A variable is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text of a Lisp program, variables are written using the syntax for symbols.
In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of its use as a function name. See section Symbol Components.
The Lisp objects that constitute a Lisp program determine the textual form of the program--it is simply the read syntax for those Lisp objects. This is why, for example, a variable in a textual Lisp program is written using the read syntax for the symbol that represents the variable.
The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.
You specify a value for a symbol with setq. For example,
(setq x '(a b))
gives the variable x the value (a b). Note that
setq does not evaluate its first argument, the name of the
variable, but it does evaluate the second argument, the new value.
Once the variable has a value, you can refer to it by using the symbol by itself as an expression. Thus,
x => (a b)
assuming the setq form shown above has already been executed.
If you do set the same variable again, the new value replaces the old one:
x
=> (a b)
(setq x 4)
=> 4
x
=> 4
In Emacs Lisp, certain symbols normally evaluate to themselves. These
include nil and t, as well as any symbol whose name starts
with `:'. These symbols cannot be rebound, nor can their values be
changed. Any attempt to set or bind nil or t signals a
setting-constant error. The same is true for a symbol whose name
starts with `:', except that you are allowed to set such a symbol to
itself.
nil == 'nil
=> nil
(setq nil 500)
error--> Attempt to set constant symbol: nil
nil, you are allowed to set and bind symbols
whose names start with `:' as you wish. This is to make it
possible to run old Lisp programs which do that.
Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only until a certain part of the program finishes. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The let
special form explicitly establishes new local values for specified
variables; these last until exit from the let form.
Establishing a local value saves away the previous value (or lack of one) of the variable. When the life span of the local value is over, the previous value is restored. In the mean time, we say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see section Scope).
If you set a variable (such as with setq) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entry to a function, or a special form such as let, creates the
local binding; exit from the function or from the let removes the
local binding. As long as the local binding lasts, the variable's value
is stored within it. Use of setq or set while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This rule is called
dynamic scoping; see section Scoping Rules for Variable Bindings.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the most-local existing
binding, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms let and let* exist to create
local bindings.
let-form
returns the value of the last form in forms.
Each of the bindings is either (i) a symbol, in which case
that symbol is bound to nil; or (ii) a list of the form
(symbol value-form), in which case symbol is
bound to the result of evaluating value-form. If value-form
is omitted, nil is used.
All of the value-forms in bindings are evaluated in the
order they appear and before binding any of the symbols to them.
Here is an example of this: Z is bound to the old value of
Y, which is 2, not the new value of Y, which is 1.
(setq Y 2)
=> 2
(let ((Y 1)
(Z Y))
(list Y Z))
=> (1 2)
let, but it binds each variable right
after computing its local value, before computing the local value for
the next variable. Therefore, an expression in bindings can
reasonably refer to the preceding symbols bound in this let*
form. Compare the following example with the example above for
let.
(setq Y 2)
=> 2
(let* ((Y 1)
(Z Y)) ; Use the just-established value of Y.
(list Y Z))
=> (1 1)
Here is a complete list of the other facilities that create local bindings:
condition-case (see section Errors).
Variables can also have buffer-local bindings (see section Buffer-Local Variables) and frame-local bindings (see section Frame-Local Variables); a few variables have terminal-local bindings (see section Multiple Displays). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on "where" you are in Emacs, rather than localized in time.
unwind-protect cleanups (see section Nonlocal Exits)
that are allowed before signaling an error (with data "Variable
binding depth exceeds max-specpdl-size").
This limit, with the associated error when it is exceeded, is one way
that Lisp avoids infinite recursion on an ill-defined function.
max-lisp-eval-depth provides another limit on depth of nesting.
See section Eval.
The default value is 600. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is void. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable error rather than
a value.
Note that a value of nil is not the same as void. The symbol
nil is a Lisp object and can be the value of a variable just as any
other object can be; but it is a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound.
void-variable, unless and until you set it again.
makunbound returns symbol.
(makunbound 'x) ; Make the global value of x void.
=> x
x
error--> Symbol's value as variable is void: x
If symbol is locally bound, makunbound affects the most
local existing binding. This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values. In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous local or global binding is
reexposed as usual, and the variable is no longer void unless the newly
reexposed binding was void all along.
(setq x 1) ; Put a value in the global binding.
=> 1
(let ((x 2)) ; Locally bind it.
(makunbound 'x) ; Void the local binding.
x)
error--> Symbol's value as variable is void: x
x ; The global binding is unchanged.
