Comment by tmtvl

Check out Common Lisp, it's gradually typed, so if you want you can just write your code like this:

  (defun fib (n)
    (declare (type (Integer 0 100) n))
    (the (Integer 0 *)
         (if (< n 2)
             n
             (+ (fib (- n 1))
                (fib (- n 2).)