The cl-psetq form is just like setq, except that multiple
assignments are done in parallel rather than sequentially.
This special form (actually a macro) is used to assign to several
variables simultaneously. Given only one symbol and form,
it has the same effect as setq. Given several symbol
and form pairs, it evaluates all the forms in advance
and then stores the corresponding variables afterwards.
(setq x 2 y 3)
(setq x (+ x y) y (* x y))
x
⇒ 5
y ; y was computed after x was set.
⇒ 15
(setq x 2 y 3)
(cl-psetq x (+ x y) y (* x y))
x
⇒ 5
y ; y was computed before x was set.
⇒ 6
The simplest use of cl-psetq is (cl-psetq x y y x), which
exchanges the values of two variables. (The cl-rotatef form
provides an even more convenient way to swap two variables;
see Modify Macros.)
cl-psetq always returns nil.