=> 1
(let ((x 2)) ; Locally bind it.
(let ((x 3)) ; And again.
(makunbound 'x) ; Void the innermost-local binding.
x)) ; And refer: it's void.
error--> Symbol's value as variable is void: x
(let ((x 2))
(let ((x 3))
(makunbound 'x)) ; Void inner binding, then remove it.
x) ; Now outer let binding is visible.
=> 2
A variable that has been made void with makunbound is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp to test whether a variable is
currently void.
boundp returns t if variable (a symbol) is not void;
more precisely, if its current binding is not void. It returns
nil otherwise.
(boundp 'abracadabra) ; Starts out void.
=> nil
(let ((abracadabra 5)) ; Locally bind it.
(boundp 'abracadabra))
=> t
(boundp 'abracadabra) ; Still globally void.
=> nil
(setq abracadabra 5) ; Make it globally nonvoid.
=> 5
(boundp 'abracadabra)
=> t
You may announce your intention to use a symbol as a global variable
with a variable definition: a special form, either defconst
or defvar.
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are intended to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as etags and
make-docfile, which create data bases of the functions and
variables in a program.
The difference between defconst and defvar is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on defconst or defvar
declarations. However, it does make a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
defvar.
If symbol is void and value is specified, defvar
evaluates it and sets symbol to the result. But if symbol
already has a value (i.e., it is not void), value is not even
evaluated, and symbol's value remains unchanged. If value
is omitted, the value of symbol is not changed in any case.
If symbol has a buffer-local binding in the current buffer,
defvar operates on the default value, which is buffer-independent,
not the current (buffer-local) binding. It sets the default value if
the default value is void. See section Buffer-Local Variables.
When you evaluate a top-level defvar form with C-M-x in
Emacs Lisp mode (eval-defun), a special feature of
eval-defun arranges to set the variable unconditionally, without
testing whether its value is void.
If the doc-string argument appears, it specifies the documentation
for the variable. (This opportunity to specify documentation is one of
the main benefits of defining the variable.) The documentation is
stored in the symbol's variable-documentation property. The
Emacs help functions (see section Documentation) look for this property.
If the first character of doc-string is `*', it means that
this variable is considered a user option. This lets users set the
variable conveniently using the commands set-variable and
edit-options. However, it is better to use defcustom
instead of defvar for user option variables, so you can specify
customization information. See section Writing Customization Definitions.
Here are some examples. This form defines foo but does not
initialize it:
(defvar foo)
=> foo
This example initializes the value of bar to 23, and gives
it a documentation string:
(defvar bar 23
"The normal weight of a bar.")
=> bar
The following form changes the documentation string for bar,
making it a user option, but does not change the value, since bar
already has a value. (The addition (1+ nil) would get an error
if it were evaluated, but since it is not evaluated, there is no error.)
(defvar bar (1+ nil)
"*The normal weight of a bar.")
=> bar
bar
=> 23
Here is an equivalent expression for the defvar special form:
(defvar symbol value doc-string)
==
(progn
(if (not (boundp 'symbol))
(setq symbol value))
(if 'doc-string
(put 'symbol 'variable-documentation 'doc-string))
'symbol)
The defvar form returns symbol, but it is normally used
at top level in a file where its value does not matter.
defconst.
defconst always evaluates value, and sets the value of
symbol to the result if value is given. If symbol
does have a buffer-local binding in the current buffer, defconst
sets the default value, not the buffer-local value. (But you should not
be making buffer-local bindings for a symbol that is defined with
defconst.)
Here, pi is a constant that presumably ought not to be changed
by anyone (attempts by the Indiana State Legislature notwithstanding).
As the second form illustrates, however, this is only advisory.
(defconst pi 3.1415 "Pi to five places.")
=> pi
(setq pi 3)
=> pi
pi
=> 3
t if variable is a user option--a
variable intended to be set by the user for customization--and
nil otherwise. (Variables other than user options exist for the
internal purposes of Lisp programs, and users need not know about them.)
User option variables are distinguished from other variables by the
first character of the variable-documentation property. If the
property exists and is a string, and its first character is `*',
then the variable is a user option.
If a user option variable has a variable-interactive property,
the set-variable command uses that value to control reading the
new value for the variable. The property's value is used as if it were
to interactive (see section Using interactive). However, this feature
is largely obsoleted by defcustom (see section Writing Customization Definitions).
Warning: If the defconst and defvar special
forms are used while the variable has a local binding, they set the
local binding's value; the global binding is not changed. This is not
what we really want. To prevent it, use these special forms at top
level in a file, where normally no local binding is in effect, and make
sure to load the file before making a local binding for the variable.
When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the defvar, like this:
(defvar my-mode-map
(let ((map (make-sparse-keymap)))
(define-key map "\C-c\C-a" 'my-command)
...
map)
docstring)
This method has several benefits. First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between. If it is still uninitialized,
reloading the file will initialize it properly. Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys). Third, evaluating the defvar form with
C-M-x will reinitialize the map completely.
Putting so much code in the defvar form has one disadvantage:
it puts the documentation string far away from the line which names the
variable. Here's a safe way to avoid that:
(defvar my-mode-map nil
docstring)
(if my-mode-map
nil
(let ((map (make-sparse-keymap)))
(define-key my-mode-map "\C-c\C-a" 'my-command)
...
(setq my-mode-map map)))
This has all the same advantages as putting the initialization inside
the defvar, except that you must type C-M-x twice, once on
each form, if you do want to reinitialize the variable.
But be careful not to write the code like this:
(defvar my-mode-map nil
docstring)
(if my-mode-map
nil
(setq my-mode-map (make-sparse-keymap))
(define-key my-mode-map "\C-c\C-a" 'my-command)
...)
This code sets the variable, then alters it, but it does so in more than
one step. If the user quits just after the setq, that leaves the
variable neither correctly initialized nor void nor nil. Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.
The usual way to reference a variable is to write the symbol which
names it (see section Symbol Forms). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use symbol-value.
(setq abracadabra 5)
=> 5
(setq foo 9)
=> 9
;; Here the symbol abracadabra
;; is the symbol whose value is examined.
(let ((abracadabra 'foo))
(symbol-value 'abracadabra))
=> foo
;; Here the value of abracadabra,
;; which is foo,
;; is the symbol whose value is examined.
(let ((abracadabra 'foo))
(symbol-value abracadabra))
=> 9
(symbol-value 'abracadabra)
=> 5
A void-variable error is signaled if the current binding of
symbol is void.
The usual way to change the value of a variable is with the special
form setq. When you need to compute the choice of variable at
run time, use the function set.
setq does not evaluate symbol; it sets the symbol that you
write. We say that this argument is automatically quoted. The
`q' in setq stands for "quoted."
The value of the setq form is the value of the last form.
(setq x (1+ 2))
=> 3
x ; x now has a global value.
=> 3
(let ((x 5))
(setq x 6) ; The local binding of x is set.
x)
=> 6
x ; The global value is unchanged.
=> 3
Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:
(setq x 10 ; Notice thatxis set before y (1+ x)) ; the value ofyis computed. => 11
set is a function, the expression written for
symbol is evaluated to obtain the symbol to set.
The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected.
(set one 1)
error--> Symbol's value as variable is void: one
(set 'one 1)
=> 1
(set 'two 'one)
=> one
(set two 2) ; two evaluates to symbol one.
=> 2
one ; So it is one that was set.
=> 2
(let ((one 1)) ; This binding of one is set,
(set 'one 3) ; not the global value.
one)
=> 3
one
=> 2
If symbol is not actually a symbol, a wrong-type-argument
error is signaled.
(set '(x y) 'z) error--> Wrong type argument: symbolp, (x y)
Logically speaking, set is a more fundamental primitive than
setq. Any use of setq can be trivially rewritten to use
set; setq could even be defined as a macro, given the
availability of set. However, set itself is rarely used;
beginners hardly need to know about it. It is useful only for choosing
at run time which variable to set. For example, the command
set-variable, which reads a variable name from the user and then
sets the variable, needs to use set.
Common Lisp note: In Common Lisp,
setalways changes the symbol's "special" or dynamic value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are dynamic, sosetalways affects the most local existing binding.
One other function for setting a variable is designed to add an element to a list if it is not already present in the list.
The argument symbol is not implicitly quoted; add-to-list
is an ordinary function, like set and unlike setq. Quote
the argument yourself if that is what you want.
Here's a scenario showing how to use add-to-list:
(setq foo '(a b))
=> (a b)
(add-to-list 'foo 'c) ;; Add c.
=> (c a b)
(add-to-list 'foo 'b) ;; No effect.
=> (c a b)
foo ;; foo was changed.
