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1 point by akkartik 4465 days ago | link | parent

I'm just taking a constant-factor hit :) A few extra iterations doesn't change the result for either print or iso. And for cycles that loop back all the way to the start it turns out there's zero overhead.

  x <- '(1 2 3)
  lastcdr.x <- x  # 1 2 3 1 2 3 1 ...
  print x
  1
  2
  3
  ...  # exactly one iteration
But:

  x <- '(1 2 3)
  lastcdr.x <- cdr.x   # 1 2 3 2 3 2 3 ...
  print x
  1
  2
  ...


2 points by rocketnia 4464 days ago | link

  You posted:
  
  x <- '(1 2 3)
  lastcdr.x <- cdr.x   # 1 2 3 2 3 2 3 ...
  print x
  1
  2
  ...
Is that the behavior you expect? I would expect "1 2 3 ...", but it looks like the rabbit and the hare meet at the (3 ...) cons and stop.

  1 2 3 2 3 2 3
  *              Print "1".
    * *          Print "2".
      *   *      Print "...".

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1 point by akkartik 4464 days ago | link

Yeah I tried it out before posting. I had to take pains in iso to make sure we do one full traversal. With print I didn't care as much.

Update: Ack, I found a bug in iso (http://arclanguage.org/item?id=17365):

  x <- '(1 2 3)
  y <- '(1 2 4)
  (do1 nil (<- lastcdr.x x lastcdr.y y))  # Avoid printing the cycles
  (iso x y)   # not nil!

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2 points by rocketnia 4464 days ago | link

Here's a slightly different bug I found before your update. It's another case where I don't know what behavior you expect, but it probably isn't this. :-p

  (do (a <- '(1 2 3)) (lastcdr.a <- cdr.a) nil)
  => nil
  (do (b <- '(1 2 3 2 3)) (lastcdr.b <- cdr.b) nil)
  => nil
  (not+not+iso a b)
  => nil
  (not+not+iso b a)
  => 1
(I'm using not+not+iso so it returns a predictable value rather than an address.)

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1 point by rocketnia 4464 days ago | link

"the rabbit and the hare"

Whoops. I didn't mean that. XD

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