var{x}
does not have meaningful data.
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@var{buffer-pos} may be a list containing a symbol (one of the symbols
listed above) instead of just the symbol.  This is what happens after
the imaginary prefix keys for these events are inserted into the input
stream.  @xref{Key Sequence Input}.

@node Drag Events
@subsection Drag Events
@cindex drag event
@cindex mouse drag event

With Emacs, you can have a drag event without even changing your
clothes.  A @dfn{drag event} happens every time the user presses a mouse
button and then moves the mouse to a different character position before
releasing the button.  Like all mouse events, drag events are
represented in Lisp as lists.  The lists record both the starting mouse
position and the final position, like this:

@example
(@var{event-type}
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 (@var{window1} @var{buffer-pos1} (@var{x1} . @var{y1}) @var{timestamp1})
 (@var{window2} @var{buffer-pos2} (@var{x2} . @var{y2}) @var{timestamp2})
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 @var{click-count})
@end example

For a drag event, the name of the symbol @var{event-type} contains the
prefix @samp{drag-}.  The second and third elements of the event give
the starting and ending position of the drag.  Aside from that, the data
have the same meanings as in a click event (@pxref{Click Events}).  You
can access the second element of any mouse event in the same way, with
no need to distinguish drag events from others.

The @samp{drag-} prefix follows the modifier key prefixes such as
@samp{C-} and @samp{M-}.

If @code{read-key-sequence} receives a drag event which has no key
binding, and the corresponding click event does have a binding, it
changes the drag event into a click event at the drag's starting
position.  This means that you don't have to distinguish between click
and drag events unless you want to.

@node Button-Down Events
@subsection Button-Down Events
@cindex button-down event

Click and drag events happen when the user releases a mouse button.
They cannot happen earlier, because there is no way to distinguish a
click from a drag until the button is released.

If you want to take action as soon as a button is pressed, you need to
handle @dfn{button-down} events.@footnote{Button-down is the
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conservative antithesis of drag.}  These occur as soon as a button is
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pressed.  They are represented by lists which look exactly like click
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events (@pxref{Click Events}), except that the @var{event-type} symbol
name contains the prefix @samp{down-}.  The @samp{down-} prefix follows
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modifier key prefixes such as @samp{C-} and @samp{M-}.

The function @code{read-key-sequence}, and the Emacs command loop,
ignore any button-down events that don't have command bindings.  This
means that you need not worry about defining button-down events unless
you want them to do something.  The usual reason to define a button-down
event is so that you can track mouse motion (by reading motion events)
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until the button is released.  @xref{Motion Events}.
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@node Repeat Events
@subsection Repeat Events
@cindex repeat events
@cindex double-click events
@cindex triple-click events

If you press the same mouse button more than once in quick succession
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without moving the mouse, Emacs generates special @dfn{repeat} mouse
events for the second and subsequent presses.
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The most common repeat events are @dfn{double-click} events.  Emacs
generates a double-click event when you click a button twice; the event
happens when you release the button (as is normal for all click
events).

The event type of a double-click event contains the prefix
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@samp{double-}.  Thus, a double click on the second mouse button with
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@key{meta} held down comes to the Lisp program as
@code{M-double-mouse-2}.  If a double-click event has no binding, the
binding of the corresponding ordinary click event is used to execute
it.  Thus, you need not pay attention to the double click feature 
unless you really want to.

When the user performs a double click, Emacs generates first an ordinary
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click event, and then a double-click event.  Therefore, you must design
the command binding of the double click event to assume that the
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single-click command has already run.  It must produce the desired
results of a double click, starting from the results of a single click.

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This is convenient, if the meaning of a double click somehow ``builds
on'' the meaning of a single click---which is recommended user interface
design practice for double clicks.
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If you click a button, then press it down again and start moving the
mouse with the button held down, then you get a @dfn{double-drag} event
when you ultimately release the button.  Its event type contains
@samp{double-drag} instead of just @samp{drag}.  If a double-drag event
has no binding, Emacs looks for an alternate binding as if the event
were an ordinary click.

Before the double-click or double-drag event, Emacs generates a
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@dfn{double-down} event when the user presses the button down for the
second time.  Its event type contains @samp{double-down} instead of just
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@samp{down}.  If a double-down event has no binding, Emacs looks for an
alternate binding as if the event were an ordinary button-down event.
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If it finds no binding that way either, the double-down event is
ignored.
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To summarize, when you click a button and then press it again right
away, Emacs generates a double-down event, followed by either a
double-click or a double-drag.

If you click a button twice and then press it again, all in quick
succession, Emacs generates a @dfn{triple-down} event, followed by
either a @dfn{triple-click} or a @dfn{triple-drag}.  The event types of
these events contain @samp{triple} instead of @samp{double}.  If any
triple event has no binding, Emacs uses the binding that it would use
for the corresponding double event.

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If you click a button three or more times and then press it again, the
events for the presses beyond the third are all triple events.  Emacs
does not have separate event types for quadruple, quintuple, etc.@:
events.  However, you can look at the event list to find out precisely
how many times the button was pressed.
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@defun event-click-count event
This function returns the number of consecutive button presses that led
up to @var{event}.  If @var{event} is a double-down, double-click or
double-drag event, the value is 2.  If @var{event} is a triple event,
the value is 3 or greater.  If @var{event} is an ordinary mouse event
(not a repeat event), the value is 1.
@end defun

@defvar double-click-time
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To generate repeat events, successive mouse button presses must be at
the same screen position, and the number of milliseconds between
successive button presses must be less than the value of
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@code{double-click-time}.  Setting @code{double-click-time} to
@code{nil} disables multi-click detection entirely.  Setting it to
@code{t} removes the time limit; Emacs then detects multi-clicks by
position only.
@end defvar

@node Motion Events
@subsection Motion Events
@cindex motion event
@cindex mouse motion events

Emacs sometimes generates @dfn{mouse motion} events to describe motion
of the mouse without any button activity.  Mouse motion events are
represented by lists that look like this:

@example
(mouse-movement
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@end example

The second element of the list describes the current position of the
mouse, just as in a click event (@pxref{Click Events}).

The special form @code{track-mouse} enables generation of motion events
within its body.  Outside of @code{track-mouse} forms, Emacs does not
generate events for mere motion of the mouse, and these events do not
appear.

@defspec track-mouse body@dots{}
This special form executes @var{body}, with generation of mouse motion
events enabled.  Typically @var{body} would use @code{read-event}
to read the motion events and modify the display accordingly.

When the user releases the button, that generates a click event.
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Typically, @var{body} should return when it sees the click event, and
discard that event.
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@end defspec

@node Focus Events
@subsection Focus Events
@cindex focus event

Window systems provide general ways for the user to control which window
gets keyboard input.  This choice of window is called the @dfn{focus}.
When the user does something to switch between Emacs frames, that
generates a @dfn{focus event}.  The normal definition of a focus event,
in the global keymap, is to select a new frame within Emacs, as the user
would expect.  @xref{Input Focus}.

Focus events are represented in Lisp as lists that look like this:

@example
(switch-frame @var{new-frame})
@end example

@noindent
where @var{new-frame} is the frame switched to.

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Most X window window managers are set up so that just moving the mouse
into a window is enough to set the focus there.  Emacs appears to do
this, because it changes the cursor to solid in the new frame.  However,
there is no need for the Lisp program to know about the focus change
until some other kind of input arrives.  So Emacs generates a focus
event only when the user actually types a keyboard key or presses a
mouse button in the new frame; just moving the mouse between frames does
not generate a focus event.
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A focus event in the middle of a key sequence would garble the
sequence.  So Emacs never generates a focus event in the middle of a key
sequence.  If the user changes focus in the middle of a key
sequence---that is, after a prefix key---then Emacs reorders the events
so that the focus event comes either before or after the multi-event key
sequence, and not within it.

@node Event Examples
@subsection Event Examples

If the user presses and releases the left mouse button over the same
location, that generates a sequence of events like this:

@smallexample
(down-mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864320))
(mouse-1      (#<window 18 on NEWS> 2613 (0 . 38) -864180))
@end smallexample

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While holding the control key down, the user might hold down the
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second mouse button, and drag the mouse from one line to the next.
That produces two events, as shown here:

@smallexample
(C-down-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219))
(C-drag-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219)
                (#<window 18 on NEWS> 3510 (0 . 28) -729648))
@end smallexample

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While holding down the meta and shift keys, the user might press the
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second mouse button on the window's mode line, and then drag the mouse
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into another window.  That produces a pair of events like these:
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@smallexample
(M-S-down-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844))
(M-S-drag-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844)
                  (#<window 20 on carlton-sanskrit.tex> 161 (33 . 3)
                   -453816))
@end smallexample

@node Classifying Events
@subsection Classifying Events
@cindex event type

  Every event has an @dfn{event type} which classifies the event for key
binding purposes.  For a keyboard event, the event type equals the event
value; thus, the event type for a character is the character, and the
event type for a function key symbol is the symbol itself.  For events
which are lists, the event type is the symbol in the @sc{car} of the
list.  Thus, the event type is always a symbol or a character.

