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Yes, that's a very peculiar feature of Haskell. I'm slightly surprised this managed to reach the front page. In most language, you should generally prefer fold
by brmgb 7y ago
Yes, that's a very peculiar feature of Haskell. I'm slightly surprised this managed to reach the front page.
In most language, you should generally prefer foldleft to foldright as foldright is not tail-recursive.
- Ericson2314 7y agofoldl' is fine an Haskell, and one of 2 endorsed by this.
- dependenttypes 7y agoNote that you need to import Data.List if you want to use foldl' (and chances are that you will still leak memory like crazy)
- nimih 7y agoOut 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 :)