Function that changes value of a variable

[wilnerthomas]

I am trying to change the value a a variable with the following function, but 
is not working as intended.

(defun constrain (var min-n max-n)  "Ensure that value of var is between MIN-N 
and MAX-N inclusive by constraining."  (cond   ((< var min-n)  (setq var 
min-n))   ((> var max-n)  (setq var max-n))   ((eq var t)     (setq var 1))   
((eq var nil)   (setq var 0))))
Have done the following test, but the value stays 21.

(defvar pingu 21)
(constrain pingu 0 8)
(message "%d" pingu)