When you run Emacs, it enters the 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.
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 read-key-sequence. Your Lisp code can also
call this function (see section Key Sequence Input). Lisp programs can also
do input at a lower level with read-event (see section Reading One Event) or discard pending input with discard-input
(see section Miscellaneous Event Input Features).
The key sequence is translated into a command through the currently
active keymaps. See section 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 M-x, then it reads the name of another
command, which it then calls. This is done by the command
execute-extended-command (see section Interactive Call).
To execute a command requires first reading the arguments for it.
This is done by calling command-execute (see section Interactive Call). For commands written in Lisp, the interactive
specification says how to read the arguments. This may use the prefix
argument (see section Prefix Command Arguments) or may read with prompting
in the minibuffer (see section Minibuffers). For example, the command
find-file has an 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.
If the command is a string or vector (i.e., a keyboard macro) then
execute-kbd-macro is used to execute it. You can call this
function yourself (see section Keyboard Macros).
To terminate the execution of a running command, type C-g. This character causes quitting (see section Quitting).
this-command contains the command that is about to
run, and last-command describes the previous command.
See section Hooks.
this-command describes the command that just ran, and
last-command describes the command before that. See section Hooks.
An erroneous function in the pre-command-hook list could easily
make Emacs go into an infinite loop of errors. To protect you from this
sort of painful problem, Emacs sets the hook variable to nil
temporarily while running the functions in the hook. Thus, if a hook
function gets an error, the hook variable is left as nil. Emacs
does the same thing for post-command-hook.
Quitting is suppressed while running pre-command-hook and
post-command-hook; this is because otherwise a quit, happening by
chance within one of these hooks, would turn off the hook.
One inconvenient result of these protective features is that you
cannot have a function in post-command-hook or
pre-command-hook which changes the value of that variable. But
that's not a real limitation. If you want hook functions to change the
hook, simply add one fixed function to the hook, and code that function
to look in another hook variable for other functions to call. Here is
an example:
;; Set up the mechanism.
(defvar current-post-command-function nil)
(defun run-current-post-command-function ()
(if current-post-command-function
(funcall current-post-command-function)))
(add-hooks 'post-command-hook
'run-current-post-command-function)
;; Here's a hook function which replaces itself
;; with a different hook function to run next time.
(defun first-post-command-function ()
(setq current-post-command-function
'second-post-command-function))
A Lisp function becomes a command when its body contains, at top
level, a form that calls the special form 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.
interactive.
interactive
This section describes how to write the interactive form that
makes a Lisp function an interactively-callable command.
A command may be called from Lisp programs like any other function, but then the caller supplies the arguments and arg-descriptor has no effect.
The interactive form has its effect because the command loop
(actually, its subroutine 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
interactive form are executed, but at this time
interactive simply returns nil without even evaluating its
argument.
There are three possibilities for the argument arg-descriptor:
nil; then the command is called with no
arguments. This leads quickly to an error if the command requires one
or more arguments.
(interactive "bFrobnicate buffer: ")The code letter `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. The prompt string can use `%' to include previous argument values (starting with the first argument) in the prompt. This is done using
format (see section 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:
(interactive "bBuffer to rename: \nsRename buffer %s to: ")If the first character in the string is `*', then an error is signaled if the buffer is read-only. If the first character in the string is `@', 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 `*' and `@' 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 `*' or `@').
interactiveThe code character descriptions below contain a number of key words, defined here as follows:
completing-read
(see section Completion). ? displays a list of possible completions.
Here are the code character descriptions for use with interactive:
fboundp). Existing,
Completion, Prompt.
commandp). Existing,
Completion, Prompt.
default-directory (see section Operating System Environment).
