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elisp: Constant Variables

 
 11.2 Variables that Never Change
 ================================
 
 In Emacs Lisp, certain symbols normally evaluate to themselves.  These
 include ‘nil’ and ‘t’, as well as any symbol whose name starts with ‘:’
 (these are called “keywords”).  These symbols cannot be rebound, nor can
 their values be changed.  Any attempt to set or bind ‘nil’ or ‘t’
 signals a ‘setting-constant’ error.  The same is true for a keyword (a
 symbol whose name starts with ‘:’), if it is interned in the standard
 obarray, except that setting such a symbol to itself is not an error.
 
      nil ≡ 'nil
           ⇒ nil
      (setq nil 500)
      error→ Attempt to set constant symbol: nil
 
  -- Function: keywordp object
      function returns ‘t’ if OBJECT is a symbol whose name starts with
      ‘:’, interned in the standard obarray, and returns ‘nil’ otherwise.
 
    These constants are fundamentally different from the constants
 defined using the ‘defconst’ special form (Defining Variables).
 A ‘defconst’ form serves to inform human readers that you do not intend
 to change the value of a variable, but Emacs does not raise an error if
 you actually change it.
 

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