1 Variables

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. @xref{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.

1.1 Global Variables

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 normally.

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 another setq, the new value replaces the old one:

x
     ⇒ (a b)

(setq x 4)
     ⇒ 4
x
     ⇒ 4

1.2 Variables that Never Change

Emacs Lisp has two special symbols, nil and t, that always evaluate to themselves. These symbols cannot be rebound, nor can their value cells be changed. An attempt to change the value of nil or t signals a setting-constant error.

nil ≡ 'nil
     ⇒ nil

(setq nil 500)
error--> Attempt to set constant symbol: nil

1.3 Local Variables

Global variables are given values that last until explicitly superseded with new values.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 let form.

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 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 is called dynamic scoping; see 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 binding.

The special forms let and let* exist to create local bindings.

Special Form: let (bindings…) forms…

This function binds variables according to bindings and then evaluates all of the forms in textual order. The 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 any of the symbols are bound. Here is an example of this: Z is bound to the old value of Y, 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 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 which create local bindings:

• Function calls (@pxref{Functions}).
• Macro calls (@pxref{Macros}).
• condition-case (@pxref{Errors}).

Variable: max-specpdl-size

This variable defines the limit on the total number of local variable bindings and unwind-protect cleanups (@pxref{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.

The default value is 600.

max-lisp-eval-depth provides another limit on depth of nesting. @xref{Eval}.

1.4 When a Variable is “Void”

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.

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 error void-variable, unless or 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 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.

Function: boundp variable

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

1.5 Defining Global Variables

You may announce your intention to use a symbol as a global variable with a definition, using defconst or defvar.

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 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 programs will change the variable. Emacs Lisp does not restrict the ways in which a variable can be used based on defconst or defvar 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 defvar.

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 defvar.

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 (@pxref{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 such as set-variable and edit-options.

For example, this form defines foo but does not set its value:

(defvar foo)
     ⇒ foo

The following example sets 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+ 23) is not even performed.)

(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)

The 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.

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.

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 "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, 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.

Note that if the defconst and 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.

1.6 Accessing Variable Values

The usual way to reference a variable is to write the symbol which names it (@pxref{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.

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

A void-variable error is signaled if symbol has neither a local binding nor a global value.

1.7 How to Alter a Variable Value

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.

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                   ; 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 that x is set before
      y (1+ x))     ;   the value of y is 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 set.

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)         ; 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

Logically speaking, set is a more fundamental primitive that 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 needed only when the choice of variable to be set is made at run time. 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, set always changes the symbol’s special value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are special, so set always affects the most local existing binding.

1.8 Scoping Rules for Variable Bindings

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 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 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.

1.8.1 Scope

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)   ; x is bound in binder.
   (foo 5))         ; foo is some other function.

(defun user ()      ; x is used in user.
  (list x))

In a lexically scoped language, the binding of x from 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:

• If we call user directly without calling binder at all, then whatever binding of x is found, it cannot come from binder.
• If we define foo as follows and call binder, then the binding made in binder will be seen in user:

  (defun foo (lose)
    (user))
  

• If we define foo as follows and 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.

1.8.2 Extent

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 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

1.8.3 Implementation of Dynamic Scoping

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.

1.8.4 Proper Use of Dynamic Scoping

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:

• Use or bind the variable only in a few related functions, written close together in one file. Such a variable is used for communication within one program.

  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.

• Give the variable a well-defined, documented meaning, and make all appropriate functions refer to it (but not bind it or set it) wherever that meaning is relevant. For example, the variable 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.

1.9 Buffer-Local Variables

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.

1.9.1 Introduction to Buffer-Local Variables

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, but any 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 that.

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’.

But 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. @xref{Auto Major Mode}.

1.9.2 Creating and Destroying Buffer-local Bindings

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 (@pxref{Association Lists}) in which each association contains one buffer-local variable and its value. If buffer is omitted, the current buffer is used.

(setq lcl (buffer-local-variables))
⇒ ((fill-column . 75)
    (case-fold-search . t)
    …
    (mark-ring #<marker at 5454 in buffers.texi>)
    (require-final-newline . t))

Note that storing new values into the CDRs of the elements 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.

Function: kill-all-local-variables

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 fundamental-mode-abbrev-table.

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.

A 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.

1.9.3 The Default Value of a Buffer-Local Variable

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.

The functions default-value and 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 this variable.

The special forms defvar and defconst also set the default value (if they set the variable at all), rather than any local value.

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

The function default-boundp tells you whether variable’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.

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 buffer sees.

;; 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 evaluated.

(set-default (car '(a b c)) 23)
     ⇒ 23

(default-value 'a)
     ⇒ 23

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