Existing, Completion, Default, Prompt.
default-directory. Existing, Completion, Default,
Prompt.
describe-key and
global-set-key.
nil, then
read a number as with n. Requires a number. See section Prefix Command Arguments. Prompt.
user-variable-p). See section High-Level Completion Functions. Existing,
Completion, Prompt.
interactive
Here are some examples of interactive:
(defun foo1 () ;foo1takes no arguments, (interactive) ; just moves forward two words. (forward-word 2)) => foo1 (defun foo2 (n) ;foo2takes one argument, (interactive "p") ; which is the numeric prefix. (forward-word (* 2 n))) => foo2 (defun foo3 (n) ;foo3takes one argument, (interactive "nCount:") ; which is read with the Minibuffer. (forward-word (* 2 n))) => foo3 (defun three-b (b1 b2 b3) "Select three existing buffers. Put them into three windows, selecting the last one." (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)) => three-b (three-b "*scratch*" "declarations.texi" "*mail*") => nil
After the command loop has translated a key sequence into a
definition, it invokes that definition using the function
command-execute. If the definition is a function that is a
command, command-execute calls call-interactively, which
reads the arguments and calls the command. You can also call these
functions yourself.
t if object is suitable for calling interactively;
that is, if object is a command. Otherwise, returns nil.
The interactively callable objects include strings and vectors (treated
as keyboard macros), lambda expressions that contain a top-level call to
interactive, compiled function objects made from such lambda
expressions, autoload objects that are declared as interactive
(non-nil fourth argument to autoload), and some of the
primitive functions.
A symbol is commandp if its function definition is
commandp.
Keys and keymaps are not commands. Rather, they are used to look up commands (see section Keymaps).
See documentation in section Access to Documentation Strings, for a
realistic example of using commandp.
If record-flag is non-nil, then this command and its
arguments are unconditionally added to the list command-history.
Otherwise, the command is added only if it uses the minibuffer to read
an argument. See section Command History.
commandp predicate; i.e.,
it must be an interactively callable function or a keyboard macro.
A string or vector as command is executed with
execute-kbd-macro. A function is passed to
call-interactively, along with the optional record-flag.
A symbol is handled by using its function definition in its place. A
symbol with an 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.
completing-read (see section Completion). Then it uses
command-execute to call the specified command. Whatever that
command returns becomes the value of execute-extended-command.
If the command asks for a prefix argument, it receives the value
prefix-argument. If execute-extended-command is called
interactively, the current raw prefix argument is used for
prefix-argument, and thus passed on to whatever command is run.
execute-extended-command is the normal definition of M-x,
so it uses the string `M-x ' as a prompt. (It would be better
to take the prompt from the events used to invoke
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.
(execute-extended-command 1)
---------- Buffer: Minibuffer ----------
1 M-x forward-word RET
---------- Buffer: Minibuffer ----------
=> t
t if the containing function (the one that
called interactive-p) was called interactively, with the function
call-interactively. (It makes no difference whether
call-interactively was called from Lisp or directly from the
editor command loop.) If the containing function was called by Lisp
evaluation (or with apply or funcall), then it was not
called interactively.
The most common use of interactive-p is for deciding whether to
print an informative message. As a special exception,
interactive-p returns nil whenever a keyboard macro is
being run. This is to suppress the informative messages and speed
execution of the macro.
For example:
(defun foo ()
(interactive)
(and (interactive-p)
(message "foo")))
=> foo
(defun bar ()
(interactive)
(setq foobar (list (foo) (interactive-p))))
=> bar
;; Type M-x foo.
-| foo
;; Type M-x bar.
;; This does not print anything.
foobar
=> (nil t)
The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run.
The value is copied from this-command when a command returns to
the command loop, except when the command specifies a prefix argument
for the following command.
last-command, it is normally a symbol
with a function definition.
The command loop sets this variable just before running a command, and
copies its value into last-command when the command finishes
(unless the command specifies a prefix argument for the following
command).
Some commands set this variable during their execution, as a flag for
whatever command runs next. In particular, the functions that kill text
set this-command to kill-region so that any kill commands
immediately following will know to append the killed text to the
previous kill.
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 this-command to t at the
beginning of the command, and set this-command back to its proper
value at the end, like this:
(defun foo (args...)
(interactive ...)
