[Haskell] specification of sum

I was wondering... In my experience, it's worked much better to use sum' = foldl' (+) 0 than the built-in "sum" function, which leaks memory like crazy for large input lists. I'm guessing the built-in definition is sum = foldr (+) 0 But as far as I know, (+) is always strict, so foldl' seems much more natural to me. Is there a case where the build-in definition is preferable? Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx