The reason I switched from Scheme to Common Lisp was because I wanted type checking more than I wanted hygienic macros or case-sensitive (by default) symbols.

Being able to do something like:

  (deftype Digit ()
    '(Integer 0 9)

  (deftype Digit-Vector (&optional (length '*))
    `(Vector Digit ,length))

  (defun integer->digit-vector (integer)
    (declare (type Integer integer))
    (coerce (loop :for character :across (format nil "~D" integer)
                  :for digit := (char-digit-p character)
                  :when digit
                    :collect digit)
            'Digit-Vector))

And then have SBCL warn me if I try to use the resulting Digit-Vector in a function which wants a String (for example) is useful.