Hello, we have a strange performance behaviour when we use unsafePerformIO, at least with GHC 6.12.3 and 7.0.1. Please consider the example program following at the end of this post. Running the original code the execution time is about 26 seconds, while uncommenting one (or both) of the comments shrinks it to about 0.01 seconds on our machine. Is there an explanation for this effect? Regards, Bjoern -- --------------- module Main where import System.IO.Unsafe traverse [] = return () -- traverse (_:xs) = traverse xs traverse (_:xs) = traverse xs >> return () makeList 0 = [] -- makeList n = () : (makeList (n - 1)) makeList n = () : (unsafePerformIO . return) (makeList (n - 1)) main = traverse $ makeList (10^5)