=> (c a b)
An equivalent expression for (add-to-list 'var
value) is this:
(or (member value var)
(setq var (cons value var)))
A given symbol foo can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
Local bindings in Emacs Lisp have indefinite scope and dynamic extent. Scope refers to where textually in the source code the binding can be accessed. Indefinite scope means that any part of the program can potentially access the variable binding. Extent refers to when, as the program is executing, the binding exists. Dynamic extent means that the binding lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called dynamic scoping. By contrast, most programming languages use lexical scoping, in which references to a local variable must be located textually within the function or block that binds the variable.
Common Lisp note: Variables declared "special" in Common Lisp are dynamically scoped, like all variables in Emacs Lisp.
Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:
(defun binder (x) ;xis bound inbinder. (foo 5)) ;foois some other function. (defun user () ;xis used ``free'' inuser. (list x))
In a lexically scoped language, the binding of x in
binder would never be accessible in user, because
user is not textually contained within the function
binder. However, in dynamically scoped Emacs Lisp, user
may or may not refer to the binding of x established in
binder, depending on circumstances:
user directly without calling binder at all,
then whatever binding of x is found, it cannot come from
binder.
foo as follows and then call binder, then the
binding made in binder will be seen in user:
(defun foo (lose) (user))
foo as follows and then call binder,
then the binding made in binder will not be seen in
user:
(defun foo (x) (user))Here, when
foo is called by binder, it binds x.
(The binding in foo is said to shadow the one made in
binder.) Therefore, user will access the x bound
by foo instead of the one bound by binder.
Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.
Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. "Local" or "automatic" variables in most languages, including C and Pascal, have dynamic extent.
One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.
To illustrate this, the function below, make-add, returns a
function that purports to add n to its own argument m. This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to make-add exits, the variable n
is no longer bound to the actual argument 2.
(defun make-add (n)
(function (lambda (m) (+ n m)))) ; Return a function.
=> make-add
(fset 'add2 (make-add 2)) ; Define function add2
; with (make-add 2).
=> (lambda (m) (+ n m))
(add2 4) ; Try to add 2 to 4.
error--> Symbol's value as variable is void: n
Some Lisp dialects have "closures", objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures.
A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called deep binding and was used in early Lisp systems.
Suppose there is a stack of bindings, which are variable-value pairs.
At entry to a function or to a let form, we can push bindings
onto the stack for the arguments or local variables created there. We
can pop those bindings from the stack at exit from the binding
construct.
We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding.
As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite.
The actual implementation of variable scoping in GNU Emacs Lisp uses a technique called shallow binding. Each variable has a standard place in which its current value is always found--the value cell of the symbol.
In shallow binding, setting the variable works by storing a value in the value cell. Creating a new binding works by pushing the old value (belonging to a previous binding) onto a stack, and storing the new local value in the value cell. Eliminating a binding works by popping the old value off the stack, into the value cell.
We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding.
Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique:
case-fold-search is defined as "non-nil means ignore case
when searching"; various search and replace functions refer to it
directly or through their subroutines, but do not bind or set it.
Then you can bind the variable in other programs, knowing reliably what
the effect will be.
In either case, you should define the variable with defvar.
This helps other people understand your program by telling them to look
for inter-function usage. It also avoids a warning from the byte
compiler. Choose the variable's name to avoid name conflicts--don't
use short names like x.
Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports additional, unusual kinds of variable binding: buffer-local bindings, which apply only in one buffer, and frame-local bindings, which apply only in one frame. Having different values for a variable in different buffers and/or frames is an important customization method.
This section describes buffer-local bindings; for frame-local bindings, see the following section, section Frame-Local Variables. (A few variables have bindings that are local to each terminal; see section Multiple Displays.)
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.
The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.
A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding (assuming there are no frame-local bindings to complicate the matter), so the new value is visible in all the buffers that see the default binding.
The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands. For example, C mode and
Lisp mode both set the variable paragraph-start to specify that only
blank lines separate paragraphs. They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode. See section Major Modes.
The usual way to make a buffer-local binding is with
make-local-variable, which is what major mode commands typically
use. This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.
A more powerful operation is to mark the variable as
automatically buffer-local by calling
make-variable-buffer-local. You can think of this as making the
variable local in all buffers, even those yet to be created. More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so. All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the default binding untouched. This means that the default value cannot
be changed with setq in any buffer; the only way to change it is
with setq-default.
