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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 (see 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.