12.5 Defining Global Variables

A variable definition is a construct that announces your intention to use a symbol as a global variable. It uses the special forms defvar or defconst, which are documented below.

A variable definition serves three purposes. First, it informs people who read the code that the symbol is intended to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as etags, allowing them to find where the variable was defined.

The difference between defconst and defvar is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with defconst. One notable difference between the two forms is that defconst unconditionally initializes the variable, whereas defvar initializes it only if it is originally void.

To define a customizable variable, you should use defcustom (which calls defvar as a subroutine). See Defining Customization Variables.

Special Form: defvar symbol [value [doc-string]] ¶

This special form defines symbol as a variable and optionally initializes and documents it. Note that it doesn’t evaluate symbol; the symbol to be defined should appear explicitly in the defvar form. defvar also marks symbol as special, meaning that its bindings should always be dynamic (see Scoping Rules for Variable Bindings).

If value is specified, and symbol is void (i.e., it has no dynamically bound value; see When a Variable is Void), then defvar evaluates value, and initializes symbol by setting it to the result of the evaluation. But if symbol is not void, defvar does not evaluate value, and leaves symbol’s value unchanged. If value is omitted, defvar doesn’t change the value of symbol in any case.

Note that specifying a value, even nil, marks the variable as special permanently. Whereas if value is omitted, then defvar marks the variable special only locally (i.e. within the current lexical scope, or within the current file, if defvar is at the top-level). This can be useful for suppressing byte compilation warnings, see Compiler Errors.

If symbol has a buffer-local binding in the current buffer, and value is specified, defvar modifies the default value of symbol, which is buffer-independent, rather than the buffer-local binding. It sets the default value if the default value is void. See Buffer-Local Variables.

If symbol is already let bound (e.g., if the defvar form occurs in a let form), then defvar sets the toplevel default value of symbol, like set-default-toplevel-value. The let binding remains in effect until its binding construct exits. See Scoping Rules for Variable Bindings.

When you evaluate a top-level defvar form with C-M-x (eval-defun) or with C-x C-e (eval-last-sexp) in Emacs Lisp mode, a special feature of these two commands arranges to set the variable unconditionally, without testing whether its value is void.

If the doc-string argument is supplied, it specifies the documentation string for the variable (stored in the symbol’s variable-documentation property). See Documentation.

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 defvar form returns symbol, but it is normally used at top level in a file where its value does not matter.

For a more elaborate example of using defvar without a value, see Local defvar example.

Special Form: defconst symbol value [doc-string] ¶

This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the defconst.

The defconst form, like defvar, marks the variable as special, meaning that it should always be dynamically bound (see Scoping Rules for Variable Bindings). In addition, it marks the variable as risky (see File Local Variables).

defconst always evaluates value, and sets the value of symbol to the result. 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.)

An example of the use of defconst is Emacs’s definition of float-pi—the mathematical constant pi, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, defconst is only advisory.

(defconst float-pi 3.141592653589793 "The value of Pi.")
     ⇒ float-pi

(setq float-pi 3)
     ⇒ float-pi

float-pi
     ⇒ 3

Warning: If you use a defconst or defvar special form while the variable has a local binding (made with let, or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, 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.