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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.

macro setf [place form]…

This macro evaluates form and stores it 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                     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 in Common Lisp Extensions.