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elisp: Definitions

 
 8.2 Defining Symbols
 ====================
 
 A “definition” is a special kind of Lisp expression that announces your
 intention to use a symbol in a particular way.  It typically specifies a
 value or meaning for the symbol for one kind of use, plus documentation
 for its meaning when used in this way.  Thus, when you define a symbol
 as a variable, you can supply an initial value for the variable, plus
 documentation for the variable.
 
    ‘defvar’ and ‘defconst’ are special forms that define a symbol as a
 “global variable”—a variable that can be accessed at any point in a Lisp
 program.  Variables, for details about variables.  To define a
 customizable variable, use the ‘defcustom’ macro, which also calls
 ‘defvar’ as a subroutine (Customization).
 
    In principle, you can assign a variable value to any symbol with
 ‘setq’, whether not it has first been defined as a variable.  However,
 you ought to write a variable definition for each global variable that
 you want to use; otherwise, your Lisp program may not act correctly if
 it is evaluated with lexical scoping enabled (Variable Scoping).
 
    ‘defun’ defines a symbol as a function, creating a lambda expression
 and storing it in the function cell of the symbol.  This lambda
 expression thus becomes the function definition of the symbol.  (The
 term “function definition”, meaning the contents of the function cell,
 is derived from the idea that ‘defun’ gives the symbol its definition as
 a function.)  ‘defsubst’ and ‘defalias’ are two other ways of defining a
 function.  Functions.
 
    ‘defmacro’ defines a symbol as a macro.  It creates a macro object
 and stores it in the function cell of the symbol.  Note that a given
 symbol can be a macro or a function, but not both at once, because both
 macro and function definitions are kept in the function cell, and that
 cell can hold only one Lisp object at any given time.  Macros.
 
    As previously noted, Emacs Lisp allows the same symbol to be defined
 both as a variable (e.g., with ‘defvar’) and as a function or macro
 (e.g., with ‘defun’).  Such definitions do not conflict.
 
    These definitions also act as guides for programming tools.  For
 example, the ‘C-h f’ and ‘C-h v’ commands create help buffers containing
 links to the relevant variable, function, or macro definitions.  
 (emacs)Name Help.
 

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