I'd like to make an iterator that yields pairs:

(0 1)
(1 1)
(1 2)
(2 2)
(2 3)
(3 3)

I've tried variants of the following:

(iter-defun iter-rectangle-sizes ()
  (let ((rect '(0 1)))
    (while t
      (let ((a (car rect))
            (b (nth 1 rect)))
        (if (> b a)
            (setq rect (list (1+ a) b))
          (setq rect (list a (1+ b))))
        (iter-yield rect)))))

But I'm getting this error that don't understand when I try to create the iterator:

ELISP> (setf rectangles (iter-rectangle-sizes))
 ((cps-state-atom-68 closure #1 nil
                     (setf cps-current-value-40
                               '(0 1)
                             (setf cps-current-state-41 cps-state-let*-67))))
  (cps-state-let*-67 closure #1 nil
                     (setf cps-binding-cps-binding-rect-43-44 cps-current-value-40 cps-current-state-41 cps-state-atom-66))
  (cps-state-atom-66 closure #1 nil
                     (setf cps-current-value-40
                           (prog1 cps-binding-cps-binding-rect-43-44
                             (setf cps-current-state-41 cps-state-let*-65))))
  (cps-state-let*-65 closure #1 nil
                     (setf cps-binding-rect-45 cps-current-value-40 cps-current-state-41 cps-state-atom-47))
  (cps-state-while-46 closure #1 nil
                        (setf cps-current-state-41
                              (if cps-current-value-40 cps-state-atom-64 cps-state-terminal-42))))
  (cps-state-atom-64 closure #1 nil
                     (setf cps-current-value-40
                               (car cps-binding-rect-45)
                             (setf cps-current-state-41 cps-state-let*-63))))
  (cps-state-let*-63 closure #1 nil
                     (setf cps-binding-cps-binding-a-48-50 cps-current-value-40 cps-current-state-41 cps-state-atom-62))
  (cps-state-atom-62 closure #1 nil
                     (setf cps-current-value-40
                               (nth 1 cps-binding-rect-45)
                             (setf cps-current-state-41 cps-state-let*-61))))
  (cps-state-let*-61 closure #1 nil
                     (setf cps-binding-cps-binding-b-49-51 cps-current-value-40 cps-current-state-41 cps-state-atom-60))
  (cps-state-atom-60 closure #1 nil
                     (setf cps-current-value-40
                           (prog1 cps-binding-cps-binding-a-48-50
                             (setf cps-current-state-41 cps-state-let*-59))))
  (cps-state-let*-59 closure #1 nil
                     (setf cps-binding-a-52 cps-current-value-40 cps-current-state-41 cps-state-atom-58))
  (cps-state-atom-58 closure #1 nil
                     (setf cps-current-value-40
                           (prog1 cps-binding-cps-binding-b-49-51
                             (setf cps-current-state-41 cps-state-let*-57))))
  (cps-state-let*-57 closure #1 nil
                     (setf cps-binding-b-53 cps-current-value-40 cps-current-state-41 cps-state-atom-56))
  (cps-state-atom-56 closure #1 nil
                     (setf cps-current-value-40
                                   (> cps-binding-b-53 cps-binding-a-52)
                                   (setq cps-binding-rect-45
                                          (1+ cps-binding-a-52)
                                 (setq cps-binding-rect-45
                                       (list cps-binding-a-52
                                             (1+ cps-binding-b-53))))
                             (setf cps-current-state-41 cps-state-atom-55))))
  (cps-state-atom-55 closure #1 nil
                     (setf cps-current-value-40
                           (prog1 cps-binding-rect-45
                             (setf cps-current-state-41 cps-state-iter-yield-54))))
  (cps-state-iter-yield-54 closure #1 nil
                             (setf cps-current-state-41 cps-state-atom-47)
                             (throw 'cps--yield cps-current-value-40)))
  (cps-state-atom-47 closure #1 nil
                     (setf cps-current-value-40
                           (prog1 t
                             (setf cps-current-state-41 cps-state-while-46))))
  (cps-state-terminal-42 closure #1 nil
                         (signal 'iter-end-of-sequence cps-current-value-40))
  (cps-current-state-41 closure #1 nil
                        (setf cps-current-value-40
                                  '(0 1)
                                (setf cps-current-state-41 cps-state-let*-67))))
 (op value)
  ((eq op :close)
   (setf cps-current-state-41 cps-state-terminal-42 cps-current-value-40 nil))
  ((eq op :next)
   (setf cps-current-value-40 value)
       ((yielded nil))
             (catch 'cps--yield
               (while t
                 (funcall cps-current-state-41)))
           (setf yielded t))
       (unless yielded
         (setf cps-current-state-41 cps-state-terminal-42 cps-current-value-40 nil)))))
   (error "unknown iterator operation %S" op))))

I could leave without generators, but I'm curious about what I'm doing wrong here.

By the way, I'm a beginner with elisp, so don't hesitate to critic iter-rectangle-sizes, even if it's not directly related to the error I'm seeing.


There is no error, the (error "unknown iterator operation %S" op) is part of the closure. Simply type (iter-next rectangles) repeatedly, you will see the expected result. cl-loop also provides a syntax sugar for generators:

(cl-loop repeat 5
         for r iter-by (iter-rectangle-sizes)
         collect r)
;; => ((1 1) (1 2) (2 2) (2 3) (3 3))
  • omg, I spent 30 minutes trying to "fix" this, and I didn't even try to use the iterator... Thanks @xuchunyang! May 16 '19 at 16:20

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