2 ms·
Out of curiosity, what causes foldl' to leak memory in most cases? I generally use it out of habit if I know I will consume the entire list and don't have a goo
by nimih 7y ago
Out of curiosity, what causes foldl' to leak memory in most cases? I generally use it out of habit if I know I will consume the entire list and don't have a good reason to use foldr, so it would be nice to know if I'm shooting myself in the foot.
- 08-15 7y agoFoldl' doesn't leak, but it sure looks like it does, if you think the folded function is strict in the accumulator argument, but isn't. For example, this attempt at computing the average of a list uncurry (/) . foldl' (\(s,l) x -> (s+x,l+1)) (0,0) leaks, because during the recursion, there is no reason to scrutinize the intermediate pairs. This version is fine: uncurry (/) . foldl' (\(!s,!l) x -> (s+x,l+1)) (0,0)
- dependenttypes 7y ago{-# LANGUAGE Strict, StrictData, BangPatterns #-} should imply the !, right?
- 08-15 7y agoI don't think it does. 'Strict' doesn't add an implicit ! to nested patterns (so (s,l) becomes !(s,l), not (!s,!l)), and 'StrictData' doesn't change anything about the tuple, which comes from a different module. (Disclaimer: I haven't actually used 'Strict', this is my interpretation of the user guide.)
- nimih 7y agoThanks, this is a helpful example :)