gistfile1.cl

;;; generator っぽいものだとスッキリしそうなケースを思いついたので書いて
;;; みたけど、あまりスッキリしなかった

(define-condition stop-iteration (condition)
  ()
  (:report (lambda (c s) (format s "Stop Iteration"))))
=> t

(defun plist-iterator (plist)
  "generato iterator from plist."
  (lambda ()
    (block #1=#:iterator
      (while (>= (length plist) 2)
        (return-from #1#
          (multiple-value-prog1
              (values (car plist) (cadr plist))
            (setf plist (cddr plist)))))
      (signal 'stop-iteration))))
=> plist-iterator

;; 手動で funcall してみる
(setf iter (plist-iterator '(foo 1 bar 2 baz 3)))
=> #<lexical-closure: (anonymous)>

(funcall iter)
=> foo
=> 1

(funcall iter)
=> bar
=> 2

(funcall iter)
=> baz
=> 3

(funcall iter)
>> Stop Iteration


;; 止まるまで funcall し続ける
(setf iter (plist-iterator '(foo 1 bar 2 baz 3)))
=> #<lexical-closure: (anonymous)>

(handler-case
    (loop
      (multiple-value-bind (key value)
          (funcall iter)
        (format t "~S = ~S~%" key value)))
  (stop-iteration (e) nil))
foo = 1
bar = 2
baz = 3
=> nil

gistfile2.cl

;;; macro で generator 定義だけ少しスッキリさせた

(defmacro defgenerator (name (&rest args) &body body)
  "Yield!!"
  `(defun ,name (,@args)
     (lambda ()
       (block #1=#:iterator
         (macrolet ((yield (value)
                      `(return-from #1# ,value)))
           ,@body
           (signal 'stop-iteration))))))
=> defgenerator

(defgenerator plist-iterator (plist)
  (while (>= (length plist) 2)
    (yield (multiple-value-prog1
               (values (first plist) (second plist))
             (setf plist (cddr plist))))))
=> plist-iterator

(setf iter (plist-iterator '(foo 1 bar 2 baz 3)))
=> #<lexical-closure: (anonymous)>

(handler-case
    (loop
      (multiple-value-bind (key value)
          (funcall iter)
        (format t "~S = ~S~%" key value)))
  (stop-iteration (e) nil))
foo = 1
bar = 2
baz = 3
=> nil