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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 for 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 whether the same symbol has a function definition. See section Symbol Components.
The textual form of a program is determined by its Lisp object representation; it is the read syntax for the Lisp object which constitutes the program. This is why a variable in a textual Lisp program is written as 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 the
first argument of
setq, the name of the variable, is not
evaluated, but the second argument, the desired value, is evaluated
Once the variable has a value, you can refer to it by using the symbol by itself as an expression. Thus,
x => (a b)
setq form shown above has already been executed.
If you do another
setq, the new value replaces the old one:
x => (a b) (setq x 4) => 4 x => 4
Emacs Lisp has two special symbols,
always evaluate to themselves. These symbols cannot be rebound, nor can
their value cells be changed. An attempt to change the value of
t signals a
nil == 'nil => nil (setq nil 500) error--> Attempt to set constant symbol: nil
Global variables are given values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only while within a certain part of the program. 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 which last until the function exits. Similarly, the
let special form explicitly establishes new local values for
specified variables; these last until exit from the
When a local value is established, the previous value (or lack of one) of the variable is saved away. 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
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 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 also call the
current binding the most-local existing binding, for emphasis.
Ordinary evaluation of a symbol always returns the value of its current
The special forms
let* exist to create
Special Form: let (bindings...) forms...
This function binds variables according to bindings and then
evaluates all of the forms in textual order. The
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
nil is used.
All of the value-forms in bindings are evaluated in the
order they appear and before any of the symbols are bound. Here
is an example of this:
Z is bound to the old value of
which is 2, not the new value, 1.
(setq Y 2) => 2 (let ((Y 1) (Z Y)) (list Y Z)) => (1 2)
Special Form: let* (bindings...) forms...
This special form is like
let, except that each symbol in
bindings is bound as soon as its new value is computed, before the
computation of the values of the following local bindings. Therefore,
an expression in bindings may reasonably refer to the preceding
symbols bound in this
let* form. Compare the following example
with the example above for
(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 which create local bindings:
condition-case(see section Errors).
This variable defines the limit on the total number of local variable
unwind-protect cleanups (see section Nonlocal Exits)
that are allowed before signaling an error (with data
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.
The default value is 600.
max-lisp-eval-depth provides another limit on depth of nesting.
See section Eval.
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
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
Function: makunbound symbol
This function makes the current binding of symbol void. This
causes any future attempt to use this symbol as a variable to signal the
void-variable, unless or until you set it again.
makunbound returns symbol.
(makunbound 'x) ; Make the global value ; of
xvoid. => 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 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
letbinding is visible. => 2
A variable that has been made void with
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
Function: boundp variable
t if variable (a symbol) is not void;
more precisely, if its current binding is not void. It returns
(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 definition, using
In Emacs Lisp, definitions serve three purposes. First, they inform
the user who reads the code that certain symbols are intended to be
used as variables. Second, they inform the Lisp system of these things,
supplying a value and documentation. Third, they provide information to
utilities such as
make-docfile, which create data
bases of the functions and variables in a program.
The difference between
defvar is primarily
a matter of intent, serving to inform human readers of whether programs
will change the variable. Emacs Lisp does not restrict the ways in
which a variable can be used based on
declarations. However, it also makes a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
One would expect user option variables to be defined with
defconst, since programs do not change them. Unfortunately, this
has bad results if the definition is in a library that is not preloaded:
defconst would override any prior value when the library is
loaded. Users would like to be able to set the option in their init
files, and override the default value given in the definition. For this
reason, user options must be defined with
Special Form: defvar symbol [value [doc-string]]
This special form informs a person reading your code that symbol
will be used as a variable that the programs are likely to set or
change. It is also used for all user option variables except in the
preloaded parts of Emacs. Note that symbol is not evaluated;
the symbol to be defined must appear explicitly in the
If symbol already has a value (i.e., it is not void), value is not even evaluated, and symbol's value remains unchanged. If symbol is void and value is specified, it is evaluated and symbol is set to the result. (If value is not specified, the value of symbol is not changed in any case.)
If symbol has a buffer-local binding in the current buffer,
defvar sets the default value, not the local value.
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 to be a user option. This affects commands
For example, this form defines
foo but does not set its value:
(defvar foo) => foo
The following example sets the value of
gives it a documentation string:
(defvar bar 23 "The normal weight of a bar.") => bar
The following form changes the documentation string for
making it a user option, but does not change the value, since
already has a value. (The addition
(1+ 23) is not even
(defvar bar (1+ 23) "*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)) (put 'symbol 'variable-documentation 'doc-string) 'symbol)
defvar form returns symbol, but it is normally used
at top level in a file where its value does not matter.
