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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 ; An object, 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 buffer (list 'print (current-buffer)))
⇒ (print #<buffer eval.texi>)
;; Evaluate it.
(eval buffer)
-| #<buffer eval.texi>
⇒ #<buffer eval.texi>