Warning: When a variable has buffer-local values in one or
more buffers, you can get Emacs very confused by binding the variable
with let, changing to a different current buffer in which a
different binding is in effect, and then exiting the let. This
can scramble the values of the buffer-local and default bindings.
To preserve your sanity, avoid using a variable in that way. If you
use save-excursion around each piece of code that changes to a
different current buffer, you will not have this problem
(see section Excursions). Here is an example of what to avoid:
(setq foo 'b)
(set-buffer "a")
(make-local-variable 'foo)
(setq foo 'a)
(let ((foo 'temp))
(set-buffer "b")
body...)
foo => 'a ; The old buffer-local value from buffer `a'
; is now the default value.
(set-buffer "a")
foo => 'temp ; The local let value that should be gone
; is now the buffer-local value in buffer `a'.
But save-excursion as shown here avoids the problem:
(let ((foo 'temp))
(save-excursion
(set-buffer "b")
body...))
Note that references to foo in body access the
buffer-local binding of buffer `b'.
When a file specifies local variable values, these become buffer-local values when you visit the file. See section `File Variables' in The GNU Emacs Manual.
The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.
;; In buffer `b1':
(setq foo 5) ; Affects all buffers.
=> 5
(make-local-variable 'foo) ; Now it is local in `b1'.
=> foo
foo ; That did not change
=> 5 ; the value.
(setq foo 6) ; Change the value
=> 6 ; in `b1'.
foo
=> 6
;; In buffer `b2', the value hasn't changed.
(save-excursion
(set-buffer "b2")
foo)
=> 5
Making a variable buffer-local within a let-binding for that
variable does not work reliably, unless the buffer in which you do this
is not current either on entry to or exit from the let. This is
because let does not distinguish between different kinds of
bindings; it knows only which variable the binding was made for.
If the variable is terminal-local, this function signals an error. Such variables cannot have buffer-local bindings as well. See section Multiple Displays.
Note: do not use make-local-variable for a hook
variable. Instead, use make-local-hook. See section Hooks.
A peculiar wrinkle of this feature is that binding the variable (with
let or other binding constructs) does not create a buffer-local
binding for it. Only setting the variable (with set or
setq) does so.
The value returned is variable.
Warning: Don't assume that you should use
make-variable-buffer-local for user-option variables, simply
because users might want to customize them differently in
different buffers. Users can make any variable local, when they wish
to. It is better to leave the choice to them.
The time to use make-variable-buffer-local is when it is crucial
that no two buffers ever share the same binding. For example, when a
variable is used for internal purposes in a Lisp program which depends
on having separate values in separate buffers, then using
make-variable-buffer-local can be the best solution.
t if variable is buffer-local in buffer
buffer (which defaults to the current buffer); otherwise,
nil.
(make-local-variable 'foobar)
(makunbound 'foobar)
(make-local-variable 'bind-me)
(setq bind-me 69)
(setq lcl (buffer-local-variables))
;; First, built-in variables local in all buffers:
=> ((mark-active . nil)
(buffer-undo-list . nil)
(mode-name . "Fundamental")
...
;; Next, non-built-in buffer-local variables.
;; This one is buffer-local and void:
foobar
;; This one is buffer-local and nonvoid:
(bind-me . 69))
Note that storing new values into the CDRs of cons cells in this list does not change the buffer-local values of the variables.
If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it.
kill-local-variable returns variable.
This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively.
This function also resets certain other information pertaining to the
buffer: it sets the local keymap to nil, the syntax table to the
value of (standard-syntax-table), the case table to
(standard-case-table), and the abbrev table to the value of
fundamental-mode-abbrev-table.
The very first thing this function does is run the normal hook
change-major-mode-hook (see below).
Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.
kill-all-local-variables returns nil.
kill-all-local-variables runs this normal hook
before it does anything else. This gives major modes a way to arrange
for something special to be done if the user switches to a different
major mode. For best results, make this variable buffer-local, so that
it will disappear after doing its job and will not interfere with the
subsequent major mode. See section Hooks.
A buffer-local variable is permanent if the variable name (a
symbol) has a permanent-local property that is non-nil.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.
The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.
The functions default-value and setq-default access and
change a variable's default value regardless of whether the current
buffer has a buffer-local binding. For example, you could use
setq-default to change the default setting of
paragraph-start for most buffers; and this would work even when
you are in a C or Lisp mode buffer that has a buffer-local value for
this variable.