Special Form: defconst symbol [value [doc-string]]
This special form informs a person reading your code that symbol
has a global value, established here, that will not normally be changed
or locally bound by the execution of the program. The user, however,
may be welcome to change it. Note that symbol is not evaluated;
the symbol to be defined must appear explicitly in the
defconst always evaluates value and sets the global value
of symbol to the result, provided value is given. If
symbol has a buffer-local binding in the current buffer,
defconst sets the default value, not the local value.
Please note: don't use
defconst for user option
variables in libraries that are not normally loaded. The user should be
able to specify a value for such a variable in the `.emacs' file,
so that it will be in effect if and when the library is loaded later.
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 "Pi to one place.") => pi (setq pi 4) => pi pi => 4
Function: user-variable-p variable
This function returns
t if variable is a user option,
intended to be set by the user for customization,
(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.
Note that if the
defvar special forms are
used while the variable has a local binding, the local binding's value
is set, and the global binding is not changed. This would be confusing.
But the normal way to use these special forms is at top level in a file,
where no local binding should be in effect.
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
Function: symbol-value symbol
This function returns the value of symbol. This is the value in the innermost local binding of the symbol, or its global value if it has no local bindings.
(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
void-variable error is signaled if symbol has neither a
local binding nor a global value.
The usual way to change the value of a variable is with the special
setq. When you need to compute the choice of variable at
run time, use the function
Special Form: setq [symbol form]...
This special form is the most common method of changing a variable's value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The most-local existing binding of the symbol is changed.
The value of the
setq form is the value of the last form.
(setq x (1+ 2)) => 3 x ;
xnow has a global value. => 3 (let ((x 5)) (setq x 6) ; The local binding of
xis 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 that
xis set before y (1+ x)) ; the value of
yis computed. => 11
Function: set symbol value
This function sets symbol's value to value, then
returns value. Since
set is a function, the expression
written for symbol is evaluated to obtain the symbol to be
The most-local existing binding of the variable is the binding that is
set; shadowed bindings are not affected. If symbol is not
actually a symbol, a
wrong-type-argument error is signaled.
(set one 1) error--> Symbol's value as variable is void: one (set 'one 1) => 1 (set 'two 'one) => one (set two 2) ;
twoevaluates to symbol
one. => 2 one ; So it is
onethat was set. => 2 (let ((one 1)) ; This binding of
oneis set, (set 'one 3) ; not the global value. one) => 3 one => 2
set is a more fundamental primitive that
setq. Any use of
setq can be trivially rewritten to use
setq could even be defined as a macro, given the
set itself is rarely used;
beginners hardly need to know about it. It is needed only when the
choice of variable to be set is made at run time. For example, the
set-variable, which reads a variable name from the user
and then sets the variable, needs to use
Common Lisp note: in Common Lisp,
setalways changes the symbol's special value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are special, so
setalways affects the most local existing binding.
A given symbol
foo may 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
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 textually within the function or block that binds the variable.
Common Lisp note: variables declared "special" in Common Lisp are dynamically scoped like 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 in
binder. (foo 5)) ;
foois some other function. (defun user () ;
xis used in
user. (list x))
In a lexically scoped language, the binding of
binder would never be accessible in
user is not textually contained within the function
binder. However, in dynamically scoped Emacs Lisp,
may or may not refer to the binding of
x established in
binder, depending on circumstances:
userdirectly without calling
binderat all, then whatever binding of
xis found, it cannot come from
fooas follows and call
binder, then the binding made in
binderwill be seen in
(defun foo (lose) (user))
fooas follows and call
binder, then the binding made in
binderwill not be seen in
(defun foo (x) (user))
foo is called by
binder, it binds
(The binding in
foo is said to shadow the one made in
user will access the
foo instead of the one bound by
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 work as intended in
Emacs Lisp, because after the call to
make-add exits, the
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
(make-add 2). => (lambda (m) (+ n m)) (add2 4) ; Try to add 2 to 4. error--> Symbol's value as variable is void: n
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: variable-value pairs. At entry
to a function or to a
let form, we can push bindings on 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. When a new local binding is created, the local value is stored in the value cell, and the old value (belonging to a previous binding) is pushed on a stack. When a binding is eliminated, the old value is popped off the stack and stored in 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:
You should write comments to inform other programmers that they can see all uses of the variable before them, and to advise them not to add uses elsewhere.
case-fold-searchis defined as "non-
nilmeans 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.
Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports another, unusual kind of variable binding: buffer-local bindings, which apply only to one buffer. Emacs Lisp is meant for programming editing commands, and having different values for a variable in different buffers is an important customization method.
