binary rational number to decimal representation - scheme

bin-rep->number.scm

(define find-point
  (lambda (str) 
    (let ((last-pos (- (string-length str) 1)))
      (let ((point? (char=? #\. (string-ref str last-pos))))
        (cond ((= (string-length str) 1) (if point? last-pos -1))
              (point? last-pos)
              (else (find-point (substring str 0 last-pos))))))))

            
(define binary-sum
  (lambda (str exp-num)
      (let ((char-to-bin (lambda (char) (if (char=? char #\0) 0 1))) (digit-value (lambda (num) (* num (expt 2 exp-num)))))
        (let ((fst-digit-value (digit-value (char-to-bin (string-ref str 0)))))
              (if (= (string-length str) 1)
                  fst-digit-value
                  (+ (binary-sum (substring str 1) (- exp-num 1)) fst-digit-value))))))

(define string-remove-at
    (lambda (str pos)
        (string-append (substring str 0 pos) (substring str (+ pos 1)))))

(define bin-rep->number-abs
  (lambda (str)
    (let ((point-pos (find-point str)))
         (if (= -1 point-pos)
             (binary-sum str (- (string-length str) 1))
             (binary-sum (string-remove-at str point-pos) (- point-pos 1))))))

(define bin-rep->number
  (lambda (str)
    (let ((fst-char-is? (lambda (char) (char=? (string-ref str 0) char))))
      (cond ((fst-char-is? #\+) (bin-rep->number-abs (string-remove-at str 0)))
            ((fst-char-is? #\-) (* -1 (bin-rep->number-abs (string-remove-at str 0))))
            (else (bin-rep->number-abs str))))))

(bin-rep->number "+1101") ; => 13
(bin-rep->number "0") ; => 0
(bin-rep->number "10110.011") ; => 22.375
(bin-rep->number "-0.1101001") ; => -0.8203125