Info Catalog elisp: Symbols elisp: Definitions

elisp: Symbol Components

 
 8.1 Symbol Components
 =====================
 
 Each symbol has four components (or “cells”), each of which references
 another object:
 
 Print name
      The symbol’s name.
 
 Value
      The symbol’s current value as a variable.
 
 Function
      The symbol’s function definition.  It can also hold a symbol, a
      keymap, or a keyboard macro.
 
 Property list
      The symbol’s property list.
 
 The print name cell always holds a string, and cannot be changed.  Each
 of the other three cells can be set to any Lisp object.
 
    The print name cell holds the string that is the name of a symbol.
 Since symbols are represented textually by their names, it is important
 not to have two symbols with the same name.  The Lisp reader ensures
 this: every time it reads a symbol, it looks for an existing symbol with
 the specified name before it creates a new one.  To get a symbol’s name,
 use the function ‘symbol-name’ (Creating Symbols).
 
    The value cell holds a symbol’s value as a variable, which is what
 you get if the symbol itself is evaluated as a Lisp expression.  
 Variables, for details about how values are set and retrieved,
 including complications such as “local bindings” and “scoping rules”.
 Most symbols can have any Lisp object as a value, but certain special
 symbols have values that cannot be changed; these include ‘nil’ and ‘t’,
 and any symbol whose name starts with ‘:’ (those are called “keywords”).
 Constant Variables.
 
    The function cell holds a symbol’s function definition.  Often, we
 refer to “the function ‘foo’” when we really mean the function stored in
 the function cell of ‘foo’; we make the distinction explicit only when
 necessary.  Typically, the function cell is used to hold a function
 (Functions) or a macro (Macros).  However, it can also
 be used to hold a symbol (Function Indirection), keyboard macro
 (Keyboard Macros), keymap (Keymaps), or autoload object
 (Autoloading).  To get the contents of a symbol’s function cell,
 use the function ‘symbol-function’ (Function Cells).
 
    The property list cell normally should hold a correctly formatted
 property list.  To get a symbol’s property list, use the function
 ‘symbol-plist’.  Symbol Properties.
 
    The function cell or the value cell may be “void”, which means that
 the cell does not reference any object.  (This is not the same thing as
 holding the symbol ‘void’, nor the same as holding the symbol ‘nil’.)
 Examining a function or value cell that is void results in an error,
 such as ‘Symbol's value as variable is void’.
 
    Because each symbol has separate value and function cells, variables
 names and function names do not conflict.  For example, the symbol
 ‘buffer-file-name’ has a value (the name of the file being visited in
 the current buffer) as well as a function definition (a primitive
 function that returns the name of the file):
 
      buffer-file-name
           ⇒ "/gnu/elisp/symbols.texi"
      (symbol-function 'buffer-file-name)
           ⇒ #<subr buffer-file-name>
 

Info Catalog elisp: Symbols elisp: Definitions