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Global variables have values that last until explicitly superseded with new values.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.