On 24/04/2010, at 00:44, Tyson Whitehead wrote:
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]
Just replacing foldl by foldl' isn't always correct, of course. I'd just like to see real-world code where using foldl is both correct (with respect to performance) and essential. Incidentally, your example aborts with a stack overflow for a list of 5M elements but this function works fine: sum . takeWhile (<3) . reverse The other example, given by Heinrich, was reverse. The report specifies it in terms of foldl but GHC implements it differently. Roman