elisp: The Mark
30.7 The Mark
=============
Each buffer has a special marker, which is designated “the mark”. When
a buffer is newly created, this marker exists but does not point
anywhere; this means that the mark doesn’t exist in that buffer yet.
Subsequent commands can set the mark.
The mark specifies a position to bound a range of text for many
commands, such as ‘kill-region’ and ‘indent-rigidly’. These commands
typically act on the text between point and the mark, which is called
the “region”. If you are writing a command that operates on the region,
don’t examine the mark directly; instead, use ‘interactive’ with the ‘r’
specification. This provides the values of point and the mark as
arguments to the command in an interactive call, but permits other Lisp
programs to specify arguments explicitly. 
Interactive Codes.
Some commands set the mark as a side-effect. Commands should do this
only if it has a potential use to the user, and never for their own
internal purposes. For example, the ‘replace-regexp’ command sets the
mark to the value of point before doing any replacements, because this
enables the user to move back there conveniently after the replace is
finished.
Once the mark exists in a buffer, it normally never ceases to exist.
However, it may become “inactive”, if Transient Mark mode is enabled.
The buffer-local variable ‘mark-active’, if non-‘nil’, means that the
mark is active. A command can call the function ‘deactivate-mark’ to
deactivate the mark directly, or it can request deactivation of the mark
upon return to the editor command loop by setting the variable
‘deactivate-mark’ to a non-‘nil’ value.
If Transient Mark mode is enabled, certain editing commands that
normally apply to text near point, apply instead to the region when the
mark is active. This is the main motivation for using Transient Mark
mode. (Another is that this enables highlighting of the region when the
mark is active. Display.)
In addition to the mark, each buffer has a “mark ring” which is a
list of markers containing previous values of the mark. When editing
commands change the mark, they should normally save the old value of the
mark on the mark ring. The variable ‘mark-ring-max’ specifies the
maximum number of entries in the mark ring; once the list becomes this
long, adding a new element deletes the last element.
There is also a separate global mark ring, but that is used only in a
few particular user-level commands, and is not relevant to Lisp
programming. So we do not describe it here.
-- Function: mark &optional force
This function returns the current buffer’s mark position as an
integer, or ‘nil’ if no mark has ever been set in this buffer.
If Transient Mark mode is enabled, and ‘mark-even-if-inactive’ is
‘nil’, ‘mark’ signals an error if the mark is inactive. However,
if FORCE is non-‘nil’, then ‘mark’ disregards inactivity of the
mark, and returns the mark position (or ‘nil’) anyway.
-- Function: mark-marker
This function returns the marker that represents the current
buffer’s mark. It is not a copy, it is the marker used internally.
Therefore, changing this marker’s position will directly affect the
buffer’s mark. Don’t do that unless that is the effect you want.
(setq m (mark-marker))
⇒ #<marker at 3420 in markers.texi>
(set-marker m 100)
⇒ #<marker at 100 in markers.texi>
(mark-marker)
⇒ #<marker at 100 in markers.texi>
Like any marker, this marker can be set to point at any buffer you
like. If you make it point at any buffer other than the one of
which it is the mark, it will yield perfectly consistent, but
rather odd, results. We recommend that you not do it!
-- Function: set-mark position
This function sets the mark to POSITION, and activates the mark.
The old value of the mark is _not_ pushed onto the mark ring.
*Please note:* Use this function only if you want the user to see
that the mark has moved, and you want the previous mark position to
be lost. Normally, when a new mark is set, the old one should go
on the ‘mark-ring’. For this reason, most applications should use
‘push-mark’ and ‘pop-mark’, not ‘set-mark’.
Novice Emacs Lisp programmers often try to use the mark for the
wrong purposes. The mark saves a location for the user’s
convenience. An editing command should not alter the mark unless
altering the mark is part of the user-level functionality of the
command. (And, in that case, this effect should be documented.)
To remember a location for internal use in the Lisp program, store
it in a Lisp variable. For example:
(let ((beg (point)))
(forward-line 1)
(delete-region beg (point))).
-- Function: push-mark &optional position nomsg activate
This function sets the current buffer’s mark to POSITION, and
pushes a copy of the previous mark onto ‘mark-ring’. If POSITION
is ‘nil’, then the value of point is used.
The function ‘push-mark’ normally _does not_ activate the mark. To
do that, specify ‘t’ for the argument ACTIVATE.
A ‘Mark set’ message is displayed unless NOMSG is non-‘nil’.
-- Function: pop-mark
This function pops off the top element of ‘mark-ring’ and makes
that mark become the buffer’s actual mark. This does not move
point in the buffer, and it does nothing if ‘mark-ring’ is empty.
