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 on the left side 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.

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:

• A symbol. In other words, (setf x y) is exactly equivalent to (setq x y), and setq itself is strictly speaking redundant given that setf exists. Most programmers will continue to prefer setq for setting simple variables, though, for stylistic and historical reasons. The macro (setf x y) actually expands to (setq x y), so there is no performance penalty for using it in compiled code.
• A call to any of the following standard Lisp functions:

            aref      cddr      symbol-function
            car       elt       symbol-plist
            caar      get       symbol-value
            cadr      gethash
            cdr       nth
            cdar      nthcdr
  

• A call to any of the following Emacs-specific functions:

            alist-get                     process-get
            frame-parameter               process-sentinel
            terminal-parameter            window-buffer
            keymap-parent                 window-display-table
            match-data                    window-dedicated-p
            overlay-get                   window-hscroll
            overlay-start                 window-parameter
            overlay-end                   window-point
            process-buffer                window-start
            process-filter                default-value
  

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 List Variables) and pop (see List Elements) 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.