I want to do a foldl' and a foldr in parallel on a list. I assumed it would be no good idea to run foldl' and foldr separately, because then the input list must be stored completely between the calls of foldl' and foldr. I wanted to be clever and implemented a routine which does foldl' and foldr in one go. But surprisingly, at least in GHCi, my clever routine is less efficient than the naive one. Is foldl'rNaive better than I expect, or is foldl'r worse than I hope? module FoldLR where import Data.List (foldl', ) import Control.Arrow (first, second, (***), ) foldl'r, foldl'rNaive :: (b -> a -> b) -> b -> (c -> d -> d) -> d -> [(a,c)] -> (b,d) foldl'r f b0 g d0 = first ($b0) . foldr (\(a,c) ~(k,d) -> (\b -> k $! f b a, g c d)) (id,d0) foldl'rNaive f b g d xs = (foldl' f b *** foldr g d) $ unzip xs test, testNaive :: (Integer, Char) test = second last $ foldl'r (+) 0 (:) "" $ replicate 1000000 (1,'a') {- *FoldLR> test (1000000,'a') (2.65 secs, 237509960 bytes) -} testNaive = second last $ foldl'rNaive (+) 0 (:) "" $ replicate 1000000 (1,'a') {- *FoldLR> testNaive (1000000,'a') (0.50 secs, 141034352 bytes) -}