Common Lisp the Language
2nd Edition
All numbers characters strings and bit-vectors are self-evaluating forms. When such an object is evaluated that object (or possibly a copy in the case of numbers or characters) is returned as the value of the form. The empty list () which is also the false value nil is also a self-evaluating form: the value of nil is nil. Keywords (symbols written with a leading colon) also evaluate to themselves: the value of :start is :start.
X3J13 voted in January 1989 (CONSTANT-MODIFICATION) to clarify that
it is an error to destructively modify any object that appears as a constant
in executable code
whether as
a self-evaluating form or within a quote special form.