  Two events of the same type are equivalent where key bindings are
concerned; thus, they always run the same command.  That does not
necessarily mean they do the same things, however, as some commands look
at the whole event to decide what to do.  For example, some commands use
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the location of a mouse event to decide where in the buffer to act.
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  Sometimes broader classifications of events are useful.  For example,
you might want to ask whether an event involved the @key{META} key,
regardless of which other key or mouse button was used.

  The functions @code{event-modifiers} and @code{event-basic-type} are
provided to get such information conveniently.

@defun event-modifiers event
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This function returns a list of the modifiers that @var{event} has.  The
modifiers are symbols; they include @code{shift}, @code{control},
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@code{meta}, @code{alt}, @code{hyper} and @code{super}.  In addition,
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the modifiers list of a mouse event symbol always contains one of
@code{click}, @code{drag}, and @code{down}.

The argument @var{event} may be an entire event object, or just an event
type.

Here are some examples:
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@example
(event-modifiers ?a)
     @result{} nil
(event-modifiers ?\C-a)
     @result{} (control)
(event-modifiers ?\C-%)
     @result{} (control)
(event-modifiers ?\C-\S-a)
     @result{} (control shift)
(event-modifiers 'f5)
     @result{} nil
(event-modifiers 's-f5)
     @result{} (super)
(event-modifiers 'M-S-f5)
     @result{} (meta shift)
(event-modifiers 'mouse-1)
     @result{} (click)
(event-modifiers 'down-mouse-1)
     @result{} (down)
@end example

The modifiers list for a click event explicitly contains @code{click},
but the event symbol name itself does not contain @samp{click}.
@end defun

@defun event-basic-type event
This function returns the key or mouse button that @var{event}
describes, with all modifiers removed.  For example:

@example
(event-basic-type ?a)
     @result{} 97
(event-basic-type ?A)
     @result{} 97
(event-basic-type ?\C-a)
     @result{} 97
(event-basic-type ?\C-\S-a)
     @result{} 97
(event-basic-type 'f5)
     @result{} f5
(event-basic-type 's-f5)
     @result{} f5
(event-basic-type 'M-S-f5)
     @result{} f5
(event-basic-type 'down-mouse-1)
     @result{} mouse-1
@end example
@end defun

@defun mouse-movement-p object
This function returns non-@code{nil} if @var{object} is a mouse movement
event.
@end defun

@node Accessing Events
@subsection Accessing Events

  This section describes convenient functions for accessing the data in
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a mouse button or motion event.
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  These two functions return the starting or ending position of a
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mouse-button event.  The position is a list of this form:

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@example
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(@var{window} @var{buffer-position} (@var{col} . @var{row}) @var{timestamp})
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@end example
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@defun event-start event
This returns the starting position of @var{event}.

If @var{event} is a click or button-down event, this returns the
location of the event.  If @var{event} is a drag event, this returns the
drag's starting position.
@end defun

@defun event-end event
This returns the ending position of @var{event}.

If @var{event} is a drag event, this returns the position where the user
released the mouse button.  If @var{event} is a click or button-down
event, the value is actually the starting position, which is the only
position such events have.
@end defun

  These four functions take a position-list as described above, and
return various parts of it.

