Hello Cafe, While investigating a performance problem I stumbled upon what I eventually reduced to the example below: module Main where import Data.Time.Clock outside :: Int -> Int outside n = sum [i + j | i <- range, j <- range] where range = [0..n-1] inside :: Int -> Int inside n = sum [i + j | i <- [0..n-1], j <- [0..n-1]] main :: IO () main = do t0 <- getCurrentTime print $ inside 10000 t1 <- getCurrentTime print $ outside 10000 t2 <-getCurrentTime print (diffUTCTime t1 t0) print (diffUTCTime t2 t1) Compiling with -O2, up to GHC 8.2.2, both `inside` and `outside` functions would take the same amount of time to execute. Somewhere between GHC 8.2.2 and 8.6.4 something changed (possibly some new optimization) making `inside` run ~4x faster on my machine. With LLVM the difference is even bigger. It is not that `outside` got slower, but that `inside` got much faster. I'm curious to what optimizations might be happening to the `inside` function that would not fire on the outside function. Any hints? Best regards, Emilio