(let ((old-this-command this-command))
(setq this-command t)
...do the work...
(setq this-command old-this-command)))
(this-command-keys)
;; Now use C-u C-x C-e to evaluate that.
=> "^U^X^E"
One use of this variable is to figure out a good default location to pop up another menu.
self-insert-command, which uses it to decide which
character to insert.
last-command-event
;; Now use C-u C-x C-e to evaluate that.
=> 5
The value is 5 because that is the ASCII code for C-e.
The alias last-command-char exists for compatibility with
Emacs version 18.
If the value is zero, then command input is not echoed.
The Emacs command loop reads a sequence of 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.
nil if event is an input event.
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 versions 18 and earlier, characters were the only events. The event type of a character event is the character itself (an integer); see section Classifying Events.
An input character event consists of a basic code between 0 and 255, plus any or all of these modifier bits:
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 event-modifiers (see section Classifying Events).
Most keyboards also have function keys---keys that have names or
symbols that are not characters. Function keys are represented in Lisp
as symbols; the symbol's name is the function key's label, in lower
case. For example, pressing a key labeled F1 places the symbol
f1 in the input stream.
The event type of a function key event is the event symbol itself. See section Classifying Events.
Here are a few special cases in the symbol-naming convention for function keys:
backspace, tab, newline, return, delete
tab.
Most of the time, it's not useful to distinguish the two. So normally
function-key-map is set up to map tab into 9. Thus, a key
binding for character code 9 (the character C-i) also applies to
tab. Likewise for the other symbols in this group. The function
read-char likewise converts these events into characters.
In ASCII, BS is really C-h. But backspace
converts into the character code 127 (DEL), not into code 8
(BS). This is what most users prefer.
kp-add, kp-decimal, kp-divide, ...
kp-0, kp-1, ...
kp-f1, kp-f2, kp-f3, kp-f4
left, up, right, down
You can use the modifier keys ALT, CTRL, HYPER, META, SHIFT, and SUPER with function keys. The way to represent them is with prefixes in the symbol name:
Thus, the symbol for the key F3 with META held down is M-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.
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 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 (see section 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 `e' interactive code.
See section Code Characters for interactive.
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.
When the user presses a mouse button and releases it at the same location, that generates a click event. Mouse click events have this form:
(event-type (window buffer-pos (x . y) timestamp) click-count)
Here is what the elements normally mean:
mouse-1, mouse-2, ..., where the
buttons are numbered left to right.
You can also use prefixes `A-', `C-', `H-', `M-',
`S-' and `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
mouse-1, that binding would apply to all events whose
event-type is mouse-1.
(0 . 0).
The meanings of buffer-pos, x and 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.
If the location is in a scroll bar, then buffer-pos is the symbol
vertical-scroll-bar or horizontal-scroll-bar, and the pair
(x . y) is replaced with a pair (portion
. whole), where portion is the distance of the click from
the top or left end of the scroll bar, and whole is the length of
the entire scroll bar.
If the position is on a mode line or the vertical line separating
window from its neighbor to the right, then buffer-pos is
the symbol mode-line or vertical-line. For the mode line,
y does not have meaningful data. For the vertical line, x
does not have meaningful data.
In one special case, buffer-pos is a list containing a symbol (one of the symbols listed above) instead of just the symbol. This happens after the imaginary prefix keys for the event are inserted into the input stream. See section Key Sequence Input.
With Emacs, you can have a drag event without even changing your clothes. A 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:
(event-type (window1 buffer-pos1 (x1 . y1) timestamp1) (window2 buffer-pos2 (x2 . y2) timestamp2) click-count)
For a drag event, the name of the symbol event-type contains the prefix `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 (see section 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 `drag-' prefix follows the modifier key prefixes such as `C-' and `M-'.
If read-key-sequence receives a drag event that 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.
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 button-down events.(3) These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see section Click Events), except that the event-type symbol name contains the prefix `down-'. The `down-' prefix follows modifier key prefixes such as `C-' and `M-'.