@defun posn-window position
Return the window that @var{position} is in.
@end defun

@defun posn-point position
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Return the buffer position in @var{position}.  This is an integer.
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@end defun

@defun posn-x-y position
Return the pixel-based x and y coordinates column in @var{position}, as
a cons cell @code{(@var{x} . @var{y})}.
@end defun

@defun posn-col-row position
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Return the row and column (in units of characters) of @var{position}, as
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a cons cell @code{(@var{col} . @var{row})}.  These are computed from the
@var{x} and @var{y} values actually found in @var{position}.
@end defun

@defun posn-timestamp position
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Return the timestamp in @var{position}.
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@end defun

@defun scroll-bar-scale ratio total
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This function multiples (in effect) @var{ratio} by @var{total}, rounding
the result to an integer.  The argument @var{ratio} is not a number, but
rather a pair @code{(@var{num} . @var{denom})}.
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This function is handy for scaling a position on a scroll bar into a
buffer position.  Here's how to do that:
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@example
(+ (point-min)
   (scroll-bar-scale
      (posn-col-row (event-start event))
      (- (point-max) (point-min))))
@end example
@end defun

@node Strings of Events
@subsection Putting Keyboard Events in Strings

  In most of the places where strings are used, we conceptualize the
string as containing text characters---the same kind of characters found
in buffers or files.  Occasionally Lisp programs use strings which
conceptually contain keyboard characters; for example, they may be key
sequences or keyboard macro definitions.  There are special rules for
how to put keyboard characters into a string, because they are not
limited to the range of 0 to 255 as text characters are.

  A keyboard character typed using the @key{META} key is called a
@dfn{meta character}.  The numeric code for such an event includes the
2**23 bit; it does not even come close to fitting in a string.  However,
earlier Emacs versions used a different representation for these
characters, which gave them codes in the range of 128 to 255.  That did
fit in a string, and many Lisp programs contain string constants that
use @samp{\M-} to express meta characters, especially as the argument to
@code{define-key} and similar functions.

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  We provide backward compatibility to run those programs using special
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rules for how to put a keyboard character event in a string.  Here are
the rules:

@itemize @bullet
@item
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If the keyboard character value is in the range of 0 to 127, it can go
in the string unchanged.
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@item
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The meta variants of those characters, with codes in the range of 2**23
to 2**23+127, can also go in the string, but you must change their
numeric values.  You must set the 2**7 bit instead of the 2**23 bit,
resulting in a value between 128 and 255.
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@item
Other keyboard character events cannot fit in a string.  This includes
keyboard events in the range of 128 to 255.
@end itemize

  Functions such as @code{read-key-sequence} that can construct strings
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of keyboard input characters follow these rules.  They construct vectors
instead of strings, when the events won't fit in a string.
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  When you use the read syntax @samp{\M-} in a string, it produces a
code in the range of 128 to 255---the same code that you get if you
modify the corresponding keyboard event to put it in the string.  Thus,
meta events in strings work consistently regardless of how they get into
the strings.

  The reason we changed the representation of meta characters as
keyboard events is to make room for basic character codes beyond 127,
and support meta variants of such larger character codes.

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  New programs can avoid dealing with these special compatibility rules
by using vectors instead of strings for key sequences when there is any
possibility that they might contain meta characters, and by using
@code{listify-key-sequence} to access a string of events.

@defun listify-key-sequence key
This function converts the string or vector @var{key} to a list of
events which you can put in @code{unread-command-events}.  Converting a
vector is simple, but converting a string is tricky because of the
special representation used for meta characters in a string.
@end defun

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@node Reading Input
@section Reading Input

  The editor command loop reads keyboard input using the function
@code{read-key-sequence}, which uses @code{read-event}.  These and other
functions for keyboard input are also available for use in Lisp
programs.  See also @code{momentary-string-display} in @ref{Temporary
Displays}, and @code{sit-for} in @ref{Waiting}.  @xref{Terminal Input},
for functions and variables for controlling terminal input modes and
debugging terminal input.

  For higher-level input facilities, see @ref{Minibuffers}.

@menu
* Key Sequence Input::		How to read one key sequence.
* Reading One Event::		How to read just one event.
* Quoted Character Input::	Asking the user to specify a character.
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* Event Input Misc::    	How to reread or throw away input events.
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@end menu

@node Key Sequence Input
@subsection Key Sequence Input
@cindex key sequence input

  The command loop reads input a key sequence at a time, by calling
@code{read-key-sequence}.  Lisp programs can also call this function;
for example, @code{describe-key} uses it to read the key to describe.

@defun read-key-sequence prompt
@cindex key sequence
This function reads a key sequence and returns it as a string or
vector.  It keeps reading events until it has accumulated a full key
sequence; that is, enough to specify a non-prefix command using the
currently active keymaps.

If the events are all characters and all can fit in a string, then
@code{read-key-sequence} returns a string (@pxref{Strings of Events}).
Otherwise, it returns a vector, since a vector can hold all kinds of
events---characters, symbols, and lists.  The elements of the string or
vector are the events in the key sequence.

