place—a place.
rotatef modifies the values of each place by
rotating values from one place into another.
If a place produces more values than there
are store variables, the extra values are ignored. If a place
produces fewer values than there are store variables, the missing values
are set to nil.
In the form (rotatef place1 place2 ... placen),
the values in place1 through placen are read and written.
Values 2 through n
and value 1 are then stored into place1 through placen.
It is as if all the places form an end-around shift register
that is rotated one place to the left, with the value of place1
being shifted around the end to placen.
For information about the evaluation of subforms of places, see Section 5.1.1.1 (Evaluation of Subforms to Places).
(let ((n 0)
(x (list 'a 'b 'c 'd 'e 'f 'g)))
(rotatef (nth (incf n) x)
(nth (incf n) x)
(nth (incf n) x))
x) → (A C D B E F G)
define-setf-expander, defsetf, setf, shiftf, *macroexpand-hook*, Section 5.1 (Generalized Reference)
The effect of
(rotatef place1 place2 ... placen)
is roughly equivalent to
(psetf place1 place2
place2 place3
...
placen place1)
except that the latter would evaluate any subforms
of each place twice, whereas rotatef evaluates them once.