On: It never makes sense to use foldl on lists in Hask...

[brmgb]

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 → brmgb]

foldl' is fine an Haskell, and one of 2 endorsed by this.

[dependenttypes → Ericson2314]

Note 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 → dependenttypes]

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 → nimih]

Foldl' 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 → 08-15]

{-# LANGUAGE Strict, StrictData, BangPatterns #-} should imply the !, right?

[08-15 → dependenttypes]

I 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 → 08-15]

Thanks, this is a helpful example :)