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:

• 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                     overlay-start
  default-value                 overlay-get
  face-background               process-buffer
  face-font                     process-filter
  face-foreground               process-get
  face-stipple                  process-sentinel
  face-underline-p              terminal-parameter
  file-modes                    window-buffer
  frame-parameter               window-dedicated-p
  frame-parameters              window-display-table
  get-register                  window-hscroll
  getenv                        window-parameter
  keymap-parent                 window-point
  match-data                    window-start
  overlay-end

• A call of the form (substring subplace n [m]), where subplace is itself a valid generalized variable whose current value is a string, and where the value stored is also a string. The new string is spliced into the specified part of the destination string. For example:

  (setq a (list "hello" "world"))
       ⇒ ("hello" "world")
  (cadr a)
       ⇒ "world"
  (substring (cadr a) 2 4)
       ⇒ "rl"
  (setf (substring (cadr a) 2 4) "o")
       ⇒ "o"
  (cadr a)
       ⇒ "wood"
  a
       ⇒ ("hello" "wood")

• The if and cond conditionals will work as generalized variables. For instance, this will set either the foo or the bar variable to zot:

  (setf (if (zerop (random 2))
  	  foo
  	bar)
        'zot)

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.