This is just a short notice that using foldl' (+) 0 [0..100000::Int] is over 10 times slower than using flip execState 0 $ forLoop (0 :: Int) (< n) (+1) $ \i -> do x <- get put $! x + i with `loopFor` as on https://github.com/nh2/loop. Even using an IORef is twice as fast as the pure foldl' (but still 5 times slower than strict State). The benchmark is at http://htmlpreview.github.io/?https://github.com/nh2/loop/blob/master/result.... (All benchmarks are linked from https://github.com/nh2/loop.) You can see there that the problem is gone when using Vector.foldl', but only for Int - for Word32 it persists. It seems that manual looping is beneficial even when dealing with prime examples of pure code that GHC ought to optimize well. Niklas