The function read-key-sequence, and therefore the Emacs command
loop as well, 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) until the button is released. See section Motion Events.
If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special repeat mouse events for the second and subsequent presses.
The most common repeat events are 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
`double-'. Thus, a double click on the second mouse button with
meta held down comes to the Lisp program as
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 click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click.
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.
If you click a button, then press it down again and start moving the mouse with the button held down, then you get a double-drag event when you ultimately release the button. Its event type contains `double-drag' instead of just `drag'. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag.
Before the double-click or double-drag event, Emacs generates a double-down event when the user presses the button down for the second time. Its event type contains `double-down' instead of just `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. If it finds no binding that way either, the double-down event is ignored.
To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event.
If you click a button twice and then press it again, all in quick succession, Emacs generates a triple-down event, followed by either a triple-click or a triple-drag. The event types of these events contain `triple' instead of `double'. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event.
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.
double-click-time. Setting double-click-time to
nil disables multi-click detection entirely. Setting it to
t removes the time limit; Emacs then detects multi-clicks by
position only.
Emacs sometimes generates mouse motion events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this:
(mouse-movement (window buffer-pos (x . y) timestamp))
The second element of the list describes the current position of the mouse, just as in a click event (see section Click Events).
The special form track-mouse enables generation of motion events
within its body. Outside of track-mouse forms, Emacs does not
generate events for mere motion of the mouse, and these events do not
appear.
read-event
to read the motion events and modify the display accordingly.
When the user releases the button, that generates a click event. Typically, body should return when it sees the click event, and discard that event.
Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the focus. When the user does something to switch between Emacs frames, that generates a 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. See section Input Focus.
Focus events are represented in Lisp as lists that look like this:
(switch-frame new-frame)
where new-frame is the frame switched to.
Most X 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.
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.
If the user presses and releases the left mouse button over the same location, that generates a sequence of events like this:
(down-mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864320)) (mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864180))
While holding the control key down, the user might hold down the second mouse button, and drag the mouse from one line to the next. That produces two events, as shown here:
(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))
While holding down the meta and shift keys, the user might press the second mouse button on the window's mode line, and then drag the mouse into another window. That produces a pair of events like these:
(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))
Every event has an 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 that are lists, the event type is the symbol in the 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 the location of a mouse event to decide where in the buffer to act.
Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the META key, regardless of which other key or mouse button was used.
The functions event-modifiers and event-basic-type are
provided to get such information conveniently.
shift, control,
meta, alt, hyper and super. In addition,
the modifiers list of a mouse event symbol always contains one of
click, drag, and down.
The argument event may be an entire event object, or just an event type.
Here are some examples:
(event-modifiers ?a)
=> nil
(event-modifiers ?\C-a)
=> (control)
(event-modifiers ?\C-%)
=> (control)
(event-modifiers ?\C-\S-a)
=> (control shift)
(event-modifiers 'f5)
=> nil
(event-modifiers 's-f5)
=> (super)
(event-modifiers 'M-S-f5)
=> (meta shift)
(event-modifiers 'mouse-1)
=> (click)
(event-modifiers 'down-mouse-1)
=> (down)
The modifiers list for a click event explicitly contains click,
but the event symbol name itself does not contain `click'.
(event-basic-type ?a)
=> 97
(event-basic-type ?A)
=> 97
(event-basic-type ?\C-a)
=> 97
(event-basic-type ?\C-\S-a)
=> 97
(event-basic-type 'f5)
=> f5
(event-basic-type 's-f5)
=> f5
(event-basic-type 'M-S-f5)
=> f5
(event-basic-type 'down-mouse-1)
=> mouse-1
nil if object is a mouse movement
event.
This section describes convenient functions for accessing the data in a mouse button or motion event.
These two functions return the starting or ending position of a mouse-button event. The position is a list of this form:
(window buffer-position (x . y) timestamp)
If event is a click or button-down event, this returns the location of the event. If event is a drag event, this returns the drag's starting position.
