Proposal #3999: Improved folds for Data.Map and Data.IntMap
twhitehead at gmail.com
Fri Apr 23 10:44:48 EDT 2010
On April 22, 2010 21:11:55 Roman Leshchinskiy wrote:
> That doesn't really help me, though, because I can't tell why using foldl
> as opposed to foldl' is essential (or even correct) in these cases. In
> fact, at a cursory glance a lot of them look quite leaky to me.
Here's a toy example where replacing foldl with foldl' would be wrong
sumWhile g y x | g x = y + x
| otherwise = 0
foldl (sumWhile (< 3)) 6 [5,undefined,4,3,2,1,0]
Perhaps there is other code where forcing values via foldl' before they are
known to be required can be bad (due the cost or causing an exception).
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