Learning CLisp - Implement Binary Search

bin-search.cl

(defun binary-search (key collection) ;; Function with simple signature
  (defun binary-search-recursion (data element lower upper) ;; Helper function with multiple parameters
    (let* ((range (- upper lower))
          (mid (+ lower (round (/ range 2))))
          (value (nth mid data)))
          (cond
            ((= value element) mid)
            ((< range 1) -1)
            ((< value element) (binary-search-recursion data element (+ mid 1) upper))
            ((> value element) (binary-search-recursion data element lower (- mid 1))))))

  (binary-search-recursion collection key 0 (- (length collection) 1))) ;; Call the helper function and recurse

(let* ((data '(10 20 30 40 50 60 70 80 90 99))
      (value 20)
      (index (binary-search value data)))
      (format t "~d" index))

DEFUN should not be nested. Use FLET or LABELS for local functions.