setf macro is the most basic way to operate on generalized
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
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.
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
setfreturns 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
(setf x y)is exactly equivalent to
(setq x y), and
setqitself is strictly speaking redundant given that
setfexists. Most programmers will continue to prefer
setqfor 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.
aref cddr symbol-function car elt symbol-plist caar get symbol-value cadr gethash cdr nth cdar nthcdr
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,
0 foo) 7) will set
foo itself to 7.
push (see List Variables) and
(see List Elements) can manipulate generalized variables,
not just lists.
) removes and returns the first
element of the list stored in place. It is analogous to
(prog1 (car place
) (setf 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
(cons x place
)), except for evaluation of the
subforms. Note that
pop on an
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
See Generalized Variables.