On 12/29/05, David Roundy
On Thu, Dec 29, 2005 at 01:42:29PM +0100, Christian Maeder wrote:
stats elems = foldl add_elem Map.empty elems add_elem m x = Map.insertWith (+) x 1 m
main = print $ stats $ take 1000000 $ repeat 1
This program has a space leak and runs out of stack space. I'm guessing that I'm being bit here by an unnatural amount of laziness in Map.insertWith, but I can't see how to fix this. :( I'm sure it's something elementary...
David Roundy wrote: try:
stats = foldl' add_elem Map.empty add_elem m x = myinsertWith (+) x 1 m
myinsertWith f k v m = case Map.lookup k m of Nothing -> Map.insert k v m Just w -> let r = f v w in r `seq` Map.insert k r m
Thanks, that does it! I had tried something like that, but without the foldl', and had tried foldl', but without the myinsertWith. The reason I didn't spend much time using this implementation is that for large maps, it will take twice as long as Map.insertWith, so I assumed (incorrectly) that Map.insertWith should be written so that it behaves in a non-leaky manner.
I'm not sure I understand this ... Do you say that the leaky program is faster than the non-leaky one ?
Should the Map modules have an additional Map.insertWith' that behaves strictly, or might it be the case that you always want strict behavior when using insertWith?
IMO, there is always a possibility for needing the lazy version. I also have to say that making a strict version for each "...With" does not fill my heart with joy. :) Such cases call for cheapness analysis, or optimistic evaluation, or some other kind of solution implemented at the compiler level, IMHO. Cheers, JP.