setf

(setf place value [place2 value2 ...])

Generalized assignment: stores value into the location named by place and returns the value. Beyond plain variables, the supported places are the list accessors car, cdr, nth, first through fourth, rest, and the caXXXr compositions, plus elt (a runtime list/array dispatch; strings stay immutable), so you can mutate a specific slot of an existing structure in place. It expands into the appropriate primitive mutator (such as rplaca/rplacd). Place subforms are evaluated before the value, so the tail-collection idiom (setf (cdr tail) (setf tail (list x))) links the old tail.

Multiple place/value pairs assign sequentially (each pair sees the effects of the previous ones), and the last value is returned:

Beyond the built-in places, a defstruct accessor, a CLOS :accessor, and a user-defined setf-function ((defun (setf name) ...)) are also places: (setf (name arg...) value) calls the writer with the new value first. See defun for setf-function definitions.