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  ANSI Common Lisp   5 Data and Control Flow   5.3 Dictionary of Data and Control Flow

5.3.66 rotatef Macro

rotatef {place}*    nil

Arguments and Values:
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 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)

See Also:
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.

Allegro CL Implementation Details:

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