If event is a drag event, this returns the position where the user released the mouse button. If event is a click or button-down event, the value is actually the starting position, which is the only position such events have.
These four functions take a position as described above, and return various parts of it.
(x . y).
(col . row). These are computed from the
x and y values actually found in position.
(portion . whole) containing two integers whose ratio
is the fractional position.
(num . denom)---typically a
value returned by scroll-bar-event-ratio.
This function is handy for scaling a position on a scroll bar into a buffer position. Here's how to do that:
(+ (point-min)
(scroll-bar-scale
(posn-x-y (event-start event))
(- (point-max) (point-min))))
Recall that scroll bar events have two integers forming ratio in place of a pair of x and y coordinates.
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 that 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 META key is called a
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 `\M-' to express meta characters, especially as the argument to
define-key and similar functions.
We provide backward compatibility to run those programs using special rules for how to put a keyboard character event in a string. Here are the rules:
Functions such as read-key-sequence that can construct strings
of keyboard input characters follow these rules. They construct vectors
instead of strings, when the events won't fit in a string.
When you use the read syntax `\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.
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
listify-key-sequence to access a string of events.
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.
The editor command loop reads keyboard input using the function
read-key-sequence, which uses read-event. These and other
functions for keyboard input are also available for use in Lisp
programs. See also momentary-string-display in section Temporary Displays, and sit-for in section Waiting for Elapsed Time or Input. See section Terminal Input,
for functions and variables for controlling terminal input modes and
debugging terminal input.
For higher-level input facilities, see section Minibuffers.
The command loop reads input a key sequence at a time, by calling
read-key-sequence. Lisp programs can also call this function;
for example, describe-key uses it to read the key to describe.
If the events are all characters and all can fit in a string, then
read-key-sequence returns a string (see section Putting Keyboard Events in Strings).
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.
The function read-key-sequence suppresses quitting: C-g
typed while reading with this function works like any other character,
and does not set quit-flag. See section Quitting.
The argument prompt is either a string to be displayed in the echo
area as a prompt, or nil, meaning not to display a prompt.
In the example below, the prompt `?' is displayed in the echo area, and the user types C-x C-f.
(read-key-sequence "?")
---------- Echo Area ----------
?C-x C-f
---------- Echo Area ----------
=> "^X^F"
If an input character is an upper-case letter and has no key binding,
but its lower-case equivalent has one, then read-key-sequence
converts the character to lower case. Note that lookup-key does
not perform case conversion in this way.
The function 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
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
read-key-sequence translates this information into imaginary
prefix keys, all of which are symbols: mode-line,
vertical-line, horizontal-scroll-bar and
vertical-scroll-bar.
You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys.
For example, if you call read-key-sequence and then click the
mouse on the window's mode line, you get an event like this:
(read-key-sequence "Click on the mode line: ")
=> [mode-line
(mouse-1
(#<window 6 on NEWS> mode-line
(40 . 63) 5959987))]
The lowest level functions for command input are those that read a single event.
The function read-event does not display any message to indicate
it is waiting for input; use message first, if you wish to
display one. If you have not displayed a message, read-event
prompts by echoing: it displays descriptions of the events that led to
or were read by the current command. See section The Echo Area.
If cursor-in-echo-area is non-nil, then read-event
moves the cursor temporarily to the echo area, to the end of any message
displayed there. Otherwise read-event does not move the cursor.
Here is what happens if you call read-event and then press the
right-arrow function key:
(read-event)
=> right
In the first example, the user types the character 1 (ASCII
code 49). The second example shows a keyboard macro definition that
calls read-char from the minibuffer using eval-expression.
read-char reads the keyboard macro's very next character, which
is 1. Then eval-expression displays its return value in
the echo area.
(read-char)
=> 49
(symbol-function 'foo)
=> "^[^[(read-char)^M1"
(execute-kbd-macro 'foo)
-| 49
=> nil
You can use the function read-quoted-char to ask the user to
specify a character, and allow the user to specify a control or meta
character conveniently, either literally or as an octal character code.
