A nice macro from cl-nlp

define-lazy-singleton.lisp

(defmacro define-lazy-singleton (name init &optional docstring)
  "Define a function NAME, that will return a singleton object,
   initialized lazily with INIT on first call.
   Also define a symbol macro <NAME> that will expand to (NAME)."
  (with-gensyms (singleton)
    `(let (,singleton)
       (defun ,name ()
         ,docstring
         (or ,singleton
             (setf ,singleton ,init)))
       (define-symbol-macro ,(mksym name :format "<~A>") (,name)))))