gistfile1.lisp

(defparameter *is-logged-in* nil)

(defparameter *users* '((golo secret)
                        (jones anothersecret)))

(defun user-by-login (login)
  (assoc login *users*))

(defun password-of (user)
  (cadr user))

(defun log-in (login password)
  (let ((stored-password (password-of (user-by-login login))))
       (when (equal stored-password password) (setf *is-logged-in* t))))

(defun log-out ()
  (setf *is-logged-in* nil))

;; ----------------------------

> *is-logged-in*
NIL

> (log-in 'golo 'secret)
T

> *is-logged-in*
T

> (log-out)
NIL