Common Lisp. Dynamic variable binding using defun's lambda expression.

demo.lisp

(defvar *foo* nil)

(defun minor ()
  (format t "~&*foo* in minor: ~a~%" *foo*))

(defun bar (&key ((:baz *foo*) 42))
  (format t "~&*foo* in bar: ~a~%" *foo*)
  (minor))

(progn
  (format t "~&*foo* in global scope: ~a~%" *foo*)
  (bar :baz 'checking))

;; This code outputs:
;; *foo* in global scope: NIL
;; *foo* in bar: CHECKING
;; *foo* in minor: CHECKING

;; Here *foo* behaves as a special variable,
;; which makes this bar definition almost equivalent to

(defun bar (&key (baz *foo*))
  (let ((*foo* baz))
    (format t "~&*foo* in bar: ~a~%" *foo*)
    (minor)))