The special forms defvar and defconst also set the
default value (if they set the variable at all), rather than any
buffer-local or frame-local value.
symbol-value (see section Accessing Variable Values).
default-boundp tells you whether symbol's
default value is nonvoid. If (default-boundp 'foo) returns
nil, then (default-value 'foo) would get an error.
default-boundp is to default-value as boundp is to
symbol-value.
setq-default form is the value of the last form.
If a symbol is not buffer-local for the current buffer, and is not
marked automatically buffer-local, setq-default has the same
effect as setq. If symbol is buffer-local for the current
buffer, then this changes the value that other buffers will see (as long
as they don't have a buffer-local value), but not the value that the
current buffer sees.
;; In buffer `foo':
(make-local-variable 'buffer-local)
=> buffer-local
(setq buffer-local 'value-in-foo)
=> value-in-foo
(setq-default buffer-local 'new-default)
=> new-default
buffer-local
=> value-in-foo
(default-value 'buffer-local)
=> new-default
;; In (the new) buffer `bar':
buffer-local
=> new-default
(default-value 'buffer-local)
=> new-default
(setq buffer-local 'another-default)
=> another-default
(default-value 'buffer-local)
=> another-default
;; Back in buffer `foo':
buffer-local
=> value-in-foo
(default-value 'buffer-local)
=> another-default
setq-default, except that symbol is
an ordinary evaluated argument.
(set-default (car '(a b c)) 23)
=> 23
(default-value 'a)
=> 23
Just as variables can have buffer-local bindings, they can also have
frame-local bindings. These bindings belong to one frame, and are in
effect when that frame is selected. Frame-local bindings are actually
frame parameters: you create a frame-local binding in a specific frame
by calling modify-frame-parameters and specifying the variable
name as the parameter name.
To enable frame-local bindings for a certain variable, call the function
make-variable-frame-local.
If the variable is terminal-local, this function signals an error, because such variables cannot have frame-local bindings as well. See section Multiple Displays. A few variables that are implemented specially in Emacs can be (and usually are) buffer-local, but can never be frame-local.
Buffer-local bindings take precedence over frame-local bindings. Thus,
consider a variable foo: if the current buffer has a buffer-local
binding for foo, that binding is active; otherwise, if the
selected frame has a frame-local binding for foo, that binding is
active; otherwise, the default binding of foo is active.
Here is an example. First we prepare a few bindings for foo:
(setq f1 (selected-frame)) (make-variable-frame-local 'foo) ;; Make a buffer-local binding forfooin `b1'. (set-buffer (get-buffer-create "b1")) (make-local-variable 'foo) (setq foo '(b 1)) ;; Make a frame-local binding forfooin a new frame. ;; Store that frame inf2. (setq f2 (make-frame)) (modify-frame-parameters f2 '((foo . (f 2))))
Now we examine foo in various contexts. Whenever the
buffer `b1' is current, its buffer-local binding is in effect,
regardless of the selected frame:
(select-frame f1)
(set-buffer (get-buffer-create "b1"))
foo
=> (b 1)
(select-frame f2)
(set-buffer (get-buffer-create "b1"))
foo
=> (b 1)
Otherwise, the frame gets a chance to provide the binding; when frame
f2 is selected, its frame-local binding is in effect:
(select-frame f2)
(set-buffer (get-buffer "*scratch*"))
foo
=> (f 2)
When neither the current buffer nor the selected frame provides a binding, the default binding is used:
(select-frame f1)
(set-buffer (get-buffer "*scratch*"))
foo
=> nil
When the active binding of a variable is a frame-local binding, setting
the variable changes that binding. You can observe the result with
frame-parameters:
(select-frame f2)
(set-buffer (get-buffer "*scratch*"))
(setq foo 'nobody)
(assq 'foo (frame-parameters f2))
=> (foo . nobody)
We have considered the idea of bindings that are local to a category
of frames--for example, all color frames, or all frames with dark
backgrounds. We have not implemented them because it is not clear that
this feature is really useful. You can get more or less the same
results by adding a function to after-make-frame-hook, set up to
define a particular frame parameter according to the appropriate
conditions for each frame.
It would also be possible to implement window-local bindings. We don't know of many situations where they would be useful, and it seems that indirect buffers (see section Indirect Buffers) with buffer-local bindings offer a way to handle these situations more robustly.
If sufficient application is found for either of these two kinds of local bindings, we will provide it in a subsequent Emacs version.
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