The command quoted-insert uses this function.
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
character represented by those digits in octal.
Quitting is suppressed when the first character is read, so that the user can enter a C-g. See section Quitting.
If prompt is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single `-'.
In the following example, the user types in the octal number 177 (which is 127 in decimal).
(read-quoted-char "What character")
---------- Echo Area ----------
What character-177
---------- Echo Area ----------
=> 127
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.
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.
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. 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 so as to
put them in unread-command-events is to use
listify-key-sequence (see section Putting Keyboard Events in Strings).
This variable is mostly obsolete now that you can use
unread-command-events instead; it exists only to support programs
written for Emacs versions 18 and earlier.
t if
there is available input, nil otherwise. On rare occasions it
may return t when no input is available.
In the example below, the Lisp program reads the character 1,
ASCII code 49. It becomes the value of last-input-event,
while C-e (from the C-x C-e command used to evaluate this
expression) remains the value of last-command-event.
(progn (print (read-char))
(print last-command-event)
last-input-event)
-| 49
-| 5
=> 49
The alias last-input-char exists for compatibility with
Emacs version 18.
nil.
In the following example, the user may type a number of characters right
after starting the evaluation of the form. After the sleep-for
finishes sleeping, discard-input discards any characters typed
during the sleep.
(progn (sleep-for 2)
(discard-input))
=> nil
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.
sit-for pauses and updates the screen, and returns immediately if
input comes in, while sleep-for pauses without updating the
screen.
t if sit-for waited the full
time with no input arriving (see input-pending-p in section Miscellaneous Event Input Features). Otherwise, the value is nil.
The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. Not all operating systems support waiting periods other than multiples of a second; on those that do not, you get an error if you specify nonzero millisec.
Redisplay is always preempted if input arrives, and does not happen at
all if input is available before it starts. Thus, there is no way to
force screen updating if there is pending input; however, if there is no
input pending, you can force an update with no delay by using
(sit-for 0).
If nodisp is non-nil, then sit-for does not
redisplay, but it still returns as soon as input is available (or when
the timeout elapses).
The usual purpose of sit-for is to give the user time to read
text that you display.
nil.
The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. Not all operating systems support waiting periods other than multiples of a second; on those that do not, you get an error if you specify nonzero millisec.
Use sleep-for when you wish to guarantee a delay.
See section Time of Day, for functions to get the current time.
Typing C-g while a Lisp function is running causes Emacs to quit whatever it is doing. This means that control returns to the innermost active command loop.
Typing C-g while the command loop is waiting for keyboard input
does not cause a quit; it acts as an ordinary input character. In the
simplest case, you cannot tell the difference, because C-g
normally runs the command keyboard-quit, whose effect is to quit.
However, when C-g follows a prefix key, the result is an undefined
key. The effect is to cancel the prefix key as well as any prefix
argument.
In the minibuffer, C-g has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop within the minibuffer.) The reason why C-g does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. C-g following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if C-g always quit directly.
When C-g does directly quit, it does so by setting the variable
quit-flag to t. Emacs checks this variable at appropriate
times and quits if it is not nil. Setting quit-flag
non-nil in any way thus causes a quit.
At the level of C code, quitting cannot happen just anywhere; only at the
special places that check quit-flag. The reason for this is
that quitting at other places might leave an inconsistency in Emacs's
internal state. Because quitting is delayed until a safe place, quitting
cannot make Emacs crash.
Certain functions such as read-key-sequence or
read-quoted-char prevent quitting entirely even though they wait
for input. Instead of quitting, C-g serves as the requested
input. In the case of read-key-sequence, this serves to bring
about the special behavior of C-g in the command loop. In the
case of read-quoted-char, this is so that C-q can be used
to quote a C-g.
