scope.lisp

;; set two global variables
(setq x 'global-x)
(setq y 'global-y)

;; define a function that returns a nested function
(defun return-a-nested-fn (x)
  ;; the nested function takes no arguments and returns a list of two values
  (lambda () (list x y)))

;; call that function, and call the function returned
(print (funcall (return-a-nested-fn 'local-x)))

;; in Emacs Lisp, which defaults to dynamic scope, this program will print:
;; (global-x global-y)
;; This is because when the nested function is called, the value of x
;; is global-x, even though that was not that value when the function
;; was defined

;; in Common Lisp, which defaults to lexical scope, this program will print:
;; (local-x global-y)
;; This is because when the nested function was defined, the value of
;; x was local-x, and this is _closed over_ when we create the nested
;; function and return it

;; in Emacs Lisp below will print (let-local-x global-y)
;; in Common Lisp, this will still print (local-x global-y)
(let ((x 'let-local-x))
  (print (funcall (return-a-nested-fn 'local-x))))

;; Tested with Emacs and SBCL on OS X