12.17.1 The setf Macro

The setf macro is the most basic way to operate on generalized variables. The setf form is like setq, except that it accepts arbitrary place forms in the first (left) argument of each pair rather than just symbols. For example, (setf (car a) b) sets the car of a to b, doing the same operation as (setcar a b), but without you having to use two separate functions for setting and accessing this type of place.

Macro: setf [place form]…

This macro evaluates form and stores its value in place, which must be a valid generalized variable form. If there are several place and form pairs, the assignments are done sequentially just as with setq. setf returns the value of the last form.

The following Lisp forms are the forms in Emacs that will work as generalized variables, and so may appear in the place argument of setf:

setf signals an error if you pass a place form that it does not know how to handle.

Note that for nthcdr, the list argument of the function must itself be a valid place form. For example, (setf (nthcdr 0 foo) 7) will set foo itself to 7.

The macros push (see Modifying List Variables) and pop (see Accessing Elements of Lists) can manipulate generalized variables, not just lists. (pop place) removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once. (push x place) inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms. Note that push and pop on an nthcdr place can be used to insert or delete at any position in a list.

The cl-lib library defines various extensions for generalized variables, including additional setf places. See Generalized Variables in Common Lisp Extensions.