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, evaluation of 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>