A function is tail recursive if it either returns an answer directly, or the answer is exactly the result of a recursive call. Tail recursion often involves the use of an auxiliary function with extra variables as parameters.
(defun length (lst)
(lengthb lst 0)) ; init extra variable
(defun lengthb (lst answer)
(if (null lst) ; test for base case
answer ; answer for base case
(lengthb (rest lst) ; recursive call
(+ answer 1)) ) ) ; update answer
public static int length (Cons lst) {
return lengthb(lst, 0); }
public static int lengthb (Cons lst, int answer) {
if ( lst == null )
return answer;
else return lengthb(rest(lst), answer + 1); }