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9.2.1 Self-Evaluating Forms

A self-evaluating form is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string "foo" evaluates to the string "foo". Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged.

'123               ; A number, shown without evaluation.
     ⇒ 123
123                ; Evaluated as usual—result is the same.
     ⇒ 123
(eval '123)        ; Evaluated “by hand”—result is the same.
     ⇒ 123
(eval (eval '123)) ; Evaluating twice changes nothing.
     ⇒ 123

It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there’s no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:

;; Build an expression containing a buffer object.
(setq print-exp (list 'print (current-buffer)))
     ⇒ (print #<buffer eval.texi>)
;; Evaluate it.
(eval print-exp)
     -| #<buffer eval.texi>
     ⇒ #<buffer eval.texi>