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The function @code{read-key-sequence} suppresses quitting: @kbd{C-g}
typed while reading with this function works like any other character,
and does not set @code{quit-flag}.  @xref{Quitting}.
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The argument @var{prompt} is either a string to be displayed in the echo
area as a prompt, or @code{nil}, meaning not to display a prompt.

In the example below, the prompt @samp{?} is displayed in the echo area,
and the user types @kbd{C-x C-f}.

@example
(read-key-sequence "?")

@group
---------- Echo Area ----------
?@kbd{C-x C-f}
---------- Echo Area ----------

     @result{} "^X^F"
@end group
@end example
@end defun

@defvar num-input-keys
@c Emacs 19 feature
This variable's value is the number of key sequences processed so far in
this Emacs session.  This includes key sequences read from the terminal
and key sequences read from keyboard macros being executed.
@end defvar

@cindex upper case key sequence
@cindex downcasing in @code{lookup-key}
If an input character is an upper case letter and has no key binding,
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but its lower case equivalent has one, then @code{read-key-sequence}
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converts the character to lower case.  Note that @code{lookup-key} does
not perform case conversion in this way.

The function @code{read-key-sequence} also transforms some mouse events.
It converts unbound drag events into click events, and discards unbound
button-down events entirely.  It also reshuffles focus events so that they
never appear in a key sequence with any other events.

When mouse events occur in special parts of a window, such as a mode
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line or a scroll bar, the event type shows nothing special---it is the
same symbol that would normally represent that combination of mouse
button and modifier keys.  The information about the window part is
kept elsewhere in the event---in the coordinates.  But
@code{read-key-sequence} translates this information into imaginary
prefix keys, all of which are symbols: @code{mode-line},
@code{vertical-line}, @code{horizontal-scroll-bar} and
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@code{vertical-scroll-bar}.

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You can define meanings for mouse clicks in special window parts by
defining key sequences using these imaginary prefix keys.

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For example, if you call @code{read-key-sequence} and then click the
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mouse on the window's mode line, you get an event like this:
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@example
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(read-key-sequence "Click on the mode line: ")
     @result{} [mode-line
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         (mouse-1
          (#<window 6 on NEWS> mode-line
           (40 . 63) 5959987))]
@end example
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@node Reading One Event
@subsection Reading One Event

  The lowest level functions for command input are those which read a
single event.

@defun read-event
This function reads and returns the next event of command input, waiting
if necessary until an event is available.  Events can come directly from
the user or from a keyboard macro.

The function @code{read-event} does not display any message to indicate
it is waiting for input; use @code{message} first, if you wish to
display one.  If you have not displayed a message, @code{read-event}
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prompts by echoing: it displays descriptions of the events that led to
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or were read by the current command.  @xref{The Echo Area}.

If @code{cursor-in-echo-area} is non-@code{nil}, then @code{read-event}
moves the cursor temporarily to the echo area, to the end of any message
displayed there.  Otherwise @code{read-event} does not move the cursor.

Here is what happens if you call @code{read-event} and then press the
right-arrow function key:

@example
@group
(read-event)
     @result{} right
@end group
@end example
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@end defun
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@defun read-char
This function reads and returns a character of command input.  It
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discards any events that are not characters, until it gets a character.
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In the first example, the user types the character @kbd{1} (@sc{ASCII}
code 49).  The second example shows a keyboard macro definition that
calls @code{read-char} from the minibuffer using @code{eval-expression}.
@code{read-char} reads the keyboard macro's very next character, which
is @kbd{1}.  Then @code{eval-expression} displays its return value in
the echo area.
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@example
@group
(read-char)
     @result{} 49
@end group

@group
(symbol-function 'foo)
     @result{} "^[^[(read-char)^M1"
@end group
@group
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(execute-kbd-macro 'foo)
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     @print{} 49
     @result{} nil
@end group
@end example
@end defun

@node Quoted Character Input
@subsection Quoted Character Input
@cindex quoted character input

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  You can use the function @code{read-quoted-char} when to ask the user
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to specify a character, and allow the user to specify a control or meta
character conveniently with quoting or as an octal character code.  The
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command @code{quoted-insert} uses this function.
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@defun read-quoted-char &optional prompt
@cindex octal character input
@cindex control characters, reading
@cindex nonprinting characters, reading
This function is like @code{read-char}, except that if the first
character read is an octal digit (0-7), it reads up to two more octal digits
(but stopping if a non-octal digit is found) and returns the
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character represented by those digits in octal.
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Quitting is suppressed when the first character is read, so that the
user can enter a @kbd{C-g}.  @xref{Quitting}.

