Сканер и парсер

(define (list-head s i stek)
   (if (= i 0) stek (list-head (cdr s) (- i 1) (append stek (list (car s))))))

(define (findclose s i)
  (if (null? s)
      #f
      (if (eq? (car s) #\)) i (findclose (cdr s) (+ i 1)))))

(define (findopen s i)
  (if (or (not i) (= i (- 1)))
      #f
      (if (eq? (list-ref s i) #\() i (findopen s (- i 1)))))

(define (convert s)
  (if (string->number s) (string->number s) (string->symbol s)))

(define (read-string s)
  (define (skan s i stek st)
    (if (< i 0)
        (if (eq? st "")
            stek
            (cons (convert st)stek))
        (let ((a (string-ref s i)))
          (cond ((or (eq? a #\() (eq? a #\))) (skan s (- i 1) (cons a (if (eq? st "") stek (cons (convert st) stek))) ""))
            ((or (eq? a #\space) (eq? a #\newline) (eq? a #\tab))
                 (if (eq? "" st)
                     (skan s (- i 1) stek "")
                     (skan s (- i 1) (cons (convert st) stek) "")))
            (else (skan s (- i 1) stek (string-append (string a) st)))))))
  (define (list->list s)
    (let* ((r (findclose s 0))
          (l (findopen s r)))
         (if l
             (list->list (append (list-head s l '()) (list (list-head (list-tail s (+ l 1)) (- r l 1) '())) (list-tail s (+ r 1))))
             s)))
  (define (checking s k)
    (if (null? s)
        (zero? k)
        (if (< k 0)
            #f
            (cond ((eq? (car s) #\() (checking (cdr s) (+ k 1)))
                  ((eq? (car s) #\)) (checking (cdr s) (- k 1)))
            (else (checking (cdr s) k))))))
  (define (check s)
    (if (and (> (length s) 1) (not (eq? (car s) #\())) #f (checking s 0)))
  (let ((s (skan s (- (string-length s) 1) '() "")))
  (if (and (check s) (not (null? s))) (car (list->list s)) #f)))