Is there a difference between "sum $ map f xs" and "sum $! map f xs"? I would think no, since "$!" just evaluates the argument to WHNF, so it would evaluate just the first cons of the list. This little program main = print $ sum $ map bitcount [0, 4 .. 2^24-1 ] bitcount :: Int -> Int bitcount x = if x > 0 then let (d,m) = divMod x 2 in bitcount d + m else 0 runs in 1.6 seconds when compiled with -O2 on ghc-7.6.3, ghc-7.8.4, but takes 3.2 seconds on ghc-7.10.[1-3]. Why? when I change the main function (note: $ => $!) to main = print $ sum $! map bitcount [0, 4 .. 2^24-1 ] and compile with 7.10.[1-3], then it also runs in 1.6 seconds. (I can write a bug report but I want to check whether I am missing something obvious) - J.W.