I was reading this article https://wiki.haskell.org/Correctness_of_short_cut_fusion on the Haskell wiki. It presents an example (2.1.2) where destroy/unfoldr fusion behaves oddly. If I use a lazier definition of unfoldr, then this problem goes away: unfoldr :: (b -> Maybe (a, b)) -> b -> [a]
unfoldr f b = case f b of Nothing -> [] Just z -> fst z : unfoldr f (snd z)
Does this mean unfoldr is 'too strict'? Or is there a good reason for not writing it this way (performance, perhaps?)
I think the "official" version could be implemented with a lazy pattern match and it'd be the same as yours, no? Cheers, Vanessa On 12/11/18 12:55 AM, Isaac Elliott wrote:
I was reading this article https://wiki.haskell.org/Correctness_of_short_cut_fusion on the Haskell wiki. It presents an example (2.1.2) where destroy/unfoldr fusion behaves oddly. If I use a lazier definition of unfoldr, then this problem goes away:
unfoldr :: (b -> Maybe (a, b)) -> b -> [a] unfoldr f b = case f b of Nothing -> [] Just z -> fst z : unfoldr f (snd z)
Does this mean unfoldr is 'too strict'? Or is there a good reason for not writing it this way (performance, perhaps?)
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Yep, a lazy pattern match gets you the same benefit.
On Tue, 11 Dec. 2018, 7:50 pm Vanessa McHale,
I think the "official" version could be implemented with a lazy pattern match and it'd be the same as yours, no?
Cheers, Vanessa On 12/11/18 12:55 AM, Isaac Elliott wrote:
I was reading this article https://wiki.haskell.org/Correctness_of_short_cut_fusion on the Haskell wiki. It presents an example (2.1.2) where destroy/unfoldr fusion behaves oddly. If I use a lazier definition of unfoldr, then this problem goes away:
unfoldr :: (b -> Maybe (a, b)) -> b -> [a]
unfoldr f b = case f b of Nothing -> [] Just z -> fst z : unfoldr f (snd z)
Does this mean unfoldr is 'too strict'? Or is there a good reason for not writing it this way (performance, perhaps?)
_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to:http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
participants (2)
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Isaac Elliott -
Vanessa McHale