If @var{prompt} is supplied, it specifies a string for prompting the
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user.  The prompt string is always displayed in the echo area, followed
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by a single @samp{-}.

In the following example, the user types in the octal number 177 (which
is 127 in decimal).

@example
(read-quoted-char "What character")

@group
---------- Echo Area ----------
What character-@kbd{177}
---------- Echo Area ----------

     @result{} 127
@end group
@end example
@end defun

@need 3000

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@node Event Input Misc
@subsection Miscellaneous Event Input Features

This section describes how to ``peek ahead'' at events without using
them up, how to check for pending input, and how to discard pending
input.
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@defvar unread-command-events
@cindex next input
@cindex peeking at input
This variable holds a list of events waiting to be read as command
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input.  The events are used in the order they appear in the list, and
removed one by one as they are used.
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The variable is needed because in some cases a function reads a event
and then decides not to use it.  Storing the event in this variable
causes it to be processed normally, by the command loop or by the
functions to read command input.
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@cindex prefix argument unreading
For example, the function that implements numeric prefix arguments reads
any number of digits.  When it finds a non-digit event, it must unread
the event so that it can be read normally by the command loop.
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Likewise, incremental search uses this feature to unread events with no 
special meaning in a search, because these events should exit the search
and then execute normally.

The reliable and easy way to extract events from a key sequence to put
them in @code{unread-command-events} is to use
@code{listify-key-sequence} (@pxref{Strings of Events}).
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@end defvar

@defvar unread-command-char
This variable holds a character to be read as command input.
A value of -1 means ``empty''.

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This variable is mostly obsolete now that you can use
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@code{unread-command-events} instead; it exists only to support programs
written for Emacs versions 18 and earlier.
@end defvar

@defun input-pending-p
@cindex waiting for command key input
This function determines whether any command input is currently
available to be read.  It returns immediately, with value @code{t} if
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there is available input, @code{nil} otherwise.  On rare occasions it
may return @code{t} when no input is available.
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@end defun

@defvar last-input-event
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This variable records the last terminal input event read, whether
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as part of a command or explicitly by a Lisp program.

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In the example below, the Lisp program reads the character @kbd{1},
@sc{ASCII} code 49.  It becomes the value of @code{last-input-event},
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while @kbd{C-e} (from the @kbd{C-x C-e} command used to evaluate this
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expression) remains the value of @code{last-command-event}.
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@example
@group
(progn (print (read-char))
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       (print last-command-event)
       last-input-event)
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     @print{} 49
     @print{} 5
     @result{} 49
@end group
@end example

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@vindex last-input-char
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The alias @code{last-input-char} exists for compatibility with
Emacs version 18.
@end defvar

@defun discard-input
@cindex flush input
@cindex discard input
@cindex terminate keyboard macro
This function discards the contents of the terminal input buffer and
cancels any keyboard macro that might be in the process of definition.
It returns @code{nil}.

In the following example, the user may type a number of characters right
after starting the evaluation of the form.  After the @code{sleep-for}
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finishes sleeping, @code{discard-input} discards any characters typed 
during the sleep.
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@example
(progn (sleep-for 2)
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       (discard-input))
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     @result{} nil
@end example
@end defun

@node Waiting
@section Waiting for Elapsed Time or Input
@cindex pausing
@cindex waiting

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  The wait functions are designed to wait for a certain amount of time
to pass or until there is input.  For example, you may wish to pause in
the middle of a computation to allow the user time to view the display.
@code{sit-for} pauses and updates the screen, and returns immediately if
input comes in, while @code{sleep-for} pauses without updating the
screen.
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@defun sit-for seconds &optional millisec nodisp
This function performs redisplay (provided there is no pending input
from the user), then waits @var{seconds} seconds, or until input is
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available.  The value is @code{t} if @code{sit-for} waited the full
time with no input arriving (see @code{input-pending-p} in @ref{Event 
Input Misc}).  Otherwise, the value is @code{nil}.
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