It deactivates the mark.
-- User Option: transient-mark-mode
This variable, if non-‘nil’, enables Transient Mark mode. In
Transient Mark mode, every buffer-modifying primitive sets
‘deactivate-mark’. As a consequence, most commands that modify the
buffer also deactivate the mark.
When Transient Mark mode is enabled and the mark is active, many
commands that normally apply to the text near point instead apply
to the region. Such commands should use the function
‘use-region-p’ to test whether they should operate on the region.
The Region.
Lisp programs can set ‘transient-mark-mode’ to non-‘nil’, non-‘t’
values to enable Transient Mark mode temporarily. If the value is
‘lambda’, Transient Mark mode is automatically turned off after any
action, such as buffer modification, that would normally deactivate
the mark. If the value is ‘(only . OLDVAL)’, then
‘transient-mark-mode’ is set to the value OLDVAL after any
subsequent command that moves point and is not shift-translated
(shift-translation Key Sequence Input.), or after any other
action that would normally deactivate the mark.
-- User Option: mark-even-if-inactive
If this is non-‘nil’, Lisp programs and the Emacs user can use the
mark even when it is inactive. This option affects the behavior of
Transient Mark mode. When the option is non-‘nil’, deactivation of
the mark turns off region highlighting, but commands that use the
mark behave as if the mark were still active.
-- Variable: deactivate-mark
If an editor command sets this variable non-‘nil’, then the editor
command loop deactivates the mark after the command returns (if
Transient Mark mode is enabled). All the primitives that change
the buffer set ‘deactivate-mark’, to deactivate the mark when the
command is finished. Setting this variable makes it buffer-local.
To write Lisp code that modifies the buffer without causing
deactivation of the mark at the end of the command, bind
‘deactivate-mark’ to ‘nil’ around the code that does the
modification. For example:
(let (deactivate-mark)
(insert " "))
-- Function: deactivate-mark &optional force
If Transient Mark mode is enabled or FORCE is non-‘nil’, this
function deactivates the mark and runs the normal hook
‘deactivate-mark-hook’. Otherwise, it does nothing.
-- Variable: mark-active
The mark is active when this variable is non-‘nil’. This variable
is always buffer-local in each buffer. Do _not_ use the value of
this variable to decide whether a command that normally operates on
text near point should operate on the region instead. Use the
function ‘use-region-p’ for that (The Region).
-- Variable: activate-mark-hook
-- Variable: deactivate-mark-hook
These normal hooks are run, respectively, when the mark becomes
active and when it becomes inactive. The hook ‘activate-mark-hook’
is also run at the end of the command loop if the mark is active
and it is possible that the region may have changed.
-- Function: handle-shift-selection
This function implements the shift-selection behavior of
point-motion commands. (emacs)Shift Selection. It is
called automatically by the Emacs command loop whenever a command
with a ‘^’ character in its ‘interactive’ spec is invoked, before
the command itself is executed (^ Interactive Codes.).
If ‘shift-select-mode’ is non-‘nil’ and the current command was
invoked via shift translation (shift-translation Key
Sequence Input.), this function sets the mark and temporarily
activates the region, unless the region was already temporarily
activated in this way. Otherwise, if the region has been activated
temporarily, it deactivates the mark and restores the variable
‘transient-mark-mode’ to its earlier value.
-- Variable: mark-ring
The value of this buffer-local variable is the list of saved former
marks of the current buffer, most recent first.
mark-ring
⇒ (#<marker at 11050 in markers.texi>
#<marker at 10832 in markers.texi>
...)
-- User Option: mark-ring-max
The value of this variable is the maximum size of ‘mark-ring’. If
more marks than this are pushed onto the ‘mark-ring’, ‘push-mark’
discards an old mark when it adds a new one.
When Delete Selection mode (Delete Selection (emacs)Using
Region.) is enabled, commands that operate on the active region (a.k.a.
“selection”) behave slightly differently. This works by adding the
function ‘delete-selection-pre-hook’ to the ‘pre-command-hook’ (
Command Overview). That function calls ‘delete-selection-helper’ to
delete the selection as appropriate for the command. If you want to
adapt a command to Delete Selection mode, put the ‘delete-selection’
property on the function’s symbol (Symbol Plists); commands that
don’t have this property on their symbol won’t delete the selection.
This property can have one of several values to tailor the behavior to
what the command is supposed to do; see the doc strings of
‘delete-selection-pre-hook’ and ‘delete-selection-helper’ for the
details.
Info Catalog
elisp: Moving Markers
elisp: Markers
elisp: The Region