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GNU Emacs Lisp Reference Manual

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11.5 Defining Global Variables

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.

Special Form: defvar symbol [value [doc-string]]
This special form defines symbol as a variable and can also initialize and document it. The definition informs a person reading your code that symbol is used as a variable that might be set or changed. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the 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 11.10 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 24. Documentation) look for this property.

If the variable is a user option that users would want to set interactively, you should use `*' as the first character of doc-string. This lets users set the variable conveniently using the set-variable command. Note that you should nearly always use defcustom instead of defvar to define these variables, so that users can use M-x customize and related commands to set them. See section 14. 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
     => 23

Here is an equivalent expression for the defvar special form:

(defvar symbol value doc-string)
  (if (not (boundp 'symbol))
      (setq symbol value))
  (if 'doc-string
    (put 'symbol 'variable-documentation 'doc-string))

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.

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
     => 3

Function: user-variable-p variable
This function returns 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 either though being declared using defcustom(4) or by the first character of their 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 specified in interactive (see section 21.2.1 Using interactive). However, this feature is largely obsoleted by defcustom (see section 14. 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 you usually 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.

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