You can prevent quitting for a portion of a Lisp function by binding
the variable inhibit-quit to a non-nil value. Then,
although C-g still sets quit-flag to t as usual, the
usual result of this--a quit--is prevented. Eventually,
inhibit-quit will become nil again, such as when its
binding is unwound at the end of a let form. At that time, if
quit-flag is still non-nil, the requested quit happens
immediately. This behavior is ideal when you wish to make sure that
quitting does not happen within a "critical section" of the program.
In some functions (such as read-quoted-char), C-g is
handled in a special way that does not involve quitting. This is done
by reading the input with inhibit-quit bound to t, and
setting quit-flag to nil before inhibit-quit
becomes nil again. This excerpt from the definition of
read-quoted-char shows how this is done; it also shows that
normal quitting is permitted after the first character of input.
(defun read-quoted-char (&optional prompt)
"...documentation..."
(let ((count 0) (code 0) char)
(while (< count 3)
(let ((inhibit-quit (zerop count))
(help-form nil))
(and prompt (message "%s-" prompt))
(setq char (read-char))
(if inhibit-quit (setq quit-flag nil)))
...)
(logand 255 code)))
nil, then Emacs quits immediately, unless
inhibit-quit is non-nil. Typing C-g ordinarily sets
quit-flag non-nil, regardless of inhibit-quit.
quit-flag
is set to a value other than nil. If inhibit-quit is
non-nil, then quit-flag has no special effect.
quit condition with (signal 'quit
nil). This is the same thing that quitting does. (See signal
in section Errors.)
You can specify a character other than C-g to use for quitting.
See the function set-input-mode in section Terminal Input.
Most Emacs commands can use a prefix argument, a number
specified before the command itself. (Don't confuse prefix arguments
with prefix keys.) The prefix argument is at all times represented by a
value, which may be nil, meaning there is currently no prefix
argument. Each command may use the prefix argument or ignore it.
There are two representations of the prefix argument: raw and numeric. The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation.
Here are the possible values of a raw prefix argument:
nil, meaning there is no prefix argument. Its numeric value is
1, but numerous commands make a distinction between nil and the
integer 1.
-. This indicates that M-- or C-u - was
typed, without following digits. The equivalent numeric value is
-1, but some commands make a distinction between the integer
-1 and the symbol -.
We illustrate these possibilities by calling the following function with various prefixes:
(defun display-prefix (arg) "Display the value of the raw prefix arg." (interactive "P") (message "%s" arg))
Here are the results of calling display-prefix with various
raw prefix arguments:
M-x display-prefix -| nil
C-u M-x display-prefix -| (4)
C-u C-u M-x display-prefix -| (16)
C-u 3 M-x display-prefix -| 3
M-3 M-x display-prefix -| 3 ; (Same as C-u 3.)
C-u - M-x display-prefix -| -
M-- M-x display-prefix -| - ; (Same as C-u -.)
C-u - 7 M-x display-prefix -| -7
M-- 7 M-x display-prefix -| -7 ; (Same as C-u -7.)
Emacs uses two variables to store the prefix argument:
prefix-arg and current-prefix-arg. Commands such as
universal-argument that set up prefix arguments for other
commands store them in prefix-arg. In contrast,
current-prefix-arg conveys the prefix argument to the current
command, so setting it has no effect on the prefix arguments for future
commands.
Normally, commands specify which representation to use for the prefix
argument, either numeric or raw, in the interactive declaration.
(See section Using interactive.) Alternatively, functions may look at the
value of the prefix argument directly in the variable
current-prefix-arg, but this is less clean.
nil, the value 1 is returned; if it is any other symbol,
the value -1 is returned; if it is a number, that number is
returned; if it is a list, the CAR of that list (which should be a
number) is returned.
(interactive "P").
Do not call the functions universal-argument,
digit-argument, or negative-argument unless you intend to
let the user enter the prefix argument for the next command.
The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it recursive editing. A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command.
The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.)
All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop.
Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer's local map; if you switch windows, you get the usual Emacs commands.
