When a symbol is evaluated, it is treated as a variable. The result is the variable’s value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see Variables.
In the following example, we set the value of a symbol with
setq. Then we evaluate the symbol, and get back the value that
setq stored.
(setq a 123)
⇒ 123
(eval 'a)
⇒ 123
a
⇒ 123
The symbols nil and t are treated specially, so that the
value of nil is always nil, and the value of t is
always t; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval treats them like any other symbol. A symbol whose name
starts with ‘:’ also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See Variables that Never Change.