lazy.lisp

(defmacro lazy (&body body)
	   (let ((forced (gensym))
		 (value (gensym)))
	     `(let ((,forced nil)
		    (,value nil))
		(lambda ()
		  (unless ,forced
		    (setf ,value (progn ,@body))
		    (setf ,forced t))
		  ,value))))
(defun force (lazy-value)
	   (funcall lazy-value))
(defmacro lazy-cons (a d)
  `(lazy (cons ,a ,d)))
(defun lazy-car (x)
  (car (force x)))
(defun lazy-cdr (x)
	   (cdr (force x)))
(defparameter *integers*
	   (labels ((f (n)
		      (lazy-cons n (f (1+ n)))))
	     (f 1)))
(defun lazy-nil ()
  (lazy nil))
(defun lazy-null (x)
	   (not (force x)))
(defun make-lazy (lst)
	   (lazy (when lst
		   (cons (car lst) (make-lazy (cdr lst))))))
(defun take (n lst)
	   (unless (or (zerop n) (lazy-null lst))
	     (cons (lazy-car lst) (take (1- n) (lazy-cdr lst)))))
(defun take-all (lst)
	   (unless (lazy-null lst)
	     (cons (lazy-car lst) (take-all (lazy-cdr lst)))))
(defun lazy-mapcar (fun lst)
	   (lazy (unless (lazy-null lst)
		   (cons (funcall fun (lazy-car lst))
			 (lazy-mapcar fun (lazy-cdr lst))))))
(defun lazy-mapcan (fun lst)
	   (labels ((f (lst-cur)
		      (if (lazy-null lst-cur)
			  (force (lazy-mapcan fun (lazy-cdr lst)))
			  (cons (lazy-car lst-cur) (lazy (f (lazy-cdr lst-cur)))))))
	     (lazy (unless (lazy-null lst)
		     (f (funcall fun (lazy-car lst)))))))
(defun lazy-find-if (fun lst)
	   (unless (lazy-null lst)
	     (let ((x (lazy-car lst)))
	       (if (funcall fun x)
		   x
		   (lazy-find-if fun (lazy-cdr lst))))))
(defun lazy-nth (n lst)
	   (if (zerop n)
	       (lazy-car lst)
	       (lazy-nth (1- n) (lazy-cdr lst))))