; 1.) Write the procedure so that it is in tail position. (You may need a helper function)
; 2.) Convert the function into CPS

(define (filter pred list)
  (cond [(null? list) '()]
        [(pred (car list)) (cons (car list) (filter pred (cdr list)))]
        [else (filter pred (cdr lst))]))

;one answer:
(define (filter pred list)
  (reverse (filter-helper pred list '())))

(define (filter-helper pred list x)
    (cond [(null? list) x]
          [(pred (car list)) (filter-helper pred (cdr list) (cons (car list) x))]
          [else (filter-helper pred (cdr list) x)]))

;CPS
;one answer
(define (filter-cps f l k)
    (if (null? l) (k '())
        (f (car l)
            (lambda (v)
                (if v (filter-cps f (cdr l) (lambda (v2) (cons (car l) v2)))
                      (filter-cps f (cdr l) k))))))