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 the global binding is unchanged; this means that the change is visible in that buffer alone.
A variable may have buffer-local bindings in some buffers but not in
others. The global binding is shared by all the buffers that don't have
their own bindings. Thus, if you set the variable in a buffer that does
not have a buffer-local binding for it, the new value is visible in all
buffers except those with buffer-local bindings. (Here we are assuming
that there are no
let-style local bindings to complicate the issue.)
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.
The usual way to make a buffer-local binding is with
make-local-variable, which is what major mode commands use. This
affects just the current buffer; all other buffers (including those yet to
be created) continue to share the global value.
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 global value of the variable as usual,
setq creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the (default) global binding untouched. The global value can no longer
be changed with
setq; you need to use
setq-default to do
Warning: when a variable has 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. To preserve your
sanity, it is wise to avoid such situations. If you use
save-excursion around each piece of code that changes to a
different current buffer, you will not have this problem. Here is an
example of incorrect code:
(setq foo 'b) (set-buffer "a") (make-local-variable 'foo) (setq foo 'a) (let ((foo 'temp)) (set-buffer "b") ...) foo => 'a ; The old buffer-local value from buffer `a' ; is now the default value. (set-buffer "a") foo => 'temp ; The local value that should be gone ; is now the buffer-local value in buffer `a'.
save-excursion as shown here avoids the problem:
(let ((foo 'temp)) (save-excursion (set-buffer "b") ...))
Local variables in a file you edit are also represented by buffer-local bindings for the buffer that holds the file within Emacs. See section How Emacs Chooses a Major Mode.
Command: make-local-variable variable
This function creates a buffer-local binding in the current buffer for variable (a symbol). Other buffers are not affected. The value returned is variable.
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
Command: make-variable-buffer-local variable
This function marks variable (a symbol) automatically buffer-local, so that any attempt to set it will make it local to the current buffer at the time.
The value returned is variable.
Function: buffer-local-variables &optional buffer
This function tells you what the buffer-local variables are in buffer buffer. It returns an association list (see section Association Lists) in which each association contains one buffer-local variable and its value. When a buffer-local variable is void in buffer, then it appears directly in the resulting list. If buffer is omitted, the current buffer is used.
(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 local variables. ;; This one is local and void: foobar ;; This one is local and nonvoid: (bind-me . 69))
Note that storing new values into the CDRs of cons cells in this list does not change the local values of the variables.
Command: kill-local-variable variable
This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the global (default) binding of variable becomes visible in this buffer. Usually this results in a change in the value of variable, since the global value is usually different from the buffer-local value just eliminated.
It is possible to kill the local binding of a variable that automatically becomes local when set. This causes the variable to show its global value in the current buffer. However, if you set the variable again, this will once again create a local value.
kill-local-variable returns variable.
This function eliminates all the buffer-local variable bindings of the current buffer except for variables marker as "permanent". As a result, the buffer will see the default values of most variables.
This function also resets certain other information pertaining to the
buffer: its local keymap is set to
nil, its syntax table is set
to the value of
standard-syntax-table, and its abbrev table is
set to the value of
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.
A local variable is permanent if the variable name (a symbol) has a
permanent-local property that is non-
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 except when specifically overridden.
setq-default allow you
to access and change the 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 which has a buffer-local value for
The special forms
defconst also set the
default value (if they set the variable at all), rather than any local
Function: default-value symbol
This function returns symbol's default value. This is the value
that is seen in buffers that do not have their own values for this
variable. If symbol is not buffer-local, this is equivalent to
symbol-value (see section Accessing Variable Values).
Function: default-boundp variable
default-boundp tells you whether variable's
default value is nonvoid. If
(default-boundp 'foo) returns
(default-value 'foo) would get an error.
default-boundp is to
boundp is to
Special Form: setq-default symbol value
This sets the default value of symbol to value.
symbol is not evaluated, but value is. The value of the
setq-default form is value.
If a symbol is not buffer-local for the current buffer, and is not
marked automatically buffer-local, this 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
;; In buffer `foo': (make-local-variable 'local) => local (setq local 'value-in-foo) => value-in-foo (setq-default local 'new-default) => new-default local => value-in-foo (default-value 'local) => new-default ;; In (the new) buffer `bar': local => new-default (default-value 'local) => new-default (setq local 'another-default) => another-default (default-value 'local) => another-default ;; Back in buffer `foo': local => value-in-foo (default-value 'local) => another-default
Function: set-default symbol value
This function is like
setq-default, except that symbol is
(set-default (car '(a b c)) 23) => 23 (default-value 'a) => 23
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