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### 11.5 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 Variable Definitions.

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

  This special form defines symbol as a variable. Note that symbol is not evaluated; the symbol to be defined should appear explicitly in the `defvar` form. The variable is marked as special, meaning that it should always be dynamically bound (see Variable Scoping).

  If value is specified, and symbol is void (i.e., it has no dynamically bound value; see Void Variables), then value is evaluated and symbol is set to the result. But if symbol is not void, value is not evaluated, and symbol’s value is left 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` acts on the default value, 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 lexically bound (e.g., if the `defvar` form occurs in a `let` form with lexical binding enabled), then `defvar` sets the dynamic value. The lexical binding remains in effect until its binding construct exits. See Variable Scoping.

  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 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.

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 Variable Scoping). 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.

Next: Tips for Defining, Previous: Void Variables, Up: Variables [Contents][Index]