To invoke a recursive editing level, call the function
recursive-edit. This function contains the command loop; it also
contains a call to catch with tag exit, which makes it
possible to exit the recursive editing level by throwing to exit
(see section Explicit Nonlocal Exits: catch and throw). If you throw a value other than t,
then recursive-edit returns normally to the function that called
it. The command C-M-c (exit-recursive-edit) does this.
Throwing a t value causes recursive-edit to quit, so that
control returns to the command loop one level up. This is called
aborting, and is done by C-] (abort-recursive-edit).
Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The e command in Rmail uses this technique.) Or, if you wish to give the user different text to edit "recursively", create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The m command in Rmail does this.)
Recursive edits are useful in debugging. You can insert a call to
debug into a function definition as a sort of breakpoint, so that
you can look around when the function gets there. debug invokes
a recursive edit but also provides the other features of the debugger.
Recursive editing levels are also used when you type C-r in
query-replace or use C-x q (kbd-macro-query).
In the following example, the function simple-rec first
advances point one word, then enters a recursive edit, printing out a
message in the echo area. The user can then do any editing desired, and
then type C-M-c to exit and continue executing simple-rec.
(defun simple-rec ()
(forward-word 1)
(message "Recursive edit in progress")
(recursive-edit)
(forward-word 1))
=> simple-rec
(simple-rec)
=> nil
(throw 'exit
nil).
quit
after exiting the recursive edit. Its definition is effectively
(throw 'exit t). See section Quitting.
Disabling a command marks the command as requiring user confirmation before it can be executed. Disabling is used for commands which might be confusing to beginning users, to prevent them from using the commands by accident.
The low-level mechanism for disabling a command is to put a
non-nil disabled property on the Lisp symbol for the
command. These properties are normally set up by the user's
`.emacs' file with Lisp expressions such as this:
(put 'upcase-region 'disabled t)
For a few commands, these properties are present by default and may be removed by the `.emacs' file.
If the value of the disabled property is a string, the message
saying the command is disabled includes that string. For example:
(put 'delete-region 'disabled
"Text deleted this way cannot be yanked back!\n")
See section `Disabling' in The GNU Emacs Manual, for the details on what happens when a disabled command is invoked interactively. Disabling a command has no effect on calling it as a function from Lisp programs.
this-command-keys to determine what the user typed to run the
command, and thus find the command itself. See section Hooks.
By default, disabled-command-hook contains a function that asks
the user whether to proceed.
The command loop keeps a history of the complex commands that have
been executed, to make it convenient to repeat these commands. A
complex command is one for which the interactive argument reading
uses the minibuffer. This includes any M-x command, any
M-ESC command, and any command whose interactive
specification reads an argument from the minibuffer. Explicit use of
the minibuffer during the execution of the command itself does not cause
the command to be considered complex.
command-history
=> ((switch-to-buffer "chistory.texi")
(describe-key "^X^[")
(visit-tags-table "~/emacs/src/")
(find-tag "repeat-complex-command"))
This history list is actually a special case of minibuffer history (see section Minibuffer History), with one special twist: the elements are expressions rather than strings.
There are a number of commands devoted to the editing and recall of
previous commands. The commands repeat-complex-command, and
list-command-history are described in the user manual
(see section `Repetition' in The GNU Emacs Manual). Within the
minibuffer, the history commands used are the same ones available in any
minibuffer.
A keyboard macro is a canned sequence of input events that can be considered a command and made the definition of a key. The Lisp representation of a keyboard macro is a string or vector containing the events. Don't confuse keyboard macros with Lisp macros (see section Macros).
If macro is a symbol, then its function definition is used in place of macro. If that is another symbol, this process repeats. Eventually the result should be a string or vector. If the result is not a symbol, string, or vector, an error is signaled.
The argument count is a repeat count; macro is executed that
many times. If count is omitted or nil, macro is
executed once. If it is 0, macro is executed over and over until it
encounters an error or a failing search.
nil.
nil if no macro is
currently executing. A command can test this variable to behave
differently when run from an executing macro. Do not set this variable
yourself.
start-kbd-macro and
end-kbd-macro set this variable--do not set it yourself.