Brent Yorgey wrote:
Raynor Vliegendhart wrote:
One of the examples I tried was:
hylo (unfoldr (\a -> Just (a,a))) head $ 42
This expression fails to determinate.
Here are two examples copumpkin tried on IRC:
<copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) length 5 <lambdabot> 5
<copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) sum 5 <lambdabot> * Exception: stack overflow
[] is a strange functor to use with hylo, since it is already recursive and its only base case (the empty list) doesn't contain any a's. Think about the intermediate structure that
hylo (unfoldr (\a -> Just (a,a))) head
is building up: it is a list of lists of lists of lists of lists of lists of.... no wonder it doesn't terminate! =)
Yep, not terminating is the correct behavior here. In particular, we have example = hylo repeat head = cata head . ana repeat and the intermediate data structure is Fix [] which is an infinite nested tower of infinite lists. Regards, apfelmus -